java ArrayList，

java ArrayList，

ArrayList继承AbstractList，实现了List。初始容量为10，最大容量为Integer.MAX_VALUE - 8（其实为Integer.MAX_VALUE）。底层基于数组进行实现。

主要函数包括：size()，isEmpty()，contains()，get()，add()，remove()。里面调用最多的方法System.arraycopy()，Arrays.copyOf()。

public E get(int index) {
        //下标越界检查
        rangeCheck(index);

        return elementData(index);
    }

public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

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

public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,size - index);
        elementData[index] = element;
        size++;
    }

//移除指定位置的元素
public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,numMoved);
        elementData[--size] = null; // clear to let GC do its work。这里将remove位置置为null，方便回收。

        return oldValue;
    }

//移除指定元素
public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    //这里就容易理解fastRemove方法，指定删除o,那也就不用返回o了。
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }
//快速移除，不检查下标，也不返回被删除的值
private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

ArrayList容量扩展：

private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //新的容量为原来的1.5倍
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            //1.5倍之后仍然小于期望的容量
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            //1.5倍之后大于Integer最大值-8，则用期望的容量和最大容量进行比较，大于则返回整数最大值，小于则返回容量最大值
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

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

最后一个jdk1.8新增的forEach方法，采用Consumer接口。

public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        @SuppressWarnings("unchecked")
        final E[] elementData = (E[]) this.elementData;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            action.accept(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }