Collection ArrayList

一、源码分析

1. ArrayList 类组成

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable    // 1. 实现了List接口，List继承Collection接口，Collection接口继承Iterable接口
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;        // 2. 默认数组容量大小(最小为10)  

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};   // 3. 共享空数组，不用赋值

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData;        // 4. ！！！存储数据数组(transient防止序列化，用自定义序列化方法，防止浪费多余空间) 

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * 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 an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;   // 5. 构造函数
    }

2. add(E)方法

* 1. 自动判断是否需要增加存储空间，如果需要，增加数组长度的一半(oldSize + oldSize>>1)；
* 2. mouCount++,如果在迭代器遍历是数据结构发生变化，报异常；
* 3. Arrays.copyOf(data,newSize) 生成新数组；
* 4. 赋值&Size++;

    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);
    }

   /**
     * 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
        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);
    }

3. add(index,element)&remove(index)方法

耗时操作

System.arraycopy(Object src,

int srcPos,

Object dest,

int destPost,

int length);

 /**
     * 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) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

    /**
     * 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) {
        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;
    }

n. 问题分析

1. arraylist 存储大小如何自动增长，增长多少？

容量检查方法判断，如果容量不够，增加size/2(默认是10);

2. ArrayList&LinkedList区别？

多数情况下，当你遇到访问元素比插入或者是删除元素更加频繁的时候，你应该使用ArrayList。
另外一方面，当你在某个特别的索引中，插入或者是删除元素更加频繁，或者你压根就不需要访问元素的时候，你会选择LinkedList。
这里的主要原因是，在ArrayList中访问元素的最糟糕的时间复杂度是”1″，而在LinkedList中可能就是”n”了。在ArrayList中增加或者删除某个元素，通常会调用System.arraycopy方法，这是一种极为消耗资源的操作，因此，在频繁的插入或者是删除元素的情况下，LinkedList的性能会更加好一点。

3. 当传递ArrayList到某个方法中，或者某个方法返回ArrayList，什么时候要考虑安全隐患？如何修复安全违规这个问题呢？

当array被当做参数传递到某个方法中，如果array在没有被复制的情况下直接被分配给了成员变量，那么就可能发生这种情况，即当原始的数组被调用的方法改变的时候，传递到这个方法中的数组也会改变。下面的这段代码展示的就是安全违规以及如何修复这个问题。

ArrayList被直接赋给成员变量——安全隐患：

修复这个安全隐患：

齊帥