ArrayList解析

1.说明：

ArrayList内部实现是以数组实现，其最重要的一点就是能够自动扩容，也就是我们常常说的"动态数组"，当超出其长度限制的时候会扩展为原来的1.5倍，如果扩展1.5倍还不满足需求，直接扩展为需求值，其默认容量为10。

2.源码解析：

(1)add函数

public boolean add(E e) {


ensureCapacityInternal(size + 1);

elementData[size++] = e;


return true;


}

(2)put函数

public E set(int index, E element) {


if (index >= size)


throw new IndexOutOfBoundsException(outOfBoundsMsg(index));



E oldValue = (E) elementData[index];


elementData[index] = element;


return oldValue;


}

(3)get函数

public E get(int index) {


if (index >= size)


throw new IndexOutOfBoundsException(outOfBoundsMsg(index));



return (E) elementData[index];


}

(4)remove函数

public E remove(int index) {


if (index >= size)


throw new IndexOutOfBoundsException(outOfBoundsMsg(index));



modCount++;


E oldValue = (E) 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



return oldValue;


}

(5)ensureCapacityInternal自动扩容机制

private void ensureCapacityInternal(int minCapacity) {


if (elementData == EMPTY_ELEMENTDATA) {


minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);


}



ensureExplicitCapacity(minCapacity);


}



private void ensureExplicitCapacity(int minCapacity) {


modCount++;



// overflow-conscious code


if (minCapacity - elementData.length > 0)


grow(minCapacity);


}

private void grow(int minCapacity) {


// overflow-conscious code


int oldCapacity = elementData.length;


// 扩展为原来的1.5倍

int newCapacity = oldCapacity + (oldCapacity >> 1);


// 如果扩展到原来的1.5倍还不满足需求，直接扩为需求值

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);


}