图解ArrayList源码

初始化数组长度为空时， 懒加载

add方法

初始无参构造器

第一次添加

public boolean add(E e) {
    // 确定容量 动态扩容 size 初始 0
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    // 将要添加的元素 添加到数组中 elementData[0] = 1 --> size = 1
    elementData[size++] = e;
    return true;
}

private void ensureCapacityInternal(int minCapacity) {
        // ensureExplicitCapacity(10)
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

/**
* elementData {}
  minCapacity 1
*/
private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        // 10  1 return 10
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    // 5
    return minCapacity;
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++; // 增长 操作次数
​
    // minCapacity 10
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

private void grow(int minCapacity) { // 10
    // overflow-conscious code
    int oldCapacity = elementData.length; // 0
    // newCapacity = 0
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        // newCapacity = 10 
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // {}   {,,,,,,,,,} 返回一个新的数组 长度为10
    elementData = Arrays.copyOf(elementData, newCapacity);
}

第二次添加

elementData = {1,,,,,,,,,};
size = 1;

public boolean add(E e) {
    // 2
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e; // elementData[1] = 2  size = 2
    return true;
}

private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    // 2
    return minCapacity;
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
​
    // overflow-conscious code 2 - 10
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

第十一次添加

elementData = {1,2,3,4,5,6,7,8,9,10};
size = 10;

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

private void ensureCapacityInternal(int minCapacity) {
    // ensureExplicitCapacity(11)
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
​
    // 11 - 10 > 0
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

private void grow(int minCapacity) { // 11
    // 10
    int oldCapacity = elementData.length;
    // 15  newCapacity 是oldCapacity的1.5倍
    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:
    // {1,2,3,4,5,6,7,8,9,10} -- > {1,2,3,4,5,6,7,8,9,10,,,,,}
    elementData = Arrays.copyOf(elementData, newCapacity);
}