大杂烩 -- ArrayList的动态增长 源码分析

基础大杂烩 -- 目录

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问题：当ArrayList中放入的元素一直增加会如何增长数组长度？？？

Class : ArrayList

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

    /**
     * 默认容量值
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 默认容量为空的数组
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 用来存储元素
     */
    transient Object[] elementData;

    /**
     * 数组大小
     *
     * @serial
     */
    private int size;

    /**
     * 添加元素
     *
     * @param e
     * @return
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);
        elementData[size++] = e;
        return true;
    }

    /**
     * 确保容量动态增长
     *
     * @param minCapacity
     */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    /**
     * 确定新的容量
     *
     * @param minCapacity
     */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * 数组最大大小
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 数组容量动态增长
     *
     */
    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);
    }

    /**
     * 计算数组最终容量
     *
     * @param minCapacity
     * @return
     */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }
}

Class : Arrays

    @SuppressWarnings("unchecked")
    public static <T> T[] copyOf(T[] original, int newLength) {
        return (T[]) copyOf(original, newLength, original.getClass());
    }

    public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
        @SuppressWarnings("unchecked")
        T[] copy = ((Object)newType == (Object)Object[].class)
            ? (T[]) new Object[newLength]
            : (T[]) Array.newInstance(newType.getComponentType(), newLength);
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

Class : System

   public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

总的来说依旧是调用System类中native方法重新创造一个数组。

啦啦啦