ArrayList源码浅析（jdk1.8）

ArrayList的实质就是动态数组。所以可以通过下标准确的找到目标元素，因此查找的效率高。但是添加或删除元素会涉及到大量元素的位置移动，所以效率低。

一、构造方法

ArrayList提供了3个构造方法

1、无参的，就是把表示集合的数组赋值为空。

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

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2、带初始化大小的构造方法，就是new一个指定大小的数组赋值给表示集合的数组。

    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);        }    }

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3、带集合的构造方法，就是把传过来的集合转成数组在赋值给表示集合的数组。

    public ArrayList(Collection<? extends E> c) {        elementData = c.toArray();        if ((size = elementData.length) != 0) {            // c.toArray might (incorrectly) not return Object[] (see 6260652)            if (elementData.getClass() != Object[].class)                elementData = Arrays.copyOf(elementData, size, Object[].class);        } else {            // replace with empty array.            this.elementData = EMPTY_ELEMENTDATA;        }    }

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二、添加元素

1、add(E e)方法，添加元素前会先判断数组是否为空，如果为空，则创建一个长度为10的空数组，

然后在把所添加的元素加入到数组中。如果不为空，则判断集合大小是否超出数组的长度，

如果超出则进行数组扩容，不然直接进行赋值。数组扩容的规则是原先数组长度加上其一半。即扩容1.5倍。

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

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2、add(int index, E element)方法，添加元素到指定位置。添加前先判断所传参数是否超出集合大小或小于0，

如果是则抛出数组越界异常，符合条件则在进行和add(E e)方法一样的数组大小的检查，看看是否扩容。

    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++;    }    private void rangeCheckForAdd(int index) {        if (index > size || index < 0)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    private void ensureCapacityInternal(int minCapacity) {        if (elementData == DEFAULTCAPACITY_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);    }    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);    }

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3、addAll(Collection<? extends E> c)方法，添加集合到集合中。把集合转成数组，然后检查数组是否进行扩容，

如果不须就直接进行添加。

    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;    }

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三、删除元素

1、remove(int index)根据下标删除元素，先检查下标是否越界，然后获取本下标的值用于返回，最后进行数组的变动。

    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;    }    private void rangeCheck(int index) {        if (index >= size)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    E elementData(int index) {        return (E) elementData[index];    }

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2、remove(Object o)根据元素删除值，for循环整个集合来检查是否有和参数相同的元素，有就根据索引进行删除。

    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;    }    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    }

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3、removeRange(int fromIndex, int toIndex)根据下标范围进行删除元素。

    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;    }

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4、removeAll(Collection<?> c)只要所传集合中的元素在本身集合中就删除，取差集的意思。

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

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四、其他方法

1、retainAll(Collection<?> c)，只要所传集合中的元素不在本身集合中就删除，取并集的意思。

    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;    }

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2、set(int index, E element)，取代指定位置的元素。

    public E set(int index, E element) {        rangeCheck(index);        E oldValue = elementData(index);        elementData[index] = element;        return oldValue;    }    private void rangeCheck(int index) {        if (index >= size)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }

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3、trimToSize()，裁剪空间。

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

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五、其他部分源码

public class ArrayList<E> extends AbstractList<E>        implements List<E>, RandomAccess, Cloneable, java.io.Serializable{    private static final long serialVersionUID = 8683452581122892189L;    /**     * 默认集合初始容量为10     */    private static final int DEFAULT_CAPACITY = 10;    private static final Object[] EMPTY_ELEMENTDATA = {};    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};    /**     * 存储集合元素的数组     */    transient Object[] elementData; // non-private to simplify nested class access    /**     * 集合大小（元素个数）     *     * @serial     */    private int size;    /**     * 构造一个带具体初始容量的空集合     * 就是new一个指定大小的object数组     * @param  initialCapacity 集合的初始容量　　  */    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);        }    }    /**     * 构造一个初始为10的空集合     */    public ArrayList() {        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;    }    /**     * 参数为集合的构造函数，集合元素被复制到数组中     *     * @param c 集合参数*/    public ArrayList(Collection<? extends E> c) {        elementData = c.toArray();        if ((size = elementData.length) != 0) {            // 防止c.toArray()返回的不是数组            if (elementData.getClass() != Object[].class)                elementData = Arrays.copyOf(elementData, size, Object[].class);        } else {            // 空数组            this.elementData = EMPTY_ELEMENTDATA;        }    }    /**     * 确保容量     */    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);        }    }    private void ensureCapacityInternal(int minCapacity) {        if (elementData == DEFAULTCAPACITY_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);    }    /**     * 最大容量     */    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;    /**     * 容量扩展     * @param minCapacity the desired minimum capacity     */    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);    }    private static int hugeCapacity(int minCapacity) {        if (minCapacity < 0) // overflow            throw new OutOfMemoryError();        return (minCapacity > MAX_ARRAY_SIZE) ?            Integer.MAX_VALUE :            MAX_ARRAY_SIZE;    }    /**     * 返回集合大小     *     * @return the number of elements in this list     */    public int size() {        return size;    }    /**     * 集合为空返回true     *     * @return <tt>true</tt> if this list contains no elements     */    public boolean isEmpty() {        return size == 0;    }    /**     * 判断集合是否包含一个元素     */    public boolean contains(Object o) {        return indexOf(o) >= 0;    }    /**     * 判断集合是否包含一个元素的具体方法　　　* 就是for循环整个数组进行比较，存在就返回下标，否则返回-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;    }    /**     * 从集合的尾部开始判断是否含有某个元素　　　* 规则同indexOf（）方法     */    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;    }    /**     * 克隆     */    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);        }    }    /**     * 集合转数组     */    public Object[] toArray() {        return Arrays.copyOf(elementData, size);    }    // 返回数组指定下标的元素    @SuppressWarnings("unchecked")    E elementData(int index) {        return (E) elementData[index];    }    /**     * 根据指定位置获取元素     */    public E get(int index) {　　　　 // 越界检查        rangeCheck(index);        return elementData(index);    }    /**     * 把某元素塞到集合指定的位置(替换)，然后返回旧的值     */    public E set(int index, E element) {　　　　  // 越界检查        rangeCheck(index);        E oldValue = elementData(index);        elementData[index] = element;        return oldValue;    }    /**     * 添加元素     */    public boolean add(E e) {　　　　  // 集合容量检查        ensureCapacityInternal(size + 1);  // Increments modCount!!        elementData[size++] = e;        return true;    }    /**     * 把元素添加到指定位置     */    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     */    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;    }    /**     * 删除指定元素     */    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    }    /**     * 清空集合     */    public void clear() {        modCount++;        // clear to let GC do its work        for (int i = 0; i < size; i++)            elementData[i] = null;        size = 0;    }    /**     * 添加集合到集合中     */    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;    }    /**     * 从指定位置添加集合     */    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;    }    /**     * 删除指定范围的集合元素     */    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;    }    /**     * 集合越界检查     */    private void rangeCheck(int index) {        if (index >= size)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    /**     * 添加元素时进行越界检查     */    private void rangeCheckForAdd(int index) {        if (index > size || index < 0)            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));    }    /**     * 越界信息提示     */    private String outOfBoundsMsg(int index) {        return "Index: "+index+", Size: "+size;    }    /**     * 根据所传集合的元素进行删除指定的元素（差集）     */    public boolean removeAll(Collection<?> c) {        Objects.requireNonNull(c);        return batchRemove(c, false);    }    /**     * 根据所传集合和本身集合取交集（交集）     */    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;    }} 

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