ArrayList源码

package com.yum.day07;
import java.util.*;
public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
    private static final long serialVersionUID = 8683452581122892189L;
    //默认容量
    private static final int DEFAULT_CAPACITY = 10;
    //空对象
    private static final Object[] EMPTY_ELEMENTDATA = {};
    // 一个空对象，如果使用默认构造函数创建，则默认对象内容默认是该值
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    // 当前数据对象存放地方，当前对象不参与序列化
    transient Object[] elementData;
    // 当前数组长度
    private int size;
    /**
     * 通过大小进行构造
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: " +
                    initialCapacity);
        }
    }
    /**
     * 构造一个空列表。
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    /**
     * 带collection的构造器
     */
    public ArrayList(Collection<? extends E> c) {
        //浅复制
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            if (elementData.getClass() != Object[].class)
                //深复制
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            //返回一个空的
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }
    //将空余空间去除
    public void trimToSize() {
        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
                ? 0
                // larger than default for default empty table. It's already
                // supposed to be at default size.
                : DEFAULT_CAPACITY;
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }
    //确保数组已使用长度（size）加1之后足够存下 下一个数据
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
    //计算容量
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }
    //将修改次数（modCount）自增1，判断是否需要扩充数组长度,判断条件就是用当前所需的数组最小长度与数组的长度对比，如果大于0，则增长数组长度。
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    /**
     * 如果当前的数组已使用空间（size）加1之后 大于数组长度，则增大数组容量，扩大为原来的1.5倍。
     */
    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);
        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;
    }
    //获得大小
    public int size() {
        return size;
    }
    //判断是否为空
    public boolean isEmpty() {
        return size == 0;
    }
    //判断元素是否存在
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
    //遍历判断
    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
            throw new InternalError(e);
        }
    }
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }
    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;
    }
    //指定元素的数据
    E elementData(int index) {
        return (E) elementData[index];
    }
    //获得指定位置的元素
    public E get(int index) {
        //检查
        rangeCheck(index);
        //返回
        return elementData(index);
    }
    //指定位置替换元素
    public E set(int index, E element) {
        rangeCheck(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    //添加
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    //带位置的添加
    public void add(int index, E element) {
        //位置检查
        rangeCheckForAdd(index);
        //确保数组已使用长度（size）加1之后足够存下 下一个数据
        ensureCapacityInternal(size + 1);
        //使用System.arraycopy 将需要插入的位置（index）后面的元素统统往后移动一位。
        System.arraycopy(elementData, index, elementData, index + 1,
                size - index);
        //指定位置放元素
        elementData[index] = element;
        size++;
    }
    //删除
    public E remove(int index) {
        //检查位置
        rangeCheck(index);
        //操作数加1
        modCount++;
        //删除的元素
        E oldValue = elementData(index);
        //
        int numMoved = size - index - 1;
        // 将指定位置（index）上的元素都往前移动一位
        if (numMoved > 0)
            System.arraycopy(elementData, index + 1, elementData, index,
                    numMoved);
        //后面的一个元素置空
        elementData[--size] = null; // clear to let GC do its work
        return oldValue;
    }
    //根据对象的位置进行删除
    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;
    }
    //删除指定位置的元素
    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
    }
    //添加操作次数（modCount），将数组内的元素都置空，等待垃圾收集器收集，不减小数组容量。
    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<? 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;
    }
    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;
    }
    //检查获得的位置是否合法
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    /**
     * 检查添加位置是否合法
     */
    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;
    }
}