public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; }
如果指定了长度,则必需大于1
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); } }
初始化时如果指定了集合,可以看到是空集合的情况下,其实也是初始化了一个空的
ArrayList。c.toArray()如果Collection是通过Arrays,toArray返回的,将是Arrays里面指定的泛型 ,而不是Object,将会返回原始的类型。
也就是通过集合初始化后的ArrayList里面的数组数据类型均为Object[]。
在我们遍历的时候,会将Object转化成我们需要的对象类型。
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; } }
添加元素时,如果长度会自动扩充,扩充为现有长度的1.5倍。
if (minCapacity - elementData.length > 0) grow(minCapacity); 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); }