Java源码分析 - ArrayList 源码分析

图片来自： http://blog.csdn.net/bondsui/article/details/8520078

ArrayList 简介

可变长数组，非线程安全。由于增加或者删除需要array.copyOf 复制数组，不适合需要频繁增删的数据类型。

三种构造方式

1. 无initialCapacity：ArrayList();
2. 有initialCapacity：ArrayList(10);
3. 有collection：ArrayList(Collection c);

自增的实现方式

ArrayList 有两个add方法，可在array下一位加入，或者在指定index加入。选择第二种方式，ArrayList 会先检测一下index是否为负数或者大于当前size。而自动变长，则由grow()方法完成。

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

calculateCapacity(elementData, size+1), 如果向空array添加数据，返回DEFAULT_CAPACITY 默认为10，否则返回已有size+1；

    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

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

ensureExplicitCapacity() 调用grow 实现增长

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

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

新长度为原有长度+原有长度的一半。如果超过最大值，调用hugeCapacity设为Integer最大值。

    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);
    }

查找的实现方式

contains(Object o), indexOf(Object o), lastIndexOf()

contains 实际上调用indexOf实现遍历，但是返回boolean类型，而indexOf 返回int。
由于java中null既不是对象也不是一种类型。对于null的比较如果使用equals() 会造成空指针异常，因此需要分成null以及非null两种情况进行比较。lastIndexOf() 与indexOf() 类似，从后向前遍历。

    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
    public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

addAll() 两种实现方式，均用arraycopy进行数组的复制

    public boolean addAll(Collection c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }
    
    public boolean addAll(int index, Collection c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount

        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);
                             
        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

删除

两种remove

    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

        return oldValue;
    }
    
    public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

私有fastRemove

内部调用，不检测或返回 index

    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
    }

removeRange(int fromIndex, int toIndex)

删除指定index内的数据

    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }