Java基础之ArrayList的扩容机制

//需要赋值ArrayList一个int类型长度值
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }
    //无参构造
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
	//入参继承了Collection对象转化为ArrayList
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

private static final Object[] EMPTY_ELEMENTDATA = {};
    
public ArrayList(int initialCapacity) {
		//initialCapacity大于0的情况下，就去new一个Object类型、长度为initialCapacity的数组
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
        //initialCapacity等于0的情况下，则生成已经创造好的Object[]；
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
        //initialCapacity小于0的情况下，直接抛异常
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    
    public ArrayList() {
    	//当你没有赋值初始长度时，同样是去赋值已经创造好的空数组，没有长度值
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

 private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    
    public boolean add(E e) {
    	//1、add的第一步先去调用ensureCapacityInternal方法传入size+1
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    
    //3、传入一开始我们new的数组和add后的数组长度
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
    	//如果elementData等于空数组，则对比DEFAULT_CAPACITY和minCapacity大小，返回大的值，DEFAULT_CAPACITY默认为10
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        //否则返回minCapacity
        return minCapacity;
    }

	//2、在ensureCapacityInternal方法内部又去调用calculateCapacity(elementData, minCapacity)方法，传入add数据后的list的size长度
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
	//4、拿到minCapacity的值后，对modCount++，比对添加后的list的长度和现在的list的长度来判断是否需要进行扩容；
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // 如果添加list的长度大于list现在的长度，则说明list的长度已经不够了，需要走grow(minCapacity)方法进行扩容
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }    

private void grow(int minCapacity) {
        // 拿到数组原来的长度
        int oldCapacity = elementData.length;
        //进行1.5倍计算
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果1.5倍的值小于add后的长度值，则把add后的长度值设置成新数组的长度
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //如果1.5倍的值大于array的最大长度则调用hugeCapacity(minCapacity)方法
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // 调用Arrays.copyOf方法传入旧数组对象和新数组的长度，进行扩容
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
            //如果add后的长度大于array的最大长度那就返回Integer的最大长度，否则返回array的最大长度，丢弃1.5倍的值，选择array的最大长度来进行扩容
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }