备战-Java 容器

2021年7月17日 3点热度 0条评论 来源: 涛姐涛哥

备战-Java 容器

 

    玉阶生白露,夜久侵罗袜。

 

简介:备战-Java 容器

一、概述

容器主要包括 Collection 和 Map 两种,Collection 存储着对象的集合,而 Map 存储着key-value 键值对(两个对象)的映射表。

Collection

1. Set

  • TreeSet:基于红黑树实现,支持有序性操作,例如根据一个范围查找元素的操作。但是查找效率不如 HashSet,HashSet 查找的时间复杂度为 O(1),TreeSet 则为 O(logN)。

  • HashSet:基于哈希表实现,支持快速查找,但不支持有序性操作。并且失去了元素的插入顺序信息,也就是说使用 Iterator 遍历 HashSet 得到的结果是不确定的。

  • LinkedHashSet:具有 HashSet 的查找效率,并且内部使用双向链表维护元素的插入顺序。

2. List

  • ArrayList:基于动态数组实现,支持随机访问。

  • Vector:和 ArrayList 类似,但它是线程安全的。

  • LinkedList:基于双向链表实现,只能顺序访问,但是可以快速地在链表中间插入和删除元素。不仅如此,LinkedList 还可以用作栈、队列和双向队列。

3. Queue

  • LinkedList:可以用它来实现双向队列。

  • PriorityQueue:基于堆结构实现,可以用它来实现优先队列。

Map

  • TreeMap:基于红黑树实现。

  • HashMap:基于哈希表实现。

  • HashTable:和 HashMap 类似,但它是线程安全的,这意味着同一时刻多个线程同时写入 HashTable 不会导致数据不一致。它是遗留类,不应该去使用它,而是使用 ConcurrentHashMap 来支持线程安全,ConcurrentHashMap 的效率会更高,因为 ConcurrentHashMap 引入了分段锁。

  • LinkedHashMap:使用双向链表来维护元素的顺序,顺序为插入顺序或者最近最少使用(LRU)顺序。(LRU算法是Least Recently Used的缩写,即最近最少使用)

二、容器中的设计模式

迭代器模式

 Collection 继承了 Iterable 接口,其中的 iterator() 方法能够产生一个 Iterator 对象,通过这个对象就可以迭代遍历 Collection 中的元素。

从 JDK 1.5 之后可以使用 foreach 方法来遍历实现了 Iterable 接口的聚合对象。

1 List<String> list = new ArrayList<>(); 2 list.add("a"); 3 list.add("b"); 4 for (String item : list) { 5  System.out.println(item); 6 }

View Code

适配器模式

java.util.Arrays.asList() 可以把数组类型转换为 List 类型。

1 @SafeVarargs 2 public static <T> List<T> asList(T... a)

值得注意的是 asList() 的参数为泛型的变长参数,不能使用基本类型数组作为参数,只能使用相应的包装类型数组。

1 Integer[] arr = {1, 2, 3}; 2 List list = Arrays.asList(arr);

也可以使用以下方式调用 asList():

List list = Arrays.asList(1, 2, 3);

三、源码分析

如果没有特别说明,以下源码分析基于 JDK 1.8。

在 IDEA 中 双击 shift 键调出 Search EveryWhere,查找源码文件,找到之后就可以阅读源码。

ArrayList

1. 概览

因为 ArrayList 是基于数组实现的,所以支持快速随机访问。RandomAccess 接口标识着该类支持快速随机访问,其默认数组大小为10

 1 /*
 2  * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.  3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.  4  *  5  *  6  *  7  *  8  *  9  *  10  *  11  *  12  *  13  *  14  *  15  *  16  *  17  *  18  *  19  *  20  *  21  *  22  *  23  *  24  */
 25 
 26 package java.util;  27 
 28 import java.util.function.Consumer;  29 import java.util.function.Predicate;  30 import java.util.function.UnaryOperator;  31 import sun.misc.SharedSecrets;  32 
 33 /**
 34  * Resizable-array implementation of the <tt>List</tt> interface. Implements  35  * all optional list operations, and permits all elements, including  36  * <tt>null</tt>. In addition to implementing the <tt>List</tt> interface,  37  * this class provides methods to manipulate the size of the array that is  38  * used internally to store the list. (This class is roughly equivalent to  39  * <tt>Vector</tt>, except that it is unsynchronized.)  40  *  41  * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,  42  * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant  43  * time. The <tt>add</tt> operation runs in <i>amortized constant time</i>,  44  * that is, adding n elements requires O(n) time. All of the other operations  45  * run in linear time (roughly speaking). The constant factor is low compared  46  * to that for the <tt>LinkedList</tt> implementation.  47  *  48  * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>. The capacity is  49  * the size of the array used to store the elements in the list. It is always  50  * at least as large as the list size. As elements are added to an ArrayList,  51  * its capacity grows automatically. The details of the growth policy are not  52  * specified beyond the fact that adding an element has constant amortized  53  * time cost.  54  *  55  * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance  56  * before adding a large number of elements using the <tt>ensureCapacity</tt>  57  * operation. This may reduce the amount of incremental reallocation.  58  *  59  * <p><strong>Note that this implementation is not synchronized.</strong>  60  * If multiple threads access an <tt>ArrayList</tt> instance concurrently,  61  * and at least one of the threads modifies the list structurally, it  62  * <i>must</i> be synchronized externally. (A structural modification is  63  * any operation that adds or deletes one or more elements, or explicitly  64  * resizes the backing array; merely setting the value of an element is not  65  * a structural modification.) This is typically accomplished by  66  * synchronizing on some object that naturally encapsulates the list.  67  *  68  * If no such object exists, the list should be "wrapped" using the  69  * {@link Collections#synchronizedList Collections.synchronizedList}  70  * method. This is best done at creation time, to prevent accidental  71  * unsynchronized access to the list:<pre>  72  * List list = Collections.synchronizedList(new ArrayList(...));</pre>  73  *  74  * <p><a name="fail-fast">  75  * The iterators returned by this class's {@link #iterator() iterator} and  76  * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:</a>  77  * if the list is structurally modified at any time after the iterator is  78  * created, in any way except through the iterator's own  79  * {@link ListIterator#remove() remove} or  80  * {@link ListIterator#add(Object) add} methods, the iterator will throw a  81  * {@link ConcurrentModificationException}. Thus, in the face of  82  * concurrent modification, the iterator fails quickly and cleanly, rather  83  * than risking arbitrary, non-deterministic behavior at an undetermined  84  * time in the future.  85  *  86  * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed  87  * as it is, generally speaking, impossible to make any hard guarantees in the  88  * presence of unsynchronized concurrent modification. Fail-fast iterators  89  * throw {@code ConcurrentModificationException} on a best-effort basis.  90  * Therefore, it would be wrong to write a program that depended on this  91  * exception for its correctness: <i>the fail-fast behavior of iterators  92  * should be used only to detect bugs.</i>  93  *  94  * <p>This class is a member of the  95  * <a href="{@docRoot}/../technotes/guides/collections/index.html">  96  * Java Collections Framework</a>.  97  *  98  * @author Josh Bloch  99  * @author Neal Gafter  100  * @see Collection  101  * @see List  102  * @see LinkedList  103  * @see Vector  104  * @since 1.2  105  */
 106 
 107 public class ArrayList<E> extends AbstractList<E>
 108         implements List<E>, RandomAccess, Cloneable, java.io.Serializable  109 {  110     private static final long serialVersionUID = 8683452581122892189L;  111 
 112     /**
 113  * Default initial capacity.  114      */
 115     private static final int DEFAULT_CAPACITY = 10;  116 
 117     /**
 118  * Shared empty array instance used for empty instances.  119      */
 120     private static final Object[] EMPTY_ELEMENTDATA = {};  121 
 122     /**
 123  * Shared empty array instance used for default sized empty instances. We  124  * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when  125  * first element is added.  126      */
 127     private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};  128 
 129     /**
 130  * The array buffer into which the elements of the ArrayList are stored.  131  * The capacity of the ArrayList is the length of this array buffer. Any  132  * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA  133  * will be expanded to DEFAULT_CAPACITY when the first element is added.  134      */
 135     transient Object[] elementData; // non-private to simplify nested class access
 136 
 137     /**
 138  * The size of the ArrayList (the number of elements it contains).  139  *  140  * @serial
 141      */
 142     private int size;  143 
 144     /**
 145  * Constructs an empty list with the specified initial capacity.  146  *  147  * @param initialCapacity the initial capacity of the list  148  * @throws IllegalArgumentException if the specified initial capacity  149  * is negative  150      */
 151     public ArrayList(int initialCapacity) {  152         if (initialCapacity > 0) {  153             this.elementData = new Object[initialCapacity];  154         } else if (initialCapacity == 0) {  155             this.elementData = EMPTY_ELEMENTDATA;  156         } else {  157             throw new IllegalArgumentException("Illegal Capacity: "+
 158  initialCapacity);  159  }  160  }  161 
 162     /**
 163  * Constructs an empty list with an initial capacity of ten.  164      */
 165     public ArrayList() {  166         this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;  167  }  168 
 169     /**
 170  * Constructs a list containing the elements of the specified  171  * collection, in the order they are returned by the collection's  172  * iterator.  173  *  174  * @param c the collection whose elements are to be placed into this list  175  * @throws NullPointerException if the specified collection is null  176      */
 177     public ArrayList(Collection<? extends E> c) {  178         elementData = c.toArray();  179         if ((size = elementData.length) != 0) {  180             // c.toArray might (incorrectly) not return Object[] (see 6260652)
 181             if (elementData.getClass() != Object[].class)  182                 elementData = Arrays.copyOf(elementData, size, Object[].class);  183         } else {  184             // replace with empty array.
 185             this.elementData = EMPTY_ELEMENTDATA;  186  }  187  }  188 
 189     /**
 190  * Trims the capacity of this <tt>ArrayList</tt> instance to be the  191  * list's current size. An application can use this operation to minimize  192  * the storage of an <tt>ArrayList</tt> instance.  193      */
 194     public void trimToSize() {  195         modCount++;  196         if (size < elementData.length) {  197             elementData = (size == 0)  198               ? EMPTY_ELEMENTDATA  199  : Arrays.copyOf(elementData, size);  200  }  201  }  202 
 203     /**
 204  * Increases the capacity of this <tt>ArrayList</tt> instance, if  205  * necessary, to ensure that it can hold at least the number of elements  206  * specified by the minimum capacity argument.  207  *  208  * @param minCapacity the desired minimum capacity  209      */
 210     public void ensureCapacity(int minCapacity) {  211         int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)  212             // any size if not default element table
 213             ? 0
 214             // larger than default for default empty table. It's already  215             // supposed to be at default size.
 216  : DEFAULT_CAPACITY;  217 
 218         if (minCapacity > minExpand) {  219  ensureExplicitCapacity(minCapacity);  220  }  221  }  222 
 223     private static int calculateCapacity(Object[] elementData, int minCapacity) {  224         if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {  225             return Math.max(DEFAULT_CAPACITY, minCapacity);  226  }  227         return minCapacity;  228  }  229 
 230     private void ensureCapacityInternal(int minCapacity) {  231  ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));  232  }  233 
 234     private void ensureExplicitCapacity(int minCapacity) {  235         modCount++;  236 
 237         // overflow-conscious code
 238         if (minCapacity - elementData.length > 0)  239  grow(minCapacity);  240  }  241 
 242     /**
 243  * The maximum size of array to allocate.  244  * Some VMs reserve some header words in an array.  245  * Attempts to allocate larger arrays may result in  246  * OutOfMemoryError: Requested array size exceeds VM limit  247      */
 248     private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;  249 
 250     /**
 251  * Increases the capacity to ensure that it can hold at least the  252  * number of elements specified by the minimum capacity argument.  253  *  254  * @param minCapacity the desired minimum capacity  255      */
 256     private void grow(int minCapacity) {  257         // overflow-conscious code
 258         int oldCapacity = elementData.length;  259         int newCapacity = oldCapacity + (oldCapacity >> 1);  260         if (newCapacity - minCapacity < 0)  261             newCapacity = minCapacity;  262         if (newCapacity - MAX_ARRAY_SIZE > 0)  263             newCapacity = hugeCapacity(minCapacity);  264         // minCapacity is usually close to size, so this is a win:
 265         elementData = Arrays.copyOf(elementData, newCapacity);  266  }  267 
 268     private static int hugeCapacity(int minCapacity) {  269         if (minCapacity < 0) // overflow
 270             throw new OutOfMemoryError();  271         return (minCapacity > MAX_ARRAY_SIZE) ?
 272  Integer.MAX_VALUE :  273  MAX_ARRAY_SIZE;  274  }  275 
 276     /**
 277  * Returns the number of elements in this list.  278  *  279  * @return the number of elements in this list  280      */
 281     public int size() {  282         return size;  283  }  284 
 285     /**
 286  * Returns <tt>true</tt> if this list contains no elements.  287  *  288  * @return <tt>true</tt> if this list contains no elements  289      */
 290     public boolean isEmpty() {  291         return size == 0;  292  }  293 
 294     /**
 295  * Returns <tt>true</tt> if this list contains the specified element.  296  * More formally, returns <tt>true</tt> if and only if this list contains  297  * at least one element <tt>e</tt> such that  298  * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.  299  *  300  * @param o element whose presence in this list is to be tested  301  * @return <tt>true</tt> if this list contains the specified element  302      */
 303     public boolean contains(Object o) {  304         return indexOf(o) >= 0;  305  }  306 
 307     /**
 308  * Returns the index of the first occurrence of the specified element  309  * in this list, or -1 if this list does not contain the element.  310  * More formally, returns the lowest index <tt>i</tt> such that  311  * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,  312  * or -1 if there is no such index.  313      */
 314     public int indexOf(Object o) {  315         if (o == null) {  316             for (int i = 0; i < size; i++)  317                 if (elementData[i]==null)  318                     return i;  319         } else {  320             for (int i = 0; i < size; i++)  321                 if (o.equals(elementData[i]))  322                     return i;  323  }  324         return -1;  325  }  326 
 327     /**
 328  * Returns the index of the last occurrence of the specified element  329  * in this list, or -1 if this list does not contain the element.  330  * More formally, returns the highest index <tt>i</tt> such that  331  * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,  332  * or -1 if there is no such index.  333      */
 334     public int lastIndexOf(Object o) {  335         if (o == null) {  336             for (int i = size-1; i >= 0; i--)  337                 if (elementData[i]==null)  338                     return i;  339         } else {  340             for (int i = size-1; i >= 0; i--)  341                 if (o.equals(elementData[i]))  342                     return i;  343  }  344         return -1;  345  }  346 
 347     /**
 348  * Returns a shallow copy of this <tt>ArrayList</tt> instance. (The  349  * elements themselves are not copied.)  350  *  351  * @return a clone of this <tt>ArrayList</tt> instance  352      */
 353     public Object clone() {  354         try {  355             ArrayList<?> v = (ArrayList<?>) super.clone();  356             v.elementData = Arrays.copyOf(elementData, size);  357             v.modCount = 0;  358             return v;  359         } catch (CloneNotSupportedException e) {  360             // this shouldn't happen, since we are Cloneable
 361             throw new InternalError(e);  362  }  363  }  364 
 365     /**
 366  * Returns an array containing all of the elements in this list  367  * in proper sequence (from first to last element).  368  *  369  * <p>The returned array will be "safe" in that no references to it are  370  * maintained by this list. (In other words, this method must allocate  371  * a new array). The caller is thus free to modify the returned array.  372  *  373  * <p>This method acts as bridge between array-based and collection-based  374  * APIs.  375  *  376  * @return an array containing all of the elements in this list in  377  * proper sequence  378      */
 379     public Object[] toArray() {  380         return Arrays.copyOf(elementData, size);  381  }  382 
 383     /**
 384  * Returns an array containing all of the elements in this list in proper  385  * sequence (from first to last element); the runtime type of the returned  386  * array is that of the specified array. If the list fits in the  387  * specified array, it is returned therein. Otherwise, a new array is  388  * allocated with the runtime type of the specified array and the size of  389  * this list.  390  *  391  * <p>If the list fits in the specified array with room to spare  392  * (i.e., the array has more elements than the list), the element in  393  * the array immediately following the end of the collection is set to  394  * <tt>null</tt>. (This is useful in determining the length of the  395  * list <i>only</i> if the caller knows that the list does not contain  396  * any null elements.)  397  *  398  * @param a the array into which the elements of the list are to  399  * be stored, if it is big enough; otherwise, a new array of the  400  * same runtime type is allocated for this purpose.  401  * @return an array containing the elements of the list  402  * @throws ArrayStoreException if the runtime type of the specified array  403  * is not a supertype of the runtime type of every element in  404  * this list  405  * @throws NullPointerException if the specified array is null  406      */
 407     @SuppressWarnings("unchecked")  408     public <T> T[] toArray(T[] a) {  409         if (a.length < size)  410             // Make a new array of a's runtime type, but my contents:
 411             return (T[]) Arrays.copyOf(elementData, size, a.getClass());  412         System.arraycopy(elementData, 0, a, 0, size);  413         if (a.length > size)  414             a[size] = null;  415         return a;  416  }  417 
 418     // Positional Access Operations
 419 
 420     @SuppressWarnings("unchecked")  421     E elementData(int index) {  422         return (E) elementData[index];  423  }  424 
 425     /**
 426  * Returns the element at the specified position in this list.  427  *  428  * @param index index of the element to return  429  * @return the element at the specified position in this list  430  * @throws IndexOutOfBoundsException {@inheritDoc}  431      */
 432     public E get(int index) {  433  rangeCheck(index);  434 
 435         return elementData(index);  436  }  437 
 438     /**
 439  * Replaces the element at the specified position in this list with  440  * the specified element.  441  *  442  * @param index index of the element to replace  443  * @param element element to be stored at the specified position  444  * @return the element previously at the specified position  445  * @throws IndexOutOfBoundsException {@inheritDoc}  446      */
 447     public E set(int index, E element) {  448  rangeCheck(index);  449 
 450         E oldValue = elementData(index);  451         elementData[index] = element;  452         return oldValue;  453  }  454 
 455     /**
 456  * Appends the specified element to the end of this list.  457  *  458  * @param e element to be appended to this list  459  * @return <tt>true</tt> (as specified by {@link Collection#add})  460      */
 461     public boolean add(E e) {  462         ensureCapacityInternal(size + 1);  // Increments modCount!!
 463         elementData[size++] = e;  464         return true;  465  }  466 
 467     /**
 468  * Inserts the specified element at the specified position in this  469  * list. Shifts the element currently at that position (if any) and  470  * any subsequent elements to the right (adds one to their indices).  471  *  472  * @param index index at which the specified element is to be inserted  473  * @param element element to be inserted  474  * @throws IndexOutOfBoundsException {@inheritDoc}  475      */
 476     public void add(int index, E element) {  477  rangeCheckForAdd(index);  478 
 479         ensureCapacityInternal(size + 1);  // Increments modCount!!
 480         System.arraycopy(elementData, index, elementData, index + 1,  481                          size - index);  482         elementData[index] = element;  483         size++;  484  }  485 
 486     /**
 487  * Removes the element at the specified position in this list.  488  * Shifts any subsequent elements to the left (subtracts one from their  489  * indices).  490  *  491  * @param index the index of the element to be removed  492  * @return the element that was removed from the list  493  * @throws IndexOutOfBoundsException {@inheritDoc}  494      */
 495     public E remove(int index) {  496  rangeCheck(index);  497 
 498         modCount++;  499         E oldValue = elementData(index);  500 
 501         int numMoved = size - index - 1;  502         if (numMoved > 0)  503             System.arraycopy(elementData, index+1, elementData, index,  504  numMoved);  505         elementData[--size] = null; // clear to let GC do its work
 506 
 507         return oldValue;  508  }  509 
 510     /**
 511  * Removes the first occurrence of the specified element from this list,  512  * if it is present. If the list does not contain the element, it is  513  * unchanged. More formally, removes the element with the lowest index  514  * <tt>i</tt> such that  515  * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>  516  * (if such an element exists). Returns <tt>true</tt> if this list  517  * contained the specified element (or equivalently, if this list  518  * changed as a result of the call).  519  *  520  * @param o element to be removed from this list, if present  521  * @return <tt>true</tt> if this list contained the specified element  522      */
 523     public boolean remove(Object o) {  524         if (o == null) {  525             for (int index = 0; index < size; index++)  526                 if (elementData[index] == null) {  527  fastRemove(index);  528                     return true;  529  }  530         } else {  531             for (int index = 0; index < size; index++)  532                 if (o.equals(elementData[index])) {  533  fastRemove(index);  534                     return true;  535  }  536  }  537         return false;  538  }  539 
 540     /*
 541  * Private remove method that skips bounds checking and does not  542  * return the value removed.  543      */
 544     private void fastRemove(int index) {  545         modCount++;  546         int numMoved = size - index - 1;  547         if (numMoved > 0)  548             System.arraycopy(elementData, index+1, elementData, index,  549  numMoved);  550         elementData[--size] = null; // clear to let GC do its work
 551  }  552 
 553     /**
 554  * Removes all of the elements from this list. The list will  555  * be empty after this call returns.  556      */
 557     public void clear() {  558         modCount++;  559 
 560         // clear to let GC do its work
 561         for (int i = 0; i < size; i++)  562             elementData[i] = null;  563 
 564         size = 0;  565  }  566 
 567     /**
 568  * Appends all of the elements in the specified collection to the end of  569  * this list, in the order that they are returned by the  570  * specified collection's Iterator. The behavior of this operation is  571  * undefined if the specified collection is modified while the operation  572  * is in progress. (This implies that the behavior of this call is  573  * undefined if the specified collection is this list, and this  574  * list is nonempty.)  575  *  576  * @param c collection containing elements to be added to this list  577  * @return <tt>true</tt> if this list changed as a result of the call  578  * @throws NullPointerException if the specified collection is null  579      */
 580     public boolean addAll(Collection<? extends E> c) {  581         Object[] a = c.toArray();  582         int numNew = a.length;  583         ensureCapacityInternal(size + numNew);  // Increments modCount
 584         System.arraycopy(a, 0, elementData, size, numNew);  585         size += numNew;  586         return numNew != 0;  587  }  588 
 589     /**
 590  * Inserts all of the elements in the specified collection into this  591  * list, starting at the specified position. Shifts the element  592  * currently at that position (if any) and any subsequent elements to  593  * the right (increases their indices). The new elements will appear  594  * in the list in the order that they are returned by the  595  * specified collection's iterator.  596  *  597  * @param index index at which to insert the first element from the  598  * specified collection  599  * @param c collection containing elements to be added to this list  600  * @return <tt>true</tt> if this list changed as a result of the call  601  * @throws IndexOutOfBoundsException {@inheritDoc}  602  * @throws NullPointerException if the specified collection is null  603      */
 604     public boolean addAll(int index, Collection<? extends E> c) {  605  rangeCheckForAdd(index);  606 
 607         Object[] a = c.toArray();  608         int numNew = a.length;  609         ensureCapacityInternal(size + numNew);  // Increments modCount
 610 
 611         int numMoved = size - index;  612         if (numMoved > 0)  613             System.arraycopy(elementData, index, elementData, index + numNew,  614  numMoved);  615 
 616         System.arraycopy(a, 0, elementData, index, numNew);  617         size += numNew;  618         return numNew != 0;  619  }  620 
 621     /**
 622  * Removes from this list all of the elements whose index is between  623  * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  624  * Shifts any succeeding elements to the left (reduces their index).  625  * This call shortens the list by {@code (toIndex - fromIndex)} elements.  626  * (If {@code toIndex==fromIndex}, this operation has no effect.)  627  *  628  * @throws IndexOutOfBoundsException if {@code fromIndex} or  629  * {@code toIndex} is out of range  630  * ({@code fromIndex < 0 ||  631  * fromIndex >= size() ||  632  * toIndex > size() ||  633  * toIndex < fromIndex})  634      */
 635     protected void removeRange(int fromIndex, int toIndex) {  636         modCount++;  637         int numMoved = size - toIndex;  638  System.arraycopy(elementData, toIndex, elementData, fromIndex,  639  numMoved);  640 
 641         // clear to let GC do its work
 642         int newSize = size - (toIndex-fromIndex);  643         for (int i = newSize; i < size; i++) {  644             elementData[i] = null;  645  }  646         size = newSize;  647  }  648 
 649     /**
 650  * Checks if the given index is in range. If not, throws an appropriate  651  * runtime exception. This method does *not* check if the index is  652  * negative: It is always used immediately prior to an array access,  653  * which throws an ArrayIndexOutOfBoundsException if index is negative.  654      */
 655     private void rangeCheck(int index) {  656         if (index >= size)  657             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));  658  }  659 
 660     /**
 661  * A version of rangeCheck used by add and addAll.  662      */
 663     private void rangeCheckForAdd(int index) {  664         if (index > size || index < 0)  665             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));  666  }  667 
 668     /**
 669  * Constructs an IndexOutOfBoundsException detail message.  670  * Of the many possible refactorings of the error handling code,  671  * this "outlining" performs best with both server and client VMs.  672      */
 673     private String outOfBoundsMsg(int index) {  674         return "Index: "+index+", Size: "+size;  675  }  676 
 677     /**
 678  * Removes from this list all of its elements that are contained in the  679  * specified collection.  680  *  681  * @param c collection containing elements to be removed from this list  682  * @return {@code true} if this list changed as a result of the call  683  * @throws ClassCastException if the class of an element of this list  684  * is incompatible with the specified collection  685  * (<a href="Collection.html#optional-restrictions">optional</a>)  686  * @throws NullPointerException if this list contains a null element and the  687  * specified collection does not permit null elements  688  * (<a href="Collection.html#optional-restrictions">optional</a>),  689  * or if the specified collection is null  690  * @see Collection#contains(Object)  691      */
 692     public boolean removeAll(Collection<?> c) {  693  Objects.requireNonNull(c);  694         return batchRemove(c, false);  695  }  696 
 697     /**
 698  * Retains only the elements in this list that are contained in the  699  * specified collection. In other words, removes from this list all  700  * of its elements that are not contained in the specified collection.  701  *  702  * @param c collection containing elements to be retained in this list  703  * @return {@code true} if this list changed as a result of the call  704  * @throws ClassCastException if the class of an element of this list  705  * is incompatible with the specified collection  706  * (<a href="Collection.html#optional-restrictions">optional</a>)  707  * @throws NullPointerException if this list contains a null element and the  708  * specified collection does not permit null elements  709  * (<a href="Collection.html#optional-restrictions">optional</a>),  710  * or if the specified collection is null  711  * @see Collection#contains(Object)  712      */
 713     public boolean retainAll(Collection<?> c) {  714  Objects.requireNonNull(c);  715         return batchRemove(c, true);  716  }  717 
 718     private boolean batchRemove(Collection<?> c, boolean complement) {  719         final Object[] elementData = this.elementData;  720         int r = 0, w = 0;  721         boolean modified = false;  722         try {  723             for (; r < size; r++)  724                 if (c.contains(elementData[r]) == complement)  725                     elementData[w++] = elementData[r];  726         } finally {  727             // Preserve behavioral compatibility with AbstractCollection,  728             // even if c.contains() throws.
 729             if (r != size) {  730  System.arraycopy(elementData, r,  731  elementData, w,  732                                  size - r);  733                 w += size - r;  734  }  735             if (w != size) {  736                 // clear to let GC do its work
 737                 for (int i = w; i < size; i++)  738                     elementData[i] = null;  739                 modCount += size - w;  740                 size = w;  741                 modified = true;  742  }  743  }  744         return modified;  745  }  746 
 747     /**
 748  * Save the state of the <tt>ArrayList</tt> instance to a stream (that  749  * is, serialize it).  750  *  751  * @serialData The length of the array backing the <tt>ArrayList</tt>  752  * instance is emitted (int), followed by all of its elements  753  * (each an <tt>Object</tt>) in the proper order.  754      */
 755     private void writeObject(java.io.ObjectOutputStream s)  756         throws java.io.IOException{  757         // Write out element count, and any hidden stuff
 758         int expectedModCount = modCount;  759  s.defaultWriteObject();  760 
 761         // Write out size as capacity for behavioural compatibility with clone()
 762  s.writeInt(size);  763 
 764         // Write out all elements in the proper order.
 765         for (int i=0; i<size; i++) {  766  s.writeObject(elementData[i]);  767  }  768 
 769         if (modCount != expectedModCount) {  770             throw new ConcurrentModificationException();  771  }  772  }  773 
 774     /**
 775  * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,  776  * deserialize it).  777      */
 778     private void readObject(java.io.ObjectInputStream s)  779         throws java.io.IOException, ClassNotFoundException {  780         elementData = EMPTY_ELEMENTDATA;  781 
 782         // Read in size, and any hidden stuff
 783  s.defaultReadObject();  784 
 785         // Read in capacity
 786         s.readInt(); // ignored
 787 
 788         if (size > 0) {  789             // be like clone(), allocate array based upon size not capacity
 790             int capacity = calculateCapacity(elementData, size);  791             SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);  792  ensureCapacityInternal(size);  793 
 794             Object[] a = elementData;  795             // Read in all elements in the proper order.
 796             for (int i=0; i<size; i++) {  797                 a[i] = s.readObject();  798  }  799  }  800  }  801 
 802     /**
 803  * Returns a list iterator over the elements in this list (in proper  804  * sequence), starting at the specified position in the list.  805  * The specified index indicates the first element that would be  806  * returned by an initial call to {@link ListIterator#next next}.  807  * An initial call to {@link ListIterator#previous previous} would  808  * return the element with the specified index minus one.  809  *  810  * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.  811  *  812  * @throws IndexOutOfBoundsException {@inheritDoc}  813      */
 814     public ListIterator<E> listIterator(int index) {  815         if (index < 0 || index > size)  816             throw new IndexOutOfBoundsException("Index: "+index);  817         return new ListItr(index);  818  }  819 
 820     /**
 821  * Returns a list iterator over the elements in this list (in proper  822  * sequence).  823  *  824  * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.  825  *  826  * @see #listIterator(int)  827      */
 828     public ListIterator<E> listIterator() {  829         return new ListItr(0);  830  }  831 
 832     /**
 833  * Returns an iterator over the elements in this list in proper sequence.  834  *  835  * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.  836  *  837  * @return an iterator over the elements in this list in proper sequence  838      */
 839     public Iterator<E> iterator() {  840         return new Itr();  841  }  842 
 843     /**
 844  * An optimized version of AbstractList.Itr  845      */
 846     private class Itr implements Iterator<E> {  847         int cursor;       // index of next element to return
 848         int lastRet = -1; // index of last element returned; -1 if no such
 849         int expectedModCount = modCount;  850 
 851  Itr() {}  852 
 853         public boolean hasNext() {  854             return cursor != size;  855  }  856 
 857         @SuppressWarnings("unchecked")  858         public E next() {  859  checkForComodification();  860             int i = cursor;  861             if (i >= size)  862                 throw new NoSuchElementException();  863             Object[] elementData = ArrayList.this.elementData;  864             if (i >= elementData.length)  865                 throw new ConcurrentModificationException();  866             cursor = i + 1;  867             return (E) elementData[lastRet = i];  868  }  869 
 870         public void remove() {  871             if (lastRet < 0)  872                 throw new IllegalStateException();  873  checkForComodification();  874 
 875             try {  876                 ArrayList.this.remove(lastRet);  877                 cursor = lastRet;  878                 lastRet = -1;  879                 expectedModCount = modCount;  880             } catch (IndexOutOfBoundsException ex) {  881                 throw new ConcurrentModificationException();  882  }  883  }  884 
 885  @Override  886         @SuppressWarnings("unchecked")  887         public void forEachRemaining(Consumer<? super E> consumer) {  888  Objects.requireNonNull(consumer);  889             final int size = ArrayList.this.size;  890             int i = cursor;  891             if (i >= size) {  892                 return;  893  }  894             final Object[] elementData = ArrayList.this.elementData;  895             if (i >= elementData.length) {  896                 throw new ConcurrentModificationException();  897  }  898             while (i != size && modCount == expectedModCount) {  899                 consumer.accept((E) elementData[i++]);  900  }  901             // update once at end of iteration to reduce heap write traffic
 902             cursor = i;  903             lastRet = i - 1;  904  checkForComodification();  905  }  906 
 907         final void checkForComodification() {  908             if (modCount != expectedModCount)  909                 throw new ConcurrentModificationException();  910  }  911  }  912 
 913     /**
 914  * An optimized version of AbstractList.ListItr  915      */
 916     private class ListItr extends Itr implements ListIterator<E> {  917         ListItr(int index) {  918             super();  919             cursor = index;  920  }  921 
 922         public boolean hasPrevious() {  923             return cursor != 0;  924  }  925 
 926         public int nextIndex() {  927             return cursor;  928  }  929 
 930         public int previousIndex() {  931             return cursor - 1;  932  }  933 
 934         @SuppressWarnings("unchecked")  935         public E previous() {  936  checkForComodification();  937             int i = cursor - 1;  938             if (i < 0)  939                 throw new NoSuchElementException();  940             Object[] elementData = ArrayList.this.elementData;  941             if (i >= elementData.length)  942                 throw new ConcurrentModificationException();  943             cursor = i;  944             return (E) elementData[lastRet = i];  945  }  946 
 947         public void set(E e) {  948             if (lastRet < 0)  949                 throw new IllegalStateException();  950  checkForComodification();  951 
 952             try {  953                 ArrayList.this.set(lastRet, e);  954             } catch (IndexOutOfBoundsException ex) {  955                 throw new ConcurrentModificationException();  956  }  957  }  958 
 959         public void add(E e) {  960  checkForComodification();  961 
 962             try {  963                 int i = cursor;  964                 ArrayList.this.add(i, e);  965                 cursor = i + 1;  966                 lastRet = -1;  967                 expectedModCount = modCount;  968             } catch (IndexOutOfBoundsException ex) {  969                 throw new ConcurrentModificationException();  970  }  971  }  972  }  973 
 974     /**
 975  * Returns a view of the portion of this list between the specified  976  * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive. (If  977  * {@code fromIndex} and {@code toIndex} are equal, the returned list is  978  * empty.) The returned list is backed by this list, so non-structural  979  * changes in the returned list are reflected in this list, and vice-versa.  980  * The returned list supports all of the optional list operations.  981  *  982  * <p>This method eliminates the need for explicit range operations (of  983  * the sort that commonly exist for arrays). Any operation that expects  984  * a list can be used as a range operation by passing a subList view  985  * instead of a whole list. For example, the following idiom  986  * removes a range of elements from a list:  987  * <pre>  988  * list.subList(from, to).clear();  989  * </pre>  990  * Similar idioms may be constructed for {@link #indexOf(Object)} and  991  * {@link #lastIndexOf(Object)}, and all of the algorithms in the  992  * {@link Collections} class can be applied to a subList.  993  *  994  * <p>The semantics of the list returned by this method become undefined if  995  * the backing list (i.e., this list) is <i>structurally modified</i> in  996  * any way other than via the returned list. (Structural modifications are  997  * those that change the size of this list, or otherwise perturb it in such  998  * a fashion that iterations in progress may yield incorrect results.)  999  * 1000  * @throws IndexOutOfBoundsException {@inheritDoc} 1001  * @throws IllegalArgumentException {@inheritDoc} 1002      */
1003     public List<E> subList(int fromIndex, int toIndex) { 1004  subListRangeCheck(fromIndex, toIndex, size); 1005         return new SubList(this, 0, fromIndex, toIndex); 1006  } 1007 
1008     static void subListRangeCheck(int fromIndex, int toIndex, int size) { 1009         if (fromIndex < 0) 1010             throw new IndexOutOfBoundsException("fromIndex = " + fromIndex); 1011         if (toIndex > size) 1012             throw new IndexOutOfBoundsException("toIndex = " + toIndex); 1013         if (fromIndex > toIndex) 1014             throw new IllegalArgumentException("fromIndex(" + fromIndex +
1015                                                ") > toIndex(" + toIndex + ")"); 1016  } 1017 
1018     private class SubList extends AbstractList<E> implements RandomAccess { 1019         private final AbstractList<E> parent; 1020         private final int parentOffset; 1021         private final int offset; 1022         int size; 1023 
1024         SubList(AbstractList<E> parent, 1025                 int offset, int fromIndex, int toIndex) { 1026             this.parent = parent; 1027             this.parentOffset = fromIndex; 1028             this.offset = offset + fromIndex; 1029             this.size = toIndex - fromIndex; 1030             this.modCount = ArrayList.this.modCount; 1031  } 1032 
1033         public E set(int index, E e) { 1034  rangeCheck(index); 1035  checkForComodification(); 1036             E oldValue = ArrayList.this.elementData(offset + index); 1037             ArrayList.this.elementData[offset + index] = e; 1038             return oldValue; 1039  } 1040 
1041         public E get(int index) { 1042  rangeCheck(index); 1043  checkForComodification(); 1044             return ArrayList.this.elementData(offset + index); 1045  } 1046 
1047         public int size() { 1048  checkForComodification(); 1049             return this.size; 1050  } 1051 
1052         public void add(int index, E e) { 1053  rangeCheckForAdd(index); 1054  checkForComodification(); 1055             parent.add(parentOffset + index, e); 1056             this.modCount = parent.modCount; 1057             this.size++; 1058  } 1059 
1060         public E remove(int index) { 1061  rangeCheck(index); 1062  checkForComodification(); 1063             E result = parent.remove(parentOffset + index); 1064             this.modCount = parent.modCount; 1065             this.size--; 1066             return result; 1067  } 1068 
1069         protected void removeRange(int fromIndex, int toIndex) { 1070  checkForComodification(); 1071             parent.removeRange(parentOffset + fromIndex, 1072                                parentOffset + toIndex); 1073             this.modCount = parent.modCount; 1074             this.size -= toIndex - fromIndex; 1075  } 1076 
1077         public boolean addAll(Collection<? extends E> c) { 1078             return addAll(this.size, c); 1079  } 1080 
1081         public boolean addAll(int index, Collection<? extends E> c) { 1082  rangeCheckForAdd(index); 1083             int cSize = c.size(); 1084             if (cSize==0) 1085                 return false; 1086 
1087  checkForComodification(); 1088             parent.addAll(parentOffset + index, c); 1089             this.modCount = parent.modCount; 1090             this.size += cSize; 1091             return true; 1092  } 1093 
1094         public Iterator<E> iterator() { 1095             return listIterator(); 1096  } 1097 
1098         public ListIterator<E> listIterator(final int index) { 1099  checkForComodification(); 1100  rangeCheckForAdd(index); 1101             final int offset = this.offset; 1102 
1103             return new ListIterator<E>() { 1104                 int cursor = index; 1105                 int lastRet = -1; 1106                 int expectedModCount = ArrayList.this.modCount; 1107 
1108                 public boolean hasNext() { 1109                     return cursor != SubList.this.size; 1110  } 1111 
1112                 @SuppressWarnings("unchecked") 1113                 public E next() { 1114  checkForComodification(); 1115                     int i = cursor; 1116                     if (i >= SubList.this.size) 1117                         throw new NoSuchElementException(); 1118                     Object[] elementData = ArrayList.this.elementData; 1119                     if (offset + i >= elementData.length) 1120                         throw new ConcurrentModificationException(); 1121                     cursor = i + 1; 1122                     return (E) elementData[offset + (lastRet = i)]; 1123  } 1124 
1125                 public boolean hasPrevious() { 1126                     return cursor != 0; 1127  } 1128 
1129                 @SuppressWarnings("unchecked") 1130                 public E previous() { 1131  checkForComodification(); 1132                     int i = cursor - 1; 1133                     if (i < 0) 1134                         throw new NoSuchElementException(); 1135                     Object[] elementData = ArrayList.this.elementData; 1136                     if (offset + i >= elementData.length) 1137                         throw new ConcurrentModificationException(); 1138                     cursor = i; 1139                     return (E) elementData[offset + (lastRet = i)]; 1140  } 1141 
1142                 @SuppressWarnings("unchecked") 1143                 public void forEachRemaining(Consumer<? super E> consumer) { 1144  Objects.requireNonNull(consumer); 1145                     final int size = SubList.this.size; 1146                     int i = cursor; 1147                     if (i >= size) { 1148                         return; 1149  } 1150                     final Object[] elementData = ArrayList.this.elementData; 1151                     if (offset + i >= elementData.length) { 1152                         throw new ConcurrentModificationException(); 1153  } 1154                     while (i != size && modCount == expectedModCount) { 1155                         consumer.accept((E) elementData[offset + (i++)]); 1156  } 1157                     // update once at end of iteration to reduce heap write traffic
1158                     lastRet = cursor = i; 1159  checkForComodification(); 1160  } 1161 
1162                 public int nextIndex() { 1163                     return cursor; 1164  } 1165 
1166                 public int previousIndex() { 1167                     return cursor - 1; 1168  } 1169 
1170                 public void remove() { 1171                     if (lastRet < 0) 1172                         throw new IllegalStateException(); 1173  checkForComodification(); 1174 
1175                     try { 1176                         SubList.this.remove(lastRet); 1177                         cursor = lastRet; 1178                         lastRet = -1; 1179                         expectedModCount = ArrayList.this.modCount; 1180                     } catch (IndexOutOfBoundsException ex) { 1181                         throw new ConcurrentModificationException(); 1182  } 1183  } 1184 
1185                 public void set(E e) { 1186                     if (lastRet < 0) 1187                         throw new IllegalStateException(); 1188  checkForComodification(); 1189 
1190                     try { 1191                         ArrayList.this.set(offset + lastRet, e); 1192                     } catch (IndexOutOfBoundsException ex) { 1193                         throw new ConcurrentModificationException(); 1194  } 1195  } 1196 
1197                 public void add(E e) { 1198  checkForComodification(); 1199 
1200                     try { 1201                         int i = cursor; 1202                         SubList.this.add(i, e); 1203                         cursor = i + 1; 1204                         lastRet = -1; 1205                         expectedModCount = ArrayList.this.modCount; 1206                     } catch (IndexOutOfBoundsException ex) { 1207                         throw new ConcurrentModificationException(); 1208  } 1209  } 1210 
1211                 final void checkForComodification() { 1212                     if (expectedModCount != ArrayList.this.modCount) 1213                         throw new ConcurrentModificationException(); 1214  } 1215  }; 1216  } 1217 
1218         public List<E> subList(int fromIndex, int toIndex) { 1219  subListRangeCheck(fromIndex, toIndex, size); 1220             return new SubList(this, offset, fromIndex, toIndex); 1221  } 1222 
1223         private void rangeCheck(int index) { 1224             if (index < 0 || index >= this.size) 1225                 throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); 1226  } 1227 
1228         private void rangeCheckForAdd(int index) { 1229             if (index < 0 || index > this.size) 1230                 throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); 1231  } 1232 
1233         private String outOfBoundsMsg(int index) { 1234             return "Index: "+index+", Size: "+this.size; 1235  } 1236 
1237         private void checkForComodification() { 1238             if (ArrayList.this.modCount != this.modCount) 1239                 throw new ConcurrentModificationException(); 1240  } 1241 
1242         public Spliterator<E> spliterator() { 1243  checkForComodification(); 1244             return new ArrayListSpliterator<E>(ArrayList.this, offset, 1245                                                offset + this.size, this.modCount); 1246  } 1247  } 1248 
1249  @Override 1250     public void forEach(Consumer<? super E> action) { 1251  Objects.requireNonNull(action); 1252         final int expectedModCount = modCount; 1253         @SuppressWarnings("unchecked") 1254         final E[] elementData = (E[]) this.elementData; 1255         final int size = this.size; 1256         for (int i=0; modCount == expectedModCount && i < size; i++) { 1257  action.accept(elementData[i]); 1258  } 1259         if (modCount != expectedModCount) { 1260             throw new ConcurrentModificationException(); 1261  } 1262  } 1263 
1264     /**
1265  * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em> 1266  * and <em>fail-fast</em> {@link Spliterator} over the elements in this 1267  * list. 1268  * 1269  * <p>The {@code Spliterator} reports {@link Spliterator#SIZED}, 1270  * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}. 1271  * Overriding implementations should document the reporting of additional 1272  * characteristic values. 1273  * 1274  * @return a {@code Spliterator} over the elements in this list 1275  * @since 1.8 1276      */
1277  @Override 1278     public Spliterator<E> spliterator() { 1279         return new ArrayListSpliterator<>(this, 0, -1, 0); 1280  } 1281 
1282     /** Index-based split-by-two, lazily initialized Spliterator */
1283     static final class ArrayListSpliterator<E> implements Spliterator<E> { 1284 
1285         /*
1286  * If ArrayLists were immutable, or structurally immutable (no 1287  * adds, removes, etc), we could implement their spliterators 1288  * with Arrays.spliterator. Instead we detect as much 1289  * interference during traversal as practical without 1290  * sacrificing much performance. We rely primarily on 1291  * modCounts. These are not guaranteed to detect concurrency 1292  * violations, and are sometimes overly conservative about 1293  * within-thread interference, but detect enough problems to 1294  * be worthwhile in practice. To carry this out, we (1) lazily 1295  * initialize fence and expectedModCount until the latest 1296  * point that we need to commit to the state we are checking 1297  * against; thus improving precision. (This doesn't apply to 1298  * SubLists, that create spliterators with current non-lazy 1299  * values). (2) We perform only a single 1300  * ConcurrentModificationException check at the end of forEach 1301  * (the most performance-sensitive method). When using forEach 1302  * (as opposed to iterators), we can normally only detect 1303  * interference after actions, not before. Further 1304  * CME-triggering checks apply to all other possible 1305  * violations of assumptions for example null or too-small 1306  * elementData array given its size(), that could only have 1307  * occurred due to interference. This allows the inner loop 1308  * of forEach to run without any further checks, and 1309  * simplifies lambda-resolution. While this does entail a 1310  * number of checks, note that in the common case of 1311  * list.stream().forEach(a), no checks or other computation 1312  * occur anywhere other than inside forEach itself. The other 1313  * less-often-used methods cannot take advantage of most of 1314  * these streamlinings. 1315          */
1316 
1317         private final ArrayList<E> list; 1318         private int index; // current index, modified on advance/split
1319         private int fence; // -1 until used; then one past last index
1320         private int expectedModCount; // initialized when fence set
1321 
1322         /** Create new spliterator covering the given range */
1323         ArrayListSpliterator(ArrayList<E> list, int origin, int fence, 1324                              int expectedModCount) { 1325             this.list = list; // OK if null unless traversed
1326             this.index = origin; 1327             this.fence = fence; 1328             this.expectedModCount = expectedModCount; 1329  } 1330 
1331         private int getFence() { // initialize fence to size on first use
1332             int hi; // (a specialized variant appears in method forEach)
1333             ArrayList<E> lst; 1334             if ((hi = fence) < 0) { 1335                 if ((lst = list) == null) 1336                     hi = fence = 0; 1337                 else { 1338                     expectedModCount = lst.modCount; 1339                     hi = fence = lst.size; 1340  } 1341  } 1342             return hi; 1343  } 1344 
1345         public ArrayListSpliterator<E> trySplit() { 1346             int hi = getFence(), lo = index, mid = (lo + hi) >>> 1; 1347             return (lo >= mid) ? null : // divide range in half unless too small
1348                 new ArrayListSpliterator<E>(list, lo, index = mid, 1349  expectedModCount); 1350  } 1351 
1352         public boolean tryAdvance(Consumer<? super E> action) { 1353             if (action == null) 1354                 throw new NullPointerException(); 1355             int hi = getFence(), i = index; 1356             if (i < hi) { 1357                 index = i + 1; 1358                 @SuppressWarnings("unchecked") E e = (E)list.elementData[i]; 1359  action.accept(e); 1360                 if (list.modCount != expectedModCount) 1361                     throw new ConcurrentModificationException(); 1362                 return true; 1363  } 1364             return false; 1365  } 1366 
1367         public void forEachRemaining(Consumer<? super E> action) { 1368             int i, hi, mc; // hoist accesses and checks from loop
1369             ArrayList<E> lst; Object[] a; 1370             if (action == null) 1371                 throw new NullPointerException(); 1372             if ((lst = list) != null && (a = lst.elementData) != null) { 1373                 if ((hi = fence) < 0) { 1374                     mc = lst.modCount; 1375                     hi = lst.size; 1376  } 1377                 else
1378                     mc = expectedModCount; 1379                 if ((i = index) >= 0 && (index = hi) <= a.length) { 1380                     for (; i < hi; ++i) { 1381                         @SuppressWarnings("unchecked") E e = (E) a[i]; 1382  action.accept(e); 1383  } 1384                     if (lst.modCount == mc) 1385                         return; 1386  } 1387  } 1388             throw new ConcurrentModificationException(); 1389  } 1390 
1391         public long estimateSize() { 1392             return (long) (getFence() - index); 1393  } 1394 
1395         public int characteristics() { 1396             return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED; 1397  } 1398  } 1399 
1400  @Override 1401     public boolean removeIf(Predicate<? super E> filter) { 1402  Objects.requireNonNull(filter); 1403         // figure out which elements are to be removed 1404         // any exception thrown from the filter predicate at this stage 1405         // will leave the collection unmodified
1406         int removeCount = 0; 1407         final BitSet removeSet = new BitSet(size); 1408         final int expectedModCount = modCount; 1409         final int size = this.size; 1410         for (int i=0; modCount == expectedModCount && i < size; i++) { 1411             @SuppressWarnings("unchecked") 1412             final E element = (E) elementData[i]; 1413             if (filter.test(element)) { 1414  removeSet.set(i); 1415                 removeCount++; 1416  } 1417  } 1418         if (modCount != expectedModCount) { 1419             throw new ConcurrentModificationException(); 1420  } 1421 
1422         // shift surviving elements left over the spaces left by removed elements
1423         final boolean anyToRemove = removeCount > 0; 1424         if (anyToRemove) { 1425             final int newSize = size - removeCount; 1426             for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) { 1427                 i = removeSet.nextClearBit(i); 1428                 elementData[j] = elementData[i]; 1429  } 1430             for (int k=newSize; k < size; k++) { 1431                 elementData[k] = null;  // Let gc do its work
1432  } 1433             this.size = newSize; 1434             if (modCount != expectedModCount) { 1435                 throw new ConcurrentModificationException(); 1436  } 1437             modCount++; 1438  } 1439 
1440         return anyToRemove; 1441  } 1442 
1443  @Override 1444     @SuppressWarnings("unchecked") 1445     public void replaceAll(UnaryOperator<E> operator) { 1446  Objects.requireNonNull(operator); 1447         final int expectedModCount = modCount; 1448         final int size = this.size; 1449         for (int i=0; modCount == expectedModCount && i < size; i++) { 1450             elementData[i] = operator.apply((E) elementData[i]); 1451  } 1452         if (modCount != expectedModCount) { 1453             throw new ConcurrentModificationException(); 1454  } 1455         modCount++; 1456  } 1457 
1458  @Override 1459     @SuppressWarnings("unchecked") 1460     public void sort(Comparator<? super E> c) { 1461         final int expectedModCount = modCount; 1462         Arrays.sort((E[]) elementData, 0, size, c); 1463         if (modCount != expectedModCount) { 1464             throw new ConcurrentModificationException(); 1465  } 1466         modCount++; 1467  } 1468 }

View Code

2. 扩容

添加元素时使用 ensureCapacityInternal() 方法来保证容量足够,如果不够时,需要使用 grow() 方法进行扩容,新容量的大小为 oldCapacity + (oldCapacity >> 1),即 oldCapacity+oldCapacity/2。其中 oldCapacity >> 1 需要取整,所以新容量大约是旧容量的 1.5 倍左右。(oldCapacity 为偶数就是 1.5 倍,为奇数就是 1.5 倍-0.5)

扩容操作需要调用 Arrays.copyOf() 把原数组整个复制到新数组中,这个操作代价很高,因此最好在创建 ArrayList 对象时就指定大概的容量大小,减少扩容操作的次数。

 1 public boolean add(E e) {  2     ensureCapacityInternal(size + 1);  // Increments modCount!!
 3     elementData[size++] = e;  4     return true;  5 }  6 
 7 private void ensureCapacityInternal(int minCapacity) {  8     if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {  9         minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); 10  } 11  ensureExplicitCapacity(minCapacity); 12 } 13 
14 private void ensureExplicitCapacity(int minCapacity) { 15     modCount++; 16     // overflow-conscious code
17     if (minCapacity - elementData.length > 0) 18  grow(minCapacity); 19 } 20 
21 private void grow(int minCapacity) { 22     // overflow-conscious code
23     int oldCapacity = elementData.length; 24     int newCapacity = oldCapacity + (oldCapacity >> 1); 25     if (newCapacity - minCapacity < 0) 26         newCapacity = minCapacity; 27     if (newCapacity - MAX_ARRAY_SIZE > 0) 28         newCapacity = hugeCapacity(minCapacity); 29     // minCapacity is usually close to size, so this is a win:
30     elementData = Arrays.copyOf(elementData, newCapacity); 31 }

View Code

3. 删除元素

需要调用 System.arraycopy() 将 index+1 后面的元素都复制到 index 位置上,该操作的时间复杂度为 O(N),可以看到 ArrayList 删除元素的代价是非常高的。

 1 public E remove(int index) {  2  rangeCheck(index);  3     modCount++;  4     E oldValue = elementData(index);  5     int numMoved = size - index - 1;  6     if (numMoved > 0)  7         System.arraycopy(elementData, index+1, elementData, index, numMoved);  8     elementData[--size] = null; // clear to let GC do its work
 9     return oldValue; 10 }

View Code

4. 序列化

ArrayList 基于数组实现,并且具有动态扩容特性,因此保存元素的数组不一定都会被使用,那么就没必要全部进行序列化。

保存元素的数组 elementData 使用 transient 修饰,该关键字声明数组默认不会被序列化。

transient Object[] elementData; // non-private to simplify nested class access

ArrayList 实现了 writeObject() 和 readObject() 来控制只序列化数组中有元素填充那部分内容。

 1 private void readObject(java.io.ObjectInputStream s)  2     throws java.io.IOException, ClassNotFoundException {  3     elementData = EMPTY_ELEMENTDATA;  4 
 5     // Read in size, and any hidden stuff
 6  s.defaultReadObject();  7 
 8     // Read in capacity
 9     s.readInt(); // ignored
10 
11     if (size > 0) { 12         // be like clone(), allocate array based upon size not capacity
13  ensureCapacityInternal(size); 14 
15         Object[] a = elementData; 16         // Read in all elements in the proper order.
17         for (int i=0; i<size; i++) { 18             a[i] = s.readObject(); 19  } 20  } 21 }

View Code

 1 private void writeObject(java.io.ObjectOutputStream s)  2     throws java.io.IOException{  3     // Write out element count, and any hidden stuff
 4     int expectedModCount = modCount;  5  s.defaultWriteObject();  6 
 7     // Write out size as capacity for behavioural compatibility with clone()
 8  s.writeInt(size);  9 
10     // Write out all elements in the proper order.
11     for (int i=0; i<size; i++) { 12  s.writeObject(elementData[i]); 13  } 14 
15     if (modCount != expectedModCount) { 16         throw new ConcurrentModificationException(); 17  } 18 }

View Code

序列化时需要使用 ObjectOutputStream 的 writeObject() 将对象转换为字节流并输出。而 writeObject() 方法在传入的对象存在 writeObject() 的时候会去反射调用该对象的 writeObject() 来实现序列化。反序列化使用的是 ObjectInputStream 的 readObject() 方法,原理类似。

1 ArrayList list = new ArrayList(); 2 ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(file)); 3 oos.writeObject(list);

5. modCount

modCount 用来记录 ArrayList 结构发生变化的次数。结构发生变化是指添加或者删除至少一个元素的所有操作,或者是调整内部数组的大小,仅仅只是设置元素的值不算结构发生变化。

在进行序列化或者迭代等操作时,需要比较操作前后 modCount 是否改变,如果改变了需要抛出 ConcurrentModificationException。代码参以上序列化中的 writeObject() 方法。

Vector

1. 同步

它的实现与 ArrayList 类似,但是使用了 synchronized 进行同步。

 1 public synchronized boolean add(E e) {  2     modCount++;  3     ensureCapacityHelper(elementCount + 1);  4     elementData[elementCount++] = e;  5     return true;  6 }  7 
 8 public synchronized E get(int index) {  9     if (index >= elementCount) 10         throw new ArrayIndexOutOfBoundsException(index); 11 
12     return elementData(index); 13 }

View Code

2. 扩容

Vector 的构造函数可以传入 capacityIncrement 参数,它的作用是在扩容时使容量 capacity 增长 capacityIncrement。如果这个参数的值小于等于 0,扩容时每次都令 capacity 为原来的两倍。

1 public Vector(int initialCapacity, int capacityIncrement) { 2     super(); 3     if (initialCapacity < 0) 4         throw new IllegalArgumentException("Illegal Capacity: "+
5  initialCapacity); 6     this.elementData = new Object[initialCapacity]; 7     this.capacityIncrement = capacityIncrement; 8 }

View Code

 1 private void grow(int minCapacity) {  2     // overflow-conscious code
 3     int oldCapacity = elementData.length;  4     int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
 5  capacityIncrement : oldCapacity);  6     if (newCapacity - minCapacity < 0)  7         newCapacity = minCapacity;  8     if (newCapacity - MAX_ARRAY_SIZE > 0)  9         newCapacity = hugeCapacity(minCapacity); 10     elementData = Arrays.copyOf(elementData, newCapacity); 11 }

View Code

调用没有 capacityIncrement 的构造函数时,capacityIncrement 值被设置为 0,也就是说默认情况下 Vector 每次扩容时容量都会翻倍。

1 public Vector(int initialCapacity) { 2     this(initialCapacity, 0); 3 } 4 
5 public Vector() { 6     this(10); 7 }

View Code

3. 与 ArrayList 的比较

  • Vector 是同步的,因此开销就比 ArrayList 要大,访问速度更慢。最好使用 ArrayList 而不是 Vector,因为同步操作完全可以由程序员自己来控制;
  • Vector 每次扩容请求其大小的 2 倍(也可以通过构造函数设置增长的容量),而 ArrayList 是 1.5 倍。

4. 替代方案

可以使用 Collections.synchronizedList(); 得到一个线程安全的 ArrayList。

1 List<String> list = new ArrayList<>(); 2 List<String> synList = Collections.synchronizedList(list);

也可以使用 concurrent 并发包下的 CopyOnWriteArrayList 类。

List<String> list = new CopyOnWriteArrayList<>();

CopyOnWriteArrayList

1. 读写分离

写操作在一个复制的数组上进行,读操作还是在原始数组中进行,读写分离,互不影响。

写操作需要加锁,防止并发写入时导致写入数据丢失。

写操作结束之后需要把原始数组指向新的复制数组。

 1 public boolean add(E e) {  2     final ReentrantLock lock = this.lock;  3  lock.lock();  4     try {  5         Object[] elements = getArray();  6         int len = elements.length;  7         Object[] newElements = Arrays.copyOf(elements, len + 1);  8         newElements[len] = e;  9  setArray(newElements); 10         return true; 11     } finally { 12  lock.unlock(); 13  } 14 } 15 
16 final void setArray(Object[] a) { 17     array = a; 18 }

View Code

1 @SuppressWarnings("unchecked") 2 private E get(Object[] a, int index) { 3     return (E) a[index]; 4 }

2. 适用场景

CopyOnWriteArrayList 在写操作的同时允许读操作,大大提高了读操作的性能,因此很适合读多写少的应用场景。

但是 CopyOnWriteArrayList 有其缺陷:

  • 内存占用:在写操作时需要复制一个新的数组,使得内存占用为原来的两倍左右;
  • 数据不一致:读操作不能读取实时性的数据,因为部分写操作的数据还未同步到读数组中。

所以 CopyOnWriteArrayList 不适合内存敏感以及对实时性要求很高的场景。

LinkedList

1. 概览

基于双向链表实现,使用 Node 存储链表节点信息。

1 private static class Node<E> { 2  E item; 3     Node<E> next; 4     Node<E> prev; 5 }

View Code

每个链表存储了 first 和 last 指针:

1 transient Node<E> first; 2 transient Node<E> last;

 

2. 与 ArrayList 的比较

ArrayList 基于动态数组实现,LinkedList 基于双向链表实现。ArrayList 和 LinkedList 的区别可以归结为数组和链表的区别:

  • 数组支持随机访问,但插入删除的代价很高,需要移动大量元素;
  • 链表不支持随机访问,但插入删除只需要改变指针。

HashMap

1、HashMap 底层原理备战链接

https://www.cnblogs.com/taojietaoge/p/11359542.html

2、与 Hashtable 的比较

  • Hashtable 使用 synchronized 来进行同步。
  • HashMap 可以插入键为 null 的 Entry。
  • HashMap 的迭代器是 fail-fast 迭代器。
  • HashMap 不能保证随着时间的推移 Map 中的元素次序是不变的。

ConcurrentHashMap

参考链接:https://www.cnblogs.com/taojietaoge/p/10301711.html

1. 存储结构

1 static final class HashEntry<K,V> { 2     final int hash; 3     final K key; 4     volatile V value; 5     volatile HashEntry<K,V> next; 6 }

ConcurrentHashMap 和 HashMap 实现上类似,最主要的差别是 ConcurrentHashMap 采用了分段锁(Segment),每个分段锁维护着几个桶(HashEntry),多个线程可以同时访问不同分段锁上的桶,从而使其并发度更高(并发度就是 Segment 的个数)。

Segment 继承自 ReentrantLock。

 1 static final class Segment<K,V> extends ReentrantLock implements Serializable {  2 
 3     private static final long serialVersionUID = 2249069246763182397L;  4 
 5     static final int MAX_SCAN_RETRIES =
 6         Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1;  7 
 8     transient volatile HashEntry<K,V>[] table;  9 
10     transient int count; 11 
12     transient int modCount; 13 
14     transient int threshold; 15 
16     final float loadFactor; 17 }

View Code

final Segment<K,V>[] segments;

默认的并发级别为 16,也就是说默认创建 16 个 Segment。

static final int DEFAULT_CONCURRENCY_LEVEL = 16;

2. size 操作

每个 Segment 维护了一个 count 变量来统计该 Segment 中的键值对个数。

1 /**
2  * The number of elements. Accessed only either within locks 3  * or among other volatile reads that maintain visibility. 4  */
5 transient int count;

View Code

在执行 size 操作时,需要遍历所有 Segment 然后把 count 累计起来。

ConcurrentHashMap 在执行 size 操作时先尝试不加锁,如果连续两次不加锁操作得到的结果一致,那么可以认为这个结果是正确的。

尝试次数使用 RETRIES_BEFORE_LOCK 定义,该值为 2,retries 初始值为 -1,因此尝试次数为 3。

如果尝试的次数超过 3 次,就需要对每个 Segment 加锁。

 1 /**
 2  * Number of unsynchronized retries in size and containsValue  3  * methods before resorting to locking. This is used to avoid  4  * unbounded retries if tables undergo continuous modification  5  * which would make it impossible to obtain an accurate result.  6  */
 7 static final int RETRIES_BEFORE_LOCK = 2;  8 
 9 public int size() { 10     // Try a few times to get accurate count. On failure due to 11     // continuous async changes in table, resort to locking.
12     final Segment<K,V>[] segments = this.segments; 13     int size; 14     boolean overflow; // true if size overflows 32 bits
15     long sum;         // sum of modCounts
16     long last = 0L;   // previous sum
17     int retries = -1; // first iteration isn't retry
18     try { 19         for (;;) { 20             // 超过尝试次数,则对每个 Segment 加锁
21             if (retries++ == RETRIES_BEFORE_LOCK) { 22                 for (int j = 0; j < segments.length; ++j) 23                     ensureSegment(j).lock(); // force creation
24  } 25             sum = 0L; 26             size = 0; 27             overflow = false; 28             for (int j = 0; j < segments.length; ++j) { 29                 Segment<K,V> seg = segmentAt(segments, j); 30                 if (seg != null) { 31                     sum += seg.modCount; 32                     int c = seg.count; 33                     if (c < 0 || (size += c) < 0) 34                         overflow = true; 35  } 36  } 37             // 连续两次得到的结果一致,则认为这个结果是正确的
38             if (sum == last) 39                 break; 40             last = sum; 41  } 42     } finally { 43         if (retries > RETRIES_BEFORE_LOCK) { 44             for (int j = 0; j < segments.length; ++j) 45  segmentAt(segments, j).unlock(); 46  } 47  } 48     return overflow ? Integer.MAX_VALUE : size; 49 }

View Code

3. JDK 1.8 的改动

JDK 1.7 使用分段锁机制来实现并发更新操作,核心类为 Segment,它继承自重入锁 ReentrantLock,并发度与 Segment 数量相等。

JDK 1.8 使用了 CAS 操作来支持更高的并发度,在 CAS 操作失败时使用内置锁 synchronized。

并且 JDK 1.8 的实现也在链表过长时会转换为红黑树。

LinkedHashMap

1、存储结构

继承自 HashMap,因此具有和 HashMap 一样的快速查找特性。

public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>

内部维护了一个双向链表,用来维护插入顺序或者 LRU 顺序。

1 /**
2  * The head (eldest) of the doubly linked list. 3  */
4 transient LinkedHashMap.Entry<K,V> head; 5 
6 /**
7  * The tail (youngest) of the doubly linked list. 8  */
9 transient LinkedHashMap.Entry<K,V> tail;

View Code

accessOrder 决定了顺序,默认为 false,此时维护的是插入顺序。

final boolean accessOrder;

LinkedHashMap 最重要的是以下用于维护顺序的函数,它们会在 put、get 等方法中调用。

1 void afterNodeAccess(Node<K,V> p) { } 2 void afterNodeInsertion(boolean evict) { }

2、afterNodeAccess()

当一个节点被访问时,如果 accessOrder 为 true,则会将该节点移到链表尾部。也就是说指定为 LRU 顺序之后,在每次访问一个节点时,会将这个节点移到链表尾部,保证链表尾部是最近访问的节点,那么链表首部就是最近最久未使用的节点。 

 1 void afterNodeAccess(Node<K,V> e) { // move node to last
 2     LinkedHashMap.Entry<K,V> last;  3     if (accessOrder && (last = tail) != e) {  4         LinkedHashMap.Entry<K,V> p =
 5             (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;  6         p.after = null;  7         if (b == null)  8             head = a;  9         else
10             b.after = a; 11         if (a != null) 12             a.before = b; 13         else
14             last = b; 15         if (last == null) 16             head = p; 17         else { 18             p.before = last; 19             last.after = p; 20  } 21         tail = p; 22         ++modCount; 23  } 24 }

View Code

3、afterNodeInsertion()

在 put 等操作之后执行,当 removeEldestEntry() 方法返回 true 时会移除最晚的节点,也就是链表首部节点 first。

evict 只有在构建 Map 的时候才为 false,在这里为 true。

1 void afterNodeInsertion(boolean evict) { // possibly remove eldest
2     LinkedHashMap.Entry<K,V> first; 3     if (evict && (first = head) != null && removeEldestEntry(first)) { 4         K key = first.key; 5         removeNode(hash(key), key, null, false, true); 6  } 7 }

View Code

removeEldestEntry() 默认为 false,如果需要让它为 true,需要继承 LinkedHashMap 并且覆盖这个方法的实现,这在实现 LRU 的缓存中特别有用,通过移除最近最久未使用的节点,从而保证缓存空间足够,并且缓存的数据都是热点数据。

1 protected boolean removeEldestEntry(Map.Entry<K,V> eldest) { 2     return false; 3 }

4、LRU 缓存

以下是使用 LinkedHashMap 实现的一个 LRU 缓存:

  • 设定最大缓存空间 MAX_ENTRIES 为 3;
  • 使用 LinkedHashMap 的构造函数将 accessOrder 设置为 true,开启 LRU 顺序;
  • 覆盖 removeEldestEntry() 方法实现,在节点多于 MAX_ENTRIES 就会将最近最久未使用的数据移除。

 1 class LRUCache<K, V> extends LinkedHashMap<K, V> {  2     private static final int MAX_ENTRIES = 3;  3 
 4     protected boolean removeEldestEntry(Map.Entry eldest) {  5         return size() > MAX_ENTRIES;  6  }  7 
 8  LRUCache() {  9         super(MAX_ENTRIES, 0.75f, true); 10  } 11 }

View Code

1 public static void main(String[] args) { 2     LRUCache<Integer, String> cache = new LRUCache<>(); 3     cache.put(1, "a"); 4     cache.put(2, "b"); 5     cache.put(3, "c"); 6     cache.get(1); 7     cache.put(4, "d"); 8     System.out.println(cache.keySet());  // [3, 1, 4]
9 }

View Code

WeakHashMap

1、存储结构

WeakHashMap 的 Entry 继承自 WeakReference,被 WeakReference 关联的对象在下一次垃圾回收时会被回收。

WeakHashMap 主要用来实现缓存,通过使用 WeakHashMap 来引用缓存对象,由 JVM 对这部分缓存进行回收。 

private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V>

2、ConcurrentCache

Tomcat 中的 ConcurrentCache 使用了 WeakHashMap 来实现缓存功能。

ConcurrentCache 采取的是分代缓存:

  • 经常使用的对象放入 eden 中,eden 使用 ConcurrentHashMap 实现,不用担心会被回收(伊甸园);
  • 不常用的对象放入 longterm,longterm 使用 WeakHashMap 实现,这些老对象会被垃圾收集器回收。
  • 当调用 get() 方法时,会先从 eden 区获取,如果没有找到的话再到 longterm 获取,当从 longterm 获取到就把对象放入 eden 中,从而保证经常被访问的节点不容易被回收。
  • 当调用 put() 方法时,如果 eden 的大小超过了 size,那么就将 eden 中的所有对象都放入 longterm 中,利用虚拟机回收掉一部分不经常使用的对象。

 1 public final class ConcurrentCache<K, V> {  2 
 3     private final int size;  4 
 5     private final Map<K, V> eden;  6 
 7     private final Map<K, V> longterm;  8 
 9     public ConcurrentCache(int size) { 10         this.size = size; 11         this.eden = new ConcurrentHashMap<>(size); 12         this.longterm = new WeakHashMap<>(size); 13  } 14 
15     public V get(K k) { 16         V v = this.eden.get(k); 17         if (v == null) { 18             v = this.longterm.get(k); 19             if (v != null) 20                 this.eden.put(k, v); 21  } 22         return v; 23  } 24 
25     public void put(K k, V v) { 26         if (this.eden.size() >= size) { 27             this.longterm.putAll(this.eden); 28             this.eden.clear(); 29  } 30         this.eden.put(k, v); 31  } 32 }

View Code

 

 

 

 

玉阶生白露
夜久侵罗袜
 

 

 

    原文作者:涛姐涛哥
    原文地址: https://www.cnblogs.com/taojietaoge/p/15014736.html
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系管理员进行删除。