ArrayList源码分析

一、先看下ArrayList的构造方法源码

1public ArrayList(int initialCapacity) {
2    super();
3    if (initialCapacity < 0)
4         throw new IllegalArgumentException("Illegal Capacity: "+
5                                               initialCapacity);
6    this.elementData = new Object[initialCapacity];
7}
8
9public ArrayList() {
10    this(10);
11}

由上面的源码可知，当我们新建一个ArrayList对象的时候：

1、如果我们不指定ArrayList的长度，会默认分配长度为10。

2、ArrayList的底层是一个对象数组。

二、添加元素源码

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

由添加元素源码可知：

1、首先判断容量是否已经满了，如果满了就会扩容，否则的话就将元素放到对象数组里。