elementData为实际存储数据的数组size为ArrayList的实际大小，和elementData的长度有所区别有一个特殊的地方，elementData使用transient修饰，即不可以被序列化，这么做的原因是： elementData数组的长度在大多数情况下，是大于实际数据的数量size的，所以不需要将空的元素也进行序列化操作。对elementData的序列化操作其实是由writeObject()和readObject()实现的。PRivate static final long serialVersionUID = 8683452581122892189L;//默认的容量private static final int DEFAULT_CAPACITY = 10;private static final Object[] EMPTY_ELEMENTDATA = {};//实际存储数据的其实是这个数组private transient Object[] elementData;//ArrayList的大小，和elementData的长度不一定相等private int size;
构造函数
三个构造函数最后一个函数由于c.toArray可能不会反回一个Object数组，所以特殊处理一下public ArrayList(int initialCapacity) { super(); if (initialCapacity < 0) throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); this.elementData = new Object[initialCapacity];}public ArrayList() { super(); this.elementData = EMPTY_ELEMENTDATA;}public ArrayList(Collection<? extends E> c) { elementData = c.toArray(); size = elementData.length; // c.toArray might (incorrectly) not return Object[] (see 6260652) if (elementData.getClass() != Object[].class) elementData = Arrays.copyOf(elementData, size, Object[].class);}
关于扩展容量的相关操作
方法调用关系：ensureCapacity() -> ensureExplicitCapacity() -> grow() -> hugeCapacity()核心的方法是grow()方法，如果minCapacity大于原来容量的1.5倍，容量增加到minCapacity；否则增加到1.5倍。对于ensureCapacity()这个方法，如果事先知道ArrayList所需存储的数据量大小，我们可以直接调用来设置，避免每次拷贝数组花费的额外时间，提升效率。/** * 按照size的大小缩减elementData的长度*/public void trimToSize() { //操作计数器，定义在AbstractList中 modCount++; if (size < elementData.length) { elementData = Arrays.copyOf(elementData, size); }}/** * 增加容量到minCapacity大小 */public void ensureCapacity(int minCapacity) { int minExpand = (elementData != EMPTY_ELEMENTDATA) // any size if real element table ? 0 // larger than default for empty table. It's already supposed to be // at default size. : DEFAULT_CAPACITY; if (minCapacity > minExpand) { ensureExplicitCapacity(minCapacity); }}/*** 主要为add方法所用*/private void ensureCapacityInternal(int minCapacity) { if (elementData == EMPTY_ELEMENTDATA) { minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity); } ensureExplicitCapacity(minCapacity);}private void ensureExplicitCapacity(int minCapacity) { modCount++; // overflow-conscious code 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;/** * 增加容量的核心函数。 * 如果minCapacity大于原来容量的1.5倍，容量增加到minCapacity；否则增加到1.5倍。 */private void grow(int minCapacity) { // overflow-conscious code int 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) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;}
