ArrayList源码解析

java.lang.Object 
    java.util.AbstractCollection<E> 
        java.util.AbstractList<E> 
            java.util.ArrayList<E> 

实现接口

Serializable, Cloneable, Iterable<E>, Collection<E>, List<E>, RandomAccess 

基本属性

transient Object[] elementData;  //存放数组的元素，transient表示该字段不进行序列化操作
private int size;    //已经放入数组中的元素个数，非数组的长度

源码解析

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    //可序列化版本号
    private static final long serialVersionUID = 8683452581122892189L;

    //默认的初始化数组大小 为10 .
    private static final int DEFAULT_CAPACITY = 10;

    //实例化一个空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};

    //存放List元素的数组
    private transient Object[] elementData;

    //List中元素的数量，和存放List元素的数组长度可能相等，也可能不相等
    private int size;

    //构造方法，指定初始化的数组长度
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    //无参构造方法
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

    //构造方法，参数为集合元素
    public ArrayList(Collection<? extends E> c) {
        //将集合转换成数组，并赋值给elementData数组
        elementData = c.toArray();
        size = elementData.length;
        //如果c.toArray返回的不是Object[]类型的数组，转换成Object[]类型
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }

    //改变数组的长度，使长度和List的size相等。
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = Arrays.copyOf(elementData, size);
        }
    }

    //确定ArrayList的容量
    //判断当前elementData是否是EMPTY_ELEMENTDATA,若是设置长度为10
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != EMPTY_ELEMENTDATA)
            // any size if real element table
            ? 0
            // larger than default for empty table. It's already supposed to be
            // at default size.
            : DEFAULT_CAPACITY;
        //是否需要扩容
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }
    //当前位置和默认大小之间取最大值
    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);
    }

    //数组的最大容量
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    //扩容操作
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //容量扩充1.5倍
        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:
        //生成一个长度为newCapacity数组，并将elementData数组中元素拷贝到新数组中，并将新数组的引用赋值给elementData
        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;
    }

    //返回数组中已经放入的元素个数，非数组长度
    public int size() {
        return size;
    }

    //List是否为空
    public boolean isEmpty() {
        return size == 0;
    }

    //判断是否包包含指定元素
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    //查找指定的元素，存在返回下标，不存在放回 -1 
    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;
    }

    //倒序查找元素，存在放回下标，不存在返回-1 
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    //因为实现了clone接口，所以需要重写clone()方法，实现对象的拷贝
    public Object clone() {
        try {
            @SuppressWarnings("unchecked")
                ArrayList<E> v = (ArrayList<E>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError();
        }
    }

    //将集合转化为数组
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    //转化为指定类型的数组元素，推荐使用此方法
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

    // Positional Access Operations
    //放回指定位置的数组元素
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    //返回列表中指定位置的元素
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    //设置指定位置的元素，并返回被替换的元素
    public E set(int index, E element) {
        rangeCheck(index);

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

    //添加元素
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    //将元素添加到指定位置上，从指定位置的元素开始所有元素向后移动，为新添加的元素提供位置
    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++;
    }

    //删除指定位置的元素，其他元素做相依的移动，并将最后一个元素置空，方便垃圾处理机制回收，防止内存泄露，并返回删除的元素值
    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;
    }

    //删除元素方法
    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;
    }

    //快速删除执行操作
    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
    }

    //清除列表
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

    //添加方法，添加的元素为集合
    public boolean addAll(Collection<? extends E> 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;
    }

    //从指定位置开始添加集合元素
    public boolean addAll(int index, Collection<? extends E> 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;
    }

    //范围删除方法
    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;
    }

    //下标检测方法，如果不合法，抛出IndexOutOfBoundsException异常
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * A version of rangeCheck used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //溢出信息
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    //删除所有元素
    public boolean removeAll(Collection<?> c) {
        return batchRemove(c, false);
    }

    
    public boolean retainAll(Collection<?> c) {
        return batchRemove(c, true);
    }

    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }

    //流操作方法，将对象写入输出流中
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    //流操作，读方法，将对象从流中取出
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            ensureCapacityInternal(size);

            Object[] a = elementData;
            // Read in all elements in the proper order.
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }

    //迭代方法，返回内部类实例
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    //迭代方法，返回内部类实例
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    //迭代方法，返回内部类实例
    public Iterator<E> iterator() {
        return new Itr();
    }

    //内部类，实现Iterator接口
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;
        //是否还有下一个元素，返回true or false 
        public boolean hasNext() {
            return cursor != size;
        }
        //返回元素
        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;   //获取外部类的elementData数组
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }
        //删除元素
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);  //调用外部类删除方法，删除指定位置的元素
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    //省略了ListItr、SubList两个内部类
}