ArrayList源码解析

ArrayList核心成员变量

private static final int DEFAULT_CAPACITY = 10;
private static final Object[] EMPTY_ELEMENTDATA = {};
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
transient Object[] elementData;
private int size;

ArrayList构造方法（空参）

public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

值得注意的是：

• jdk1.7初始化elementData时，是直接给了一个长度为默认长度（10）的对象数组。
• jdk1.8初始化elementData时，是给了一个长度为0的对象数组，直到用到这个数组的时候才对这个数组做扩容。
• 这种延迟加载的方法，好处是更合理的使用内存空间。

ArrayList的add方法

public boolean add(E e) {
    ensureCapacityInternal(size + 1); // Increments modCount!!
    elementData[size++] = e;
    return true;
}

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

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

private void ensureExplicitCapacity(int minCapacity) {
    // 记录arraylist 改动次数的一个计数器
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    // 扩容到1.5倍容量
    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);
}