Arrays

Arrays

一、总结

1.基于 jdk 1.8

二、asList

    /**
     * Returns a fixed-size list backed by the specified array.  (Changes to
     * the returned list "write through" to the array.)  This method acts
     * as bridge between array-based and collection-based APIs, in
     * combination with {@link Collection#toArray}.  The returned list is
     * serializable and implements {@link RandomAccess}.
     *
     * <p>This method also provides a convenient way to create a fixed-size
     * list initialized to contain several elements:
     * <pre>
     *     List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");
     * </pre>
     *
     * @param a the array by which the list will be backed
     * @return a list view of the specified array
     */
    public static <T> List<T> asList(T... a) {
	return new ArrayList<T>(a);
    }

1.参数为 T 多个泛型T
若参数为基础数据类型的数组，则将其视为一个整体，如 int [] a ，此时的size = 1
若参数为引用数据类型，则其size为数据的个数

2.ArrayList 非 java.util.ArrayList

三、Arrays.ArrayList 内部私有静态类

    /**
     * @serial include
     */
    // 继承 Abstract ，而 Abstract 实现 List ，所以 asList 才可以用 List 接收
    // 即 ArrayList 间接实现了 List
    private static class ArrayList<E> extends AbstractList<E>
    // RandomAccess 可以随机访问，Serializable 可序列化
	implements RandomAccess, java.io.Serializable
    {
        private static final long serialVersionUID = -2764017481108945198L;
	private final E[] a;

	ArrayList(E[] array) {
            if (array==null)
                throw new NullPointerException();
	    a = array;
	}
        // 以下是 ArrayList 实现的方法，无RemoveAll 方法，所以asList 的返回值List
        // 调用 removeAll时会抛出无此操作方法的异常
	public int size() {
	    return a.length;
	}

	public Object[] toArray() {
	    return a.clone();
	}

	public <T> T[] toArray(T[] a) {
	    int size = size();
	    if (a.length < size)
		return Arrays.copyOf(this.a, size,
				     (Class<? extends T[]>) a.getClass());
	    System.arraycopy(this.a, 0, a, 0, size);
	    if (a.length > size)
		a[size] = null;
	    return a;
	}

	public E get(int index) {
	    return a[index];
	}

	public E set(int index, E element) {
	    E oldValue = a[index];
	    a[index] = element;
	    return oldValue;
	}

        public int indexOf(Object o) {
            if (o==null) {
                for (int i=0; i<a.length; i++)
                    if (a[i]==null)
                        return i;
            } else {
                for (int i=0; i<a.length; i++)
                    if (o.equals(a[i]))
                        return i;
            }
            return -1;
        }

        public boolean contains(Object o) {
            return indexOf(o) != -1;
        }
    }