Java.util.ArrayList详解

java.util.ArrayList就是传说中的动态数组.

继承了关系，有此可看出ArrayList与list的collection的关系

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable

分析期特性

具有实例化性质，从实现了serializable就可以看出来

private static final long serialVersionUID = 8683452581122892189L;

接下来定义了初始容量大小，和初始锥形。arralist里面本身存储的为一个Object类型的数组

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

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

使用arraylist时需要实例化：有三种

　　/**
     * 初始化一个给定大小容量的数组List，主要用于如果本事知道这个list的大小时。
     * 能优化如果大小超过默认容量时导致arralist每次都要去增加并创建一个新的list
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    /**
     * 使用默认的大小10
     */
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

    /**
     * 根据一个collection来定义一个arralist
     *
     * @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();
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }

Arraylist有提供期可能存在过多的空容量时清除处理

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

另一个知识点。当arraylist新增时如果当前容量已满时的处理过程

　　public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // 判断容量是否超标
        elementData[size++] = e;
        return true;
    }

　　private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);// 容量超标情况
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)// 如果超标，则处理
            grow(minCapacity);
    }
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length; // 容量大小
        int newCapacity = oldCapacity + (oldCapacity >> 1); //超标增加量计算法，x+x/2=50%左右
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // 赋值原来的数组，并增加newCapacity个容量
        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;
    }