ArrayList源码解读

参数

	//默认初始容量private static final int DEFAULT_CAPACITY = 10;//空数组(用于空实例)private static final Object[] EMPTY_ELEMENTDATA = {};//用于默认大小空实例的共享空数组private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};//保存数据的数组transient Object[] elementData; // non-private to simplify nested class access//元素个数private int size;

注意：这里有两个空数组，第一个空数组是容量为0的时候的数组，第二个空数组是使用空参构造器的时候的数组

构造方法

	//带有参数的构造器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);}}//这就是与上面不一样的地方public ArrayList() {this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;}public ArrayList(Collection<? extends E> c) {Object[] a = c.toArray();if ((size = a.length) != 0) {if (c.getClass() == ArrayList.class) {elementData = a;} else {elementData = Arrays.copyOf(a, size, Object[].class);}} else {// replace with empty array.elementData = EMPTY_ELEMENTDATA;}}

扩容方法

如果有必要增加此ArrayList实例的容量以确保它至少能容纳元素的数量

	public void ensureCapacity(int minCapacity) {//这里就开始判处出前面两个空数组的区别了//如果这个list是无参构造的化没那么minExpand就为10，否则为0int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)? : DEFAULT_CAPACITY;//如果最小容量大于已有的最大容量if (minCapacity > minExpand) {ensureExplicitCapacity(minCapacity);}}

如果最小所需容量大于了实际可存储容量那么就需要扩容

判断是否真的需要扩容

   private void ensureExplicitCapacity(int minCapacity) {modCount++;if (minCapacity - elementData.length > 0)grow(minCapacity);}

这里就是扩容的具体办法其中分为几个情况

最开始新容量等于旧容量的1.5倍

1. 新容量大等于最小所需容量，那么最后的新容量就等于旧容量的1.5倍

2. 新容量小于最小所需容量，那么最后的新容量就等于最小所需容量

3. 新容量大于List中规定最大容量，判断最小所需容量是否大于了List中规定的最小容量，如果没有则新容量还是等于最小容量，如果大于了则新容量等于Integer的最大值

  private void grow(int minCapacity) {// overflow-conscious codeint oldCapacity = elementData.length;int newCapacity = oldCapacity + (oldCapacity >> 1);if (newCapacity - minCapacity < 0)newCapacity = minCapacity;if (newCapacity - MAX_ARRAY_SIZE > 0)newCapacity = hugeCapacity(minCapacity);// minCapacity is usually close to size, so this is a win:elementData = Arrays.copyOf(elementData, newCapacity);}private static int hugeCapacity(int minCapacity) {if (minCapacity < 0) // overflowthrow new OutOfMemoryError();return (minCapacity > MAX_ARRAY_SIZE) ?Integer.MAX_VALUE :MAX_ARRAY_SIZE;}

 	private void ensureCapacityInternal(int minCapacity) {ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));}

计算最小容量是否会等于10

    private static int calculateCapacity(Object[] elementData, int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {return Math.max(DEFAULT_CAPACITY, minCapacity);}return minCapacity;}

增删改查方法

增删改查的方法都是十分简单的所以不过多赘述

但是要注意在增加和删除方法的每一次调用的时候都会使modCount++

Iterator迭代器

在ArrayList内部有个Itr的内部类

private class Itr implements Iterator<E> {//当前游标位置int cursor; //上一个游标位置int lastRet = -1; //修改次数int expectedModCount = modCount;Itr() {}//是否还存在下一个public boolean hasNext() {return cursor != size;}@SuppressWarnings("unchecked")public E next() {//检查期待修改次数与实际的修改次数是否发生变化//如果有变化就抛出异常checkForComodification();int i = cursor;if (i >= size) //判断是否越界throw new NoSuchElementException();Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length) //判断是否越界throw new ConcurrentModificationException();cursor = i + 1; //游标变为下一个return (E) elementData[lastRet = i];}public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();try {ArrayList.this.remove(lastRet);cursor = lastRet;lastRet = -1;expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}@Override@SuppressWarnings("unchecked")public void forEachRemaining(Consumer<? super E> consumer) {Objects.requireNonNull(consumer);final int size = ArrayList.this.size;int i = cursor;if (i >= size) {return;}final Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length) {throw new ConcurrentModificationException();}while (i != size && modCount == expectedModCount) {consumer.accept((E) elementData[i++]);}// update once at end of iteration to reduce heap write trafficcursor = i;lastRet = i - 1;checkForComodification();}final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}}

快速失败

前面都说了modCount属性，然后迭代器哪里也使用到了所以这里就说一下它的作用，它的作用就是实现快速失败，每次迭代器遍历的时候都会去查询实际的modCount属性与迭代器中保存的modCount属性是否相同，如果不同那么就抛出异常，这就是快速失败

拓展：安全失败：安全失败使用的写时复制技术，这个迭代器中遍历的数据是复制的数据，所以对于原有数据的修改不会影响到复制的数据