Java集合之ArrayList示例代码分析

ArrayList简介：ArrayList是一个数组队列，相当于一个动态的数组，容量可以动态的增长；它继承了AbstractList并实现了List，RandomAccess，Cloneable、Serializable这些接口。

特性：

（1）ArrayList继承了AbstractList，实现了List，它是一个数组队列，提供了相关的添加、删除、修改、遍历等功能。

（2）ArrayList实现了RandomAccess接口，提供了随机访问功能，RandomAccess是Java中用来被List实现，为List提供快速访问功能的；还可以通过元素下标进行快速随机访问。

（3）ArrayList实现了Cloneable接口，覆盖了函数clone()，可以被克隆。

（4）ArrayList实现了Serializable接口，可以被序列化，通过网络传输。

（5）ArrayList不是线程安全的，建议在单线程中使用。

（6）ArrayList是使用elementData一个Object[] 数组来动态存储数据的。

（7）对象数组的初始大小为10，数组大小调整方式是newCapacity = oldCapacity + (oldCapacity >> 1);

arraylist支持三种遍历方式：

（1）通过迭代器进行遍历，即通过Itreator去遍历  

Integer value=Iterator iter=list.iterator()(iter.hasNext())
{
    value=(Interger)iter.next()}

（2）随机访问模式，通过索引值去遍历所有元素  

Interger value=size=list.size()fo(i=i
（3）for循环遍历  
Integer value=(Integer inte : list)
{
    value=inte}
三种遍历方式中通过索引去遍历速度最快，通过迭代器去遍历速度最慢。
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public class Hello {

    public static void main(String[] args) {

        ArrayList list = new ArrayList();
        list.add("1");
        list.add("2");
        list.add("3");
        list.add("4");
        list.add(0, "5");
        System.out.println("the first element is: "+ list.get(0));
        list.remove("3");
        System.out.println("Arraylist size=: "+ list.size());
        System.out.println("ArrayList contains 3 is: "+ list.contains(3));
        list.set(1, "10");
        for(Iterator iter = list.iterator(); iter.hasNext(); )
        {
            System.out.println("next is: "+ iter.next());
        }
        String[] arr = (String[])list.toArray(new String[0]);
        for (String str:arr)
            System.out.println("str: "+ str);
        list.clear();
        System.out.println("ArrayList is empty: "+ list.isEmpty());
    }
}
运行结果：
the first element is: 5
Arraylist size=: 4
ArrayList contains 3 is: false
next is: 5
next is: 10
next is: 2
next is: 4
str: 5
str: 10
str: 2
str: 4
ArrayList is empty: true
ArrayList源代码分析：
// Arraylist继承了AbstractList 实现了List，RandomAccess，Cloneable，Serializable接口
public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;  //序列化ID
    private static final int DEFAULT_CAPACITY = 10; //初始大小为10，会动态增加
    private static final Object[] EMPTY_ELEMENTDATA = {};//空的数组实例
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    transient Object[] elementData; // ArrayList用来存储对象的数组
    private int size;//数组中包含对象的个数
    //ArrayList构造函数，构造初始大小的elementData
    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);
        }
    }
    //构造函数，初始化大小为10的数组
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    //带有Collection的构造函数
    public ArrayList(Collection 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;
        }
    }
    //当前容量值设置为实际个数
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
                    ? EMPTY_ELEMENTDATA
                    : Arrays.copyOf(elementData, size);
        }
    }
    //判断容量，容量不够增加
    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);
    }
    //最大的容量分配
    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;
    }
    //返回ArrayList的大小
    public int size() {
        return size;
    }
    //判断ArrayList是否为空
    public boolean isEmpty() {
        return size == 0;
    }
    //判断是否含有某个对象
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }
    //查找对象，返回坐标
    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;
    }
    //返回对象相同的最后一个坐标
    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;
    }
    //返回一个影子对象克隆
    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);
        }
    }
    //返回一个包含所有ArrayList元素的数组
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }
    //返回泛型对象数组，一般使用这个不是上边那个
    @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;
    }
    //得到指定下标的对象
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }
    //得到指定下标的对象，并对下标进行判断
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }
    //替换指定下标的对象，会检查下标并返回
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    //追加对象
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    //在指定的下标后增加对象
    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++;
    }
    //删除指定坐标的对象
    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ArrayList第一次出现的对象o
    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
    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ArrayList
    public void clear() {
        modCount++;

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

        size = 0;
    }
    //将集合C追加到ArrayList中
    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;
    }
    //在ArrayList中的某个坐标后追加集合c
    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;
    }
    //删除范围中的对象
    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;
    }
    //范围检查
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }
    //在添加或者添加所有的时候范围检查
    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);
    }
    //删除c中的在ArrayList中存在或者不存在的
    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;
    }
    //序列化
    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
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