ArrayList源代码的分析

下面是ArrayList源代码实现，Java1.7版的。

ArrayList不是线程安全的，有序，可以包括重复元素。

分析如下：

1.ArrayList类继承AbstractList类，实现List、RandomAccess、Cloneable、java.io.Servializable接口。

2.ArrayList类中定义了：DEFAULT_CAPACITY、EMPTY_ELEMENTDATA、elementData、size、MAX_ARRAY_SIZE等属性。

3.ArrayList类中定义了：trimToSize、ensureCapacity、size、isEmpty、contains、indexof、lastIndexof、clone、toArray、get、set、elementData、add、remove、clear、addAll、rangeCheck、writeObject、readObject、listIterator、iterator等方法。

4.ArrayList类中定义了：Itr、ListItr、SubList一共三个内部类。Itr类实现Iterator接口，ListItr类继承Itr类并实现ListIterator接口，SubList类继承AbstractList类并实现RandomAccess接口。

4.1Itr类中定义了cursor、lastRet、expectedModCount属性，hasNext、next、remove、checkForComodification方法。

4.2ListItr类中定义了hasPrevious、nextIndex、previousIndex、previous、set、add方法。

4.3SubList类中定义了parent、parentOffset、offset、size属性，set、get、size、add、remove、removeRange、addAll、iterator、listIterator、rangeCheck、checkForComodification方法。

5.ArrayList类中：有些方法是使用Arrays.copyOf()实现复制操作，例如：trimToSize、clone、构造函数等。有些方法是使用System.arraycopy()方法实现复制操作，例如：addAll、remove、add、removeRange等。

6.ArrayList类中：定义了几个共用的方法，例如：

6.1检查角标范围方法

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

6.2当iterator()方法遍历ArrayList时，Itr类中定义的checkForComodification方法检查操作ArrayList的过程中其它线程是否也在操作ArrayList

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

6.3ArrayList中几种迭代器的实现方法

public ListIterator listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

 public ListIterator listIterator() {
        return new ListItr(0);
    }

    public Iterator iterator() {
        return new Itr();
    }

可以看出ArrayList中的迭代器是通过实例化内部类实现的。直接调用iterator方法时实例化Itr类后返回，直接调用listIterator()方法时实例化ListItr构造函数并传入参数0，直接调用listIterator(int index)方法时实例化ListItr构造函数并传入参数index。

7.RandomAccess接口的源码如下：

public interface RandomAccess {
}

8.Cloneable接口的源码如下：

public interface Cloneable {
}

9.Serializable接口的源代码如下：

public interface Serializable {
}

10.List接口的源代码如下：

public interface List extends Collection {
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator iterator();
    Object[] toArray();
     T[] toArray(T[] a);
    boolean add(E e);
    boolean remove(Object o);
    boolean containsAll(Collection c);
    boolean addAll(Collection c);
    boolean addAll(int index, Collection c);
    boolean removeAll(Collection c);
    boolean retainAll(Collection c);
    void clear();
    boolean equals(Object o);
    int hashCode();
    E get(int index);
    E set(int index, E element);
    void add(int index, E element);
    E remove(int index);
    int indexOf(Object o);
    int lastIndexOf(Object o);
    ListIterator listIterator();
    ListIterator listIterator(int index);
    List subList(int fromIndex, int toIndex);
}

11.Collection接口的源代码如下：

public interface Collection extends Iterable {
   
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator iterator();
    Object[] toArray();
     T[] toArray(T[] a);
    boolean add(E e);
    boolean remove(Object o);
    boolean containsAll(Collection c);
    boolean addAll(Collection c);
    boolean removeAll(Collection c);
    boolean retainAll(Collection c);
    void clear();
    boolean equals(Object o);
    int hashCode();
}

12.Iterable接口的源代码如下：

public interface Iterable {

     Iterator iterator();
}

13.ArrayList类的源代码如下：

public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
{   //后面所有的modCount表示：The number of times this list has been structurally modified.

    private static final int DEFAULT_CAPACITY = 10;
    private static final Object[] EMPTY_ELEMENTDATA = {};
    private transient Object[] elementData;
    private int size;
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }
    public ArrayList(Collection c) {
        elementData = c.toArray();
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = Arrays.copyOf(elementData, size);
        }
    }
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != EMPTY_ELEMENTDATA)?0:DEFAULT_CAPACITY;
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    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;
    }
    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 {
            @SuppressWarnings("unchecked")
                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();
        }
    }
    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;
    }

    @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!!
        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++;
        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;
    }
    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
    }
    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++;
        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) {
        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;
    }
    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