ArrayList实现原理
Java的数组是固定长度的，那么ArrayList如何做到随意添加成员？
概述
ArrayList可以简单的看作是动态数组，相对于普通的数组它可以动态的增加容量或者减少容量。要注意的是ArrayList并不是线程安全的，因此一般建议在单线程中使用ArrayList。
实现原理
继承关系
public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable
ArrayList继承AbstractList实现List, RandomAccess, Cloneable, java.io.Serializable接口
关键属性
/** * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. Any * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA * will be expanded to DEFAULT_CAPACITY when the first element is added. */ // 数据的数组 transient Object[] elementData; // non-private to simplify nested class access /** * The size of the ArrayList (the number of elements it contains). * * @serial */ // 实际数据的数量 private int size;
底层使用数组保存所有元素
transient 如果用transient声明一个实例变量，当对象存储时，它的值不需要维持。换句话来说就是，用transient关键字标记的成员变量不参与序列化过程
构造方法
/** * Shared empty array instance used for default sized empty instances. We * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when * first element is added. */ private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * Constructs an empty list with an initial capacity of ten. */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }
默认情况下初始化空数组（长度为0的数组）
/** * Shared empty array instance used for empty instances. */ private static final Object[] EMPTY_ELEMENTDATA = {}; /** * Constructs an empty list with the specified initial capacity. * * @param initialCapacity the initial capacity of the list * @throws IllegalArgumentException if the specified initial capacity * is negative */ public ArrayList(int initialCapacity) { if (initialCapacity > 0) { this.elementData = new Object[initialCapacity]; } else if (initialCapacity == 0) { this.elementData = EMPTY_ELEMENTDATA; } else { throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); } }
指定数组的初始容量
当指定的初始容量大于0，初始化指定大小的数组
当指定的初始容量等于0，初始化空数组
当指定的初始容量小于0，抛出IllegalArgumentException异常
/** * Constructs a list containing the elements of the specified * collection, in the order they are returned by the collection's * iterator. * * @param c the collection whose elements are to be placed into this list * @throws NullPointerException if the specified collection is null */ public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); if ((size = elementData.length) != 0) { // c.toArray might (incorrectly) not return Object[] (see 6260652) if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class); } else { // replace with empty array. this.elementData = EMPTY_ELEMENTDATA; } }
初始化指定集合的数组
当指定集合不为空即长度不为0，则复制该集合，否则初始化一个空数组
E get(int index) 获取index位置的元素
// Positional Access Operations // 返回index下标的元素且强制转化为E（List<E>中的E）类型 @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } /** * Returns the element at the specified position in this list. * * @param index index of the element to return * @return the element at the specified position in this list * @throws IndexOutOfBoundsException {@inheritDoc} */ public E get(int index) { // 检查index是否越界 rangeCheck(index); // 返回index下标的元素 return elementData(index); } /** * Checks if the given index is in range. If not, throws an appropriate * runtime exception. This method does *not* check if the index is * negative: It is always used immediately prior to an array access, * which throws an ArrayIndexOutOfBoundsException if index is negative. */ private void rangeCheck(int index) { // 检查index是否大于等于size（数组的元素数量），因为数组下标从0开始计算，所以也不能等于元素数量 // 这里没有检查index < 0的情况，因为index < 0时数组会自动抛出异常，所以并未检查index<0的情况 if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * Constructs an IndexOutOfBoundsException detail message. * Of the many possible refactorings of the error handling code, * this "outlining" performs best with both server and client VMs. */ private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; }
首先判断index是否越界，这里并没有判断是否小于0，因为下标小于0时数组会抛出异常。越界则抛出IndexOutOfBoundsException异常，反之返回数组对应index位置的元素
E set(int index, E element) 设置（覆盖）index位置的元素
/** * Replaces the element at the specified position in this list with * the specified element. * * @param index index of the element to replace * @param element element to be stored at the specified position * @return the element previously at the specified position * @throws IndexOutOfBoundsException {@inheritDoc} */ public E set(int index, E element) { rangeCheck(index); E oldValue = elementData(index); elementData[index] = element; return oldValue; }
和get一样先判断index（下标）是否越界，不越界则先获取原来index位置上的元素，接着设置（覆盖）index位置上的元素，然后返回原来的元素，反之抛出IndexOutOfBoundsException异常
boolean add(E e) 添加一个元素到列表尾
/** * Appends the specified element to the end of this list. * * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */ public boolean add(E e) { // 检查当前容量是否还可以容纳一个元素，不够则扩容 ensureCapacityInternal(size + 1); // Increments modCount!! // 添加到数组末尾 // 这个语句可以分解为 // elementData[size] = e; // size += 1; elementData[size++] = e; return true; } /** * Default initial capacity. */ private static final int DEFAULT_CAPACITY = 10; // 默认容量为10 // 如果数据等于默认数据，返回默认容量和minCapacity（所需容量最小值）的最大值，反之返回所需容量最小值 private static int calculateCapacity(Object[] elementData, int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { return Math.max(DEFAULT_CAPACITY, minCapacity); } return minCapacity; } private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } private void ensureExplicitCapacity(int minCapacity) { modCount++; // 操作数+1 // overflow-conscious code // 如果所需容量最小值大于实际数组的长度就扩大实际数组容量 if (minCapacity - elementData.length > 0) grow(minCapacity); } /** * The maximum size of array to allocate. * Some VMs reserve some header words in an array. * Attempts to allocate larger arrays may result in * OutOfMemoryError: Requested array size exceeds VM limit */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; // 数组最大容量为Integer最大值-8 /** * Increases the capacity to ensure that it can hold at least the * number of elements specified by the minimum capacity argument. * * @param minCapacity the desired minimum capacity */ private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length; // 新的容量为旧的容量的1.5倍 int newCapacity = oldCapacity + (oldCapacity >> 1); // 如果扩充容量后还是不够，则新的容量等于所需容量最小值（一般就是数组实际元素个数） if (newCapacity - minCapacity < 0) newCapacity = minCapacity; // 如果新的容量大于数组最大容量，再调用hugeCapacity计算新的容量 if (newCapacity - MAX_ARRAY_SIZE > 0) newCapacity = hugeCapacity(minCapacity); // minCapacity is usually close to size, so this is a win: // 复制原来的数据到新的数组，数组容量为新的容量 elementData = Arrays.copyOf(elementData, newCapacity); } private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); // 大于数组最大容量返回Integer最大值，反之返回数组最大容量 return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; }
添加一个元素到列表尾，当列表容量不足时自动扩容（通常是扩容至原来的1.5倍），添加成功返回true
void add(int index, E element) 在index处放置元素
/** * Inserts the specified element at the specified position in this * list. Shifts the element currently at that position (if any) and * any subsequent elements to the right (adds one to their indices). * * @param index index at which the specified element is to be inserted * @param element element to be inserted * @throws IndexOutOfBoundsException {@inheritDoc} */ public void add(int index, E element) { // 检查下标是否越界 rangeCheckForAdd(index); // 检查当前容量是否还可以在容纳一个元素，不够则扩容 ensureCapacityInternal(size + 1); // Increments modCount!! // 将elementData从index开始后面的元素往后移一位 System.arraycopy(elementData, index, elementData, index + 1, size - index); elementData[index] = element; size++; } /** * A version of rangeCheck used by add and addAll. */ private void rangeCheckForAdd(int index) { // 当index等于size时相当于添加元素到列表尾 if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); }
将elementData数组从index开始后面的元素往后移一位，接着在index处放置元素
模拟添加数据(lierabbit)到index=4过程如下
boolean addAll(Collection<? extends E> c) 添加一个集合里的所有元素到列表尾
/** * Appends all of the elements in the specified collection to the end of * this list, in the order that they are returned by the * specified collection's Iterator. The behavior of this operation is * undefined if the specified collection is modified while the operation * is in progress. (This implies that the behavior of this call is * undefined if the specified collection is this list, and this * list is nonempty.) * * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws NullPointerException if the specified collection is null */ public boolean addAll(Collection<? extends E> c) { Object[] a = c.toArray(); int numNew = a.length; // 检查当前容量是否还可以在容纳a数组的元素，不够则扩容 ensureCapacityInternal(size + numNew); // Increments modCount // 将a数组里的元素添加到elementData末尾 System.arraycopy(a, 0, elementData, size, numNew); size += numNew; // a数组不为空（长度不为0）时返回true，反之false return numNew != 0; }
将要添加的集合变为数组，然后将其复制到elementData数组末尾
boolean addAll(int index, Collection<? extends E> c) 添加一个集合里的所有元素到index位置
/** * Inserts all of the elements in the specified collection into this * list, starting at the specified position. Shifts the element * currently at that position (if any) and any subsequent elements to * the right (increases their indices). The new elements will appear * in the list in the order that they are returned by the * specified collection's iterator. * * @param index index at which to insert the first element from the * specified collection * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws IndexOutOfBoundsException {@inheritDoc} * @throws NullPointerException if the specified collection is null */ public boolean addAll(int index, Collection<? extends E> c) { // 检查下标是否越界 rangeCheckForAdd(index); Object[] a = c.toArray(); int numNew = a.length; // 检查当前容量是否还可以在容纳a数组的元素，不够则扩容 ensureCapacityInternal(size + numNew); // Increments modCount // 需要往后移动几个位置 int numMoved = size - index; if (numMoved > 0) // 从index开始，往后的元素向后移动numMoved个位置 System.arraycopy(elementData, index, elementData, index + numNew, numMoved); // 将a数组里的所有元素复制到elementData从index到index + numNew -1的位置上 System.arraycopy(a, 0, elementData, index, numNew); size += numNew; // a数组不为空（长度不为0）时返回true，反之false return numNew != 0; }
将要添加的集合变为数组，然后把elementData数组从index开始，往后的元素向后移动numMoved个位置，接着将要添加的数组里的所有元素复制到elementData从index到index + numNew -1的位置上
void trimToSize() 改变列表内部数组容量至列表实际元素数量
/** * Trims the capacity of this <tt>ArrayList</tt> instance to be the * list's current size. An application can use this operation to minimize * the storage of an <tt>ArrayList</tt> instance. */ public void trimToSize() { modCount++; // 操作数+1 // 如果数组实际元素数量小于数组长度 if (size < elementData.length) { // 如果数组实际元素数量等于0则数组被赋值为空数组，反之创建一个新的元素数量等于数组长度的数组 elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } }
当数据稳定了之后可以使用这个方法来减少内存的使用
int indexOf(Object o) 查找o元素在列表第一次出现的位置
/** * Returns the index of the first occurrence of the specified element * in this list, or -1 if this list does not contain the element. * More formally, returns the lowest index <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>, * or -1 if there is no such index. */ public int indexOf(Object o) { //元素可以为null，如果为null返回null的下标 if (o == null) { for (int i = 0; i < size; i++) if (elementData[i]==null) return i; } else { for (int i = 0; i < size; i++) if (o.equals(elementData[i])) return i; } // 没有找到对应的元素返回-1 return -1; }
ArrayList中可以存放null元素，indexof是返回elementData数组中值相同的首个元素的下标，indexof中比较方法是equals而equals是比较元素的值，因此必须对null单独查找。如果未找到该元素则返回-1
E remove(int index) 删除index位置上的元素
/** * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their * indices). * * @param index the index of the element to be removed * @return the element that was removed from the list * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { // 检查下标是否越界 rangeCheck(index); modCount++; // 操作数+1 E oldValue = elementData(index); // 获取index位置上的元素 int numMoved = size - index - 1; // 需要往前移动几个位置 if (numMoved > 0) // 从index + 1开始，往后的元素向前移动1个位置 System.arraycopy(elementData, index+1, elementData, index, numMoved); // 将数组末尾元素置空 elementData[--size] = null; // clear to let GC do its work return oldValue; }
模拟删除index=4（值为lierabbit）过程如下
boolean remove(Object o) 删除o元素
/** * Removes the first occurrence of the specified element from this list, * if it is present. If the list does not contain the element, it is * unchanged. More formally, removes the element with the lowest index * <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> * (if such an element exists). Returns <tt>true</tt> if this list * contained the specified element (or equivalently, if this list * changed as a result of the call). * * @param o element to be removed from this list, if present * @return <tt>true</tt> if this list contained the specified element */ public boolean remove(Object o) { // 元素可以为null，分开搜索o if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } // 没有找到返回false return false; } /* * Private remove method that skips bounds checking and does not * return the value removed. */ // 由于已经找到元素，则元素必定存在，则index必定合理，所以不需要在检查index是否越界 private void fastRemove(int index) { modCount++; int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // clear to let GC do its work }
通过寻找o元素，可以获得其下标，再根据下标删除o元素
forEach(Consumer<? super E> action)遍历列表
/** * The number of times this list has been <i>structurally modified</i>. * Structural modifications are those that change the size of the * list, or otherwise perturb it in such a fashion that iterations in * progress may yield incorrect results. * * <p>This field is used by the iterator and list iterator implementation * returned by the {@code iterator} and {@code listIterator} methods. * If the value of this field changes unexpectedly, the iterator (or list * iterator) will throw a {@code ConcurrentModificationException} in * response to the {@code next}, {@code remove}, {@code previous}, * {@code set} or {@code add} operations. This provides * <i>fail-fast</i> behavior, rather than non-deterministic behavior in * the face of concurrent modification during iteration. * * <p><b>Use of this field by subclasses is optional.</b> If a subclass * wishes to provide fail-fast iterators (and list iterators), then it * merely has to increment this field in its {@code add(int, E)} and * {@code remove(int)} methods (and any other methods that it overrides * that result in structural modifications to the list). A single call to * {@code add(int, E)} or {@code remove(int)} must add no more than * one to this field, or the iterators (and list iterators) will throw * bogus {@code ConcurrentModificationExceptions}. If an implementation * does not wish to provide fail-fast iterators, this field may be * ignored. */ protected transient int modCount = 0;//操作数 @Override public void forEach(Consumer<? super E> action) { // 确保不为空 Objects.requireNonNull(action); final int expectedModCount = modCount; @SuppressWarnings("unchecked") final E[] elementData = (E[]) this.elementData; final int size = this.size; for (int i=0; modCount == expectedModCount && i < size; i++) { action.accept(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * Checks that the specified object reference is not {@code null}. This * method is designed primarily for doing parameter validation in methods * and constructors, as demonstrated below: * <blockquote><pre> * public Foo(Bar bar) { * this.bar = Objects.requireNonNull(bar); * } * </pre></blockquote> * * @param obj the object reference to check for nullity * @param <T> the type of the reference * @return {@code obj} if not {@code null} * @throws NullPointerException if {@code obj} is {@code null} */ public static <T> T requireNonNull(T obj) { if (obj == null) throw new NullPointerException(); return obj; }
这里可以看到modCount的用处，当modCount发生改变后，立刻抛出ConcurrentModificationException异常。通过之前的分析可以知道当列表内容被修改时modCount会增加。也就是说如果在遍历ArrayList的过程中有其他线程修改了ArrayList，那么将抛出ConcurrentModificationException异常
ArrayList小结
- ArrayList是List接口的一个可变大小的数组的实现
- ArrayList的内部是使用一个Object对象数组来存储元素的
- 初始化ArrayList的时候，可以指定初始化容量的大小，如果不指定，就会使用默认大小，为10
- 当添加一个新元素的时候，首先会检查容量是否足够添加这个元素，如果够就直接添加，如果不够就进行扩容，扩容为原数组容量的1.5倍
- 当在index处放置一个元素的时候，会将数组index处右边的元素全部右移
- 当在index处删除一个元素的时候，会将数组index处右边的元素全部左移
原文首发：https://lierabbit.cn/2018/01/23/ArrayList%E5%AE%9E%E7%8E%B0%E5%8E%9F%E7%90%86/
