ArrayList解读之动态扩容

1 new Arraylist() 默认capacity

    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
          
            private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

从定义中我们看到默认容量大小是空
2 public ArrayList(int initialCapacity)

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);
        }
    }
        
            private static final int DEFAULT_CAPACITY = 10;

3 数据存储: transient Object[] elementData; // non-private to simplify nested class access
4 public boolean add(E e) {
ensureCapacityInternal(size + 1); // Increments modCount!!
elementData[size++] = e;
return true;
}

然后我们看看ensureCapacityInternal

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

接着往下看

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

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

接着看grow

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

可以看出动态扩容时 每次增加之后oldCapacity + (oldCapacity >> 1);