ArrayList部分源码简析

有参构造

ArrayList

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/**
 * Constructs an empty list with the specified initial capacity.
 *
 * @param  initialCapacity  the initial capacity of the list
 * @throws IllegalArgumentException if the specified initial capacity
 *         is negative
 */
public ArrayList(int initialCapacity) {
    //如果初始容量大于0
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    //等于0
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}

add方法

add

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/**
 * Appends the specified element to the end of this list.
 *
 * @param e element to be appended to this list
 * @return <tt>true</tt> (as specified by {@link Collection#add})
 */
public boolean add(E e) {
    //确保内部容量足够
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    //添加元素
    elementData[size++] = e;
    return true;
}

ensureCapacityInternal

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private void ensureCapacityInternal(int minCapacity) {
    //确保显示容量足够
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

calculateCapacity

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private static int calculateCapacity(Object[] elementData, int minCapacity) {
    //如果当前数组还未初始化，即为空数组
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        //返回默认长度10，因为此时minCapacity等于1
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    //如果当前数组已经完成初始化，则直接返回当前size+1
    return minCapacity;
}

ensureExplicitCapacity

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

    // overflow-conscious code
    //如果当前size+1大于数组长度，则进行扩容
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

grow方法

grow

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private void grow(int minCapacity) {
    // overflow-conscious code
    //当前容量
    int oldCapacity = elementData.length;
    //新容量等于1.5倍当前容量
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    //如果新容量还不足够长，则新容量直接赋值为minCapacity
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    //如果新容量大于MAX_ARRAY_SIZE（private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;）
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        //调用hugeCapacity函数
        newCapacity = hugeCapacity(minCapacity);
    
    // minCapacity is usually close to size, so this is a win:
    //完成新数组的复制
    elementData = Arrays.copyOf(elementData, newCapacity);
}

hugeCapacity

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//相当于对ArrayList数组进行保护，防止数组容量溢出
private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}