基于JDK1.8 的ArrayList源码分析

基于JDK1.8 的ArrayList源码分析，代码注释。

JDK版本：jdk1.8.0_181

package java.util;

import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import sun.misc.SharedSecrets;



public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    
    private static final int DEFAULT_CAPACITY = 10;

    
    private static final Object[] EMPTY_ELEMENTDATA = {};

    
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    
    transient Object[] elementData; // non-private to simplify nested class access

    
    private int size;

    
    public ArrayList(int initialCapacity) {
        //初始容量大于0。
        if (initialCapacity > 0) {
            //创建初始容量大小的数组。
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {//初始容量等于0。
            //创建空数组。
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            //初始容量小于0，抛出异常。
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    
    public ArrayList(Collection c) {
        //将指定集合元素的参数c赋值elementData
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            //c.toArray可能(不正确地)不返回Object[](参见JDK 6260652编号bug)
            // 《JDK1.6集合框架bug》 https://blog.csdn.net/aitangyong/article/details/30274749
            //比较object的运行时类，字节码文件。
            if (elementData.getClass() != Object[].class)
                //用来创建1个Object[]数组，这样数组中就可以存放任意对象了。
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            // 指定集合元素的参数c大小为0，创建空的ArrayList。
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    
    public void trimToSize() {
        //TODO modCount
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTdata: Arrays.copyOf(elementData, size);
        }
    }

    
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table  如果数组大小不是默认的空元素列表。
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            //指定扩容大小参数minCapacity与默认为10的DEFAULT_CAPACITY中，取一个最大值。
            //当要add进第1个元素时，minCapacity为1，在Math.max()方法比较后，minCapacity为10。
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    //确保内部容量大小。
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    //判断是否需要扩容。
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code 考虑了溢出情况的代码
        //《overflow-conscious code》 https://blog.csdn.net/lijianqingfeng/article/details/107912190
        //存储ArrayList元素的数组
        if (minCapacity - elementData.length > 0)
            //调用grow方法进行扩容。
            grow(minCapacity);
    }

    
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    
    private void grow(int minCapacity) {
        // overflow-conscious code
        //获取到旧容量，oldCapacity为旧容量。
        int oldCapacity = elementData.length;
        //相当于int newCapacity = oldCapacity + (oldCapacity / 2)，也就是newCapacity = oldCapacity * (1.5)。
        //将oldCapacity右移一位，其效果相当于oldCapacity/2。
        //位运算的速度快于整除运算，将新容量更新为旧容量的1.5倍，扩容50%。
        //ArrayList每次扩容之后容量都会变为原来的1.5倍左右。
        //newCapacity为新容量。
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //检查新容量是否大于最小需要容量。
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //检查新容量是否大于MAX_ARRAY_SIZE。
        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);
    }

    //极大的容量，最大容量为Integer.MAX_VALUE。
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    
    public int size() {
        return size;
    }

    
    public boolean isEmpty() {
        return size == 0;
    }

    
    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;
    }

    
    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;
    }

    
    public Object clone() {
        try {
            ArrayList v = (ArrayList) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable    这是不应该发生的，因为我们是可克隆的。
            throw new InternalError(e);
        }
    }

    
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    
    @SuppressWarnings("unchecked")
    public  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 访问指定位置的元素。

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    
    public E get(int index) {
        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!!
        //通过为数组赋值的方式为ArrayList列表添加元素。
        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++;
    }

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

        modCount++;
        //获取到需要移除的元素。
        E oldValue = elementData(index);
        //计算出需要向左移动多少位。
        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  清除让GC工作。

        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;
    }

    
    private void fastRemove(int index) {
        modCount++;
        //计算出需要向左移动多少位。
        int numMoved = size - index - 1;
        //从index+1开始，用后面的元素替换前面的元素。
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //将最后一个元素设为null。
        elementData[--size] = null; // clear to let GC do its work
    }

    
    public void clear() {
        modCount++;

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

        size = 0;
    }

    
    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;
    }

    
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        //要复制的数组元素的数量。把后面的元素移动到前面。
        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;
    }

    
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

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

    
    public boolean removeAll(Collection c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    
    public boolean retainAll(Collection c) {
        Objects.requireNonNull(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;
    }

    
    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