Java常见集合之ArrayList深入分析

Java常见集合之ArrayList深入分析

一、继承树

　　

二、ArrayList源码分析

2.1  继承结构和层次关系

　　

2.2 源码分析：

/*
继承自AbstractList，实现了List、RandomAccess、Cloneable、Serializable接口
    1）RandomAccess接口：用来快速随机存取，在实现了该接口后，用普通for来遍历，性能更高
    2）Cloneable接口：实现了该接口，就可以使用Object.Clone()方法了
    3）Serializable接口：实现了该接口，表明该类可以被序列化
*/
public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    //版本号
    private static final long serialVersionUID = 8683452581122892189L;
    //缺省容量
    private static final int DEFAULT_CAPACITY = 10;
    //缺省空对象数组
    private static final Object[] EMPTY_ELEMENTDATA = {};
    //默认大小的对象数组
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    //元素数组
    transient Object[] elementData;
    //数组的大小
     private int size;
     
     //构造方法
     //无参构造 Constructs an empty list with an initial capacity of ten
     public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    
    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    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);
        }
    }
    
     /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();    //转换为数组
        if ((size = elementData.length) != 0) {    //数组长度大于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;
        }
    }
    //总体而言：arrayList的构造方法就是做一件事，初始化一下储存数据的容器，
    //            其实本质就是一个数组，在其中就叫elementData
    
    //核心方法
    /**
     * Appends the specified element to the end of this list.
     * 默认直接在末尾添加元素
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {    //elementData=[1,2,3] e=6
        //确保数组的容量足够
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //数组尾部添加元素e
        elementData[size++] = e;
        return true;
    }
    
    private void ensureCapacityInternal(int minCapacity) { //minCapacity=4
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }
    
    //计算容量
    private static int calculateCapacity(Object[] elementData, int minCapacity) {//elementData=[1,2,3],minCapacity=4
        //如果元素数组等于默认容量的元素数组，就返回DEFAULT_CAPACITY和minCapacity两者的最大值
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        //否则就返回minCapacity
        return minCapacity; //4
    }
    
    private void ensureExplicitCapacity(int minCapacity) { //minCapacity=4
        modCount++;

        // overflow-conscious code
        /*
        这里有两种情况：
            情况一：elementData为空数组，minCapacity=size+1=1,1-0>0,调用grow()进行扩容
            情况二：elementData不为空数组，minCapacity=size+1，为elementData原始的数组(未调用add方法前)，
                    如果length不够用，再调用grow()进行扩容
        */
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
    
    /**
     * 增加容量以确保它至少可以容纳由最小容量参数指定的元素数
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        //将原始的elementData.length赋给oldCapacity
        int oldCapacity = elementData.length;
        //新容量=1.5*oldCapacity
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //如果elementData为空数组，那么oldCapacity和newCapacity都为0，将newCapacity直接赋值为10
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //如果newCapacity超过了最大的容量限制，将调用hugeCapacity()做进一步处理
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        //newCapacity确定好了以后，就copyof数组，进行扩容
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        //判断minCapacity是否大于最大的数组容量MAX_ARRAY_SIZE:是，返回Integer.MAX_VALUE(2147483647);
        //否则返回MAX_ARRAY_SIZE
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    
    /**
     * 在指定位置插入元素
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
     public void add(int index, E element) {
        //检查插入的位置是否合理
        rangeCheckForAdd(index);

        //插入元素之后，将index之后的元素都往后移一位
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }
    
    /**
     * 用于add和addAll方法的范围检查
     * A version of rangeCheck used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

     /**
     * 删除指定位置的元素
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        //范围检查
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index); //要删除指定index位置的值

        //计算要移动的位数
        int numMoved = size - index - 1;
        if (numMoved > 0)
            /*
            public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);
            Copies an array from the specified source array, beginning at the
            specified position, to the specified position of the destination array.                            
            从源数组的特殊位置开始拷贝到目标数组的特定位置

            System.arraycopy(elementData, index+1, elementData, index,
                             size - index - 1);    //size=8，index=5，8-5-1=2，src=[1,2,3,4,5,7,8]
            */
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }
    
     /**
     * 移除list列表中第一次出现的待删除的元素
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        //依次遍历删除
        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;
    }
    //remove函数用户移除指定下标的元素，此时会把指定下标到数组末尾的元素向前移动一个单位，
    //并把最后一个元素置为null
    
    /**
     * 将list表中指定位置的元素替换掉
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        //范围检查
        rangeCheck(index);

        //指定位置的元素值
        E oldValue = elementData(index);
        //将elementData的指定位置替换为element
        elementData[index] = element;
        return oldValue;
    }
    
    /**
     * 返回在list列表中的指定元素第一次出现的索引值
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    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;
    }
    //从头开始查找元素，可以查找null值
    
     /**
     * 返回在list列表中指定位置的元素
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        //范围检查，只检查>=size的部分
        rangeCheck(index);

        return elementData(index);
    }
    
    E elementData(int index) {
        return (E) elementData[index];
    }
    //注意：返回的值都做了向下转型（object->E）处理
}

2.3 小结：

　　1、ArrayList底层就是一个elementData数组，不过跟array相区别的是，它存在一个动态扩容机制

　　2、由于每次添加、删除元素，都涉及到数组元素的移动，因此效率不高

　　3、由于ArrayList实现了RandomAccess接口，适合做随机访问，用for循环进行查找元素，效率较高