ArrayList源码阅读笔记(基于JDk1.8)

public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
public ArrayList(int initialCapacity) {    //指定容量
        if (initialCapacity > 0) {        //指定容量>0，则按照指定的容量构建数组
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {    //指定容量=0，则构造默认空数组
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+initialCapacity);
        }
    }
public ArrayList(Collection<? extends E> c) {        //通过一个集合构造数组
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {        //参数集合不为空，则将实例构造成Object[]
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {        //参数集合为空，则将实例构造成默认空数组
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

public boolean add(E e) {
        ensureCapacityInternal(size + 1);      //扩容操作
        elementData[size++] = e;
        return true;
    }
public void add(int index, E element) {　　//添加元素到指定位置
        rangeCheckForAdd(index);    //检查index是否越界

        ensureCapacityInternal(size + 1); 
        System.arraycopy(elementData, index, elementData, index + 1,size - index);
        elementData[index] = element;
        size++;
    }
    public boolean addAll(Collection<? extends E> c) {    //将参数集合添加至实例
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }
 public boolean addAll(int index, Collection<? extends E> c) {    //将参数集合添加至实例指定位置
        rangeCheckForAdd(index);    //检查index是否越界

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

        int numMoved = size - index;        //这里要注意，插入前会将原数组从index位置分开
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

 public E set(int index, E element) {        //新增会将index位置之后的元素依次后移，而覆盖会将index位置的元素替换，对其他位置的元素没有影响
        rangeCheck(index);

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

 public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

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,        //删除元素之后会将index之后的元素依次向前一位
                             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++)        //遍历数组直到找到指定元素并删除
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)        //遍历数组直到找到指定元素并删除
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

public void clear() {
        modCount++;

        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {    //当使用第一种构造方法时，第一次执行扩容方法会进入这个分支，将容量扩展为默认大小（10）
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        if (minCapacity - elementData.length > 0)    //如果参数长度大于数组长度则会扩容
            grow(minCapacity);
    }
private void grow(int minCapacity) {    
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);    //默认每次增长为1.5倍
        if (newCapacity - minCapacity < 0)        //特例1：参数大于数组长度的1.5倍，此时按照参数扩容
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)    //特例2：数组长度的1.5倍大于MAX_ARRAY_SIZE，此时将数组扩容至MAX_ARRAY_SIZE 或Integer.MAX_VALUE
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }