ArrayList擴容

ArrayList有三種初始化方式：

　　1.指定大小初始化　　public ArrayList(int initialCapacity) 

　　2.傳入一個Collection對象初始化，並將對象中的數據添加到ArrayList中 public ArrayList(Collection<? extends E> c)

　　3.默認構造函數初始化  public ArrayList()

默認長度：

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

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

　從上面可以看出當使用無參構造函數創建ArrayList時，默認長度為0

　調用add方法添加元素時

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}//默認空數組

private int size;//元素個數

transient Object[] elementData;//存放元素的數組

private static final int DEFAULT_CAPACITY = 10;//默認容量

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

//添加元素
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}
//確保內部容量，將元素個數與數組長度比較判斷是否擴容

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

private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

    // overflow-conscious code
    if (minCapacity - elementData.length > 0)//如果最小需要容量大於數組長度，將數組擴容
        grow(minCapacity);
}
//計算最小需要容量

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

private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);//將數組長度的二進制表達形式右移，相當於原來的0.5倍，每次擴容1.5倍
    if (newCapacity - minCapacity < 0)//如果新容量小於最小需要容量，則將新容量設置為最小容量

        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)//如果新容量大於int最大值減8，則將新容量設置為int類型的最大容量
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
}
//巨大容量

private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

從上述代碼可以看出，當第一次調用add添加元素時ArrayList才會設置數組默認長度10，之后每超出數組長度就會進行擴容，每次擴容是原來的1.5倍