/**
     *从Collection中继承过来的抽象方法 
     */
    public abstract Iterator<E> iterator();
    public abstract int size();
    /**
     *直接返回 size() == 0，与size()的具体实现无关
     */
    public boolean isEmpty() {
         return size() == 0;
     }
    /**
     * 通过迭代器遍历所有元素，用equals方法比较，只要有一个符合条件的元素则返回true
     */
    public boolean contains(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return true;
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return true;
        }
        return false;
    }
    /**
     * 通过迭代器遍历集合中，转换为数组
     * 等价于
     * List<E> list = new ArrayList<E>(size());
     * for (E e : this)
     *     list.add(e);
     * return list.toArray();
     */
    public Object[] toArray() {
        // Estimate size of array; be prepared to see more or fewer elements
        Object[] r = new Object[size()];
        Iterator<E> it = iterator();
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) // fewer elements than expected
                return Arrays.copyOf(r, i);
            r[i] = it.next();
        }
        return it.hasNext() ? finishToArray(r, it) : r;
    }
    /**
     * 同上面的方法类似，比上面的方法常用，优点在于返回的数组不用做类型转换
     * 当参数数组容量不够时，将返回一个新的数组
     */
    public <T> T[] toArray(T[] a) {
        // Estimate size of array; be prepared to see more or fewer elements
        int size = size();
        T[] r = a.length >= size ? a :
                  (T[])java.lang.reflect.Array
                  .newInstance(a.getClass().getComponentType(), size);
        Iterator<E> it = iterator();

        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) { // fewer elements than expected
                if (a == r) {
                    r[i] = null; // null-terminate
                } else if (a.length < i) {
                    return Arrays.copyOf(r, i);
                } else {
                    System.arraycopy(r, 0, a, 0, i);
                    if (a.length > i) {
                        a[i] = null;
                    }
                }
                return a;
            }
            r[i] = (T)it.next();
        }
        // more elements than expected
        return it.hasNext() ? finishToArray(r, it) : r;
    }
    /**
     * 默认不支持add
     */
    public boolean add(E e) {
        throw new UnsupportedOperationException();
    }
    /**
     * 迭代器遍历集合，通过equals找到符合条件的元素，将其移除
     * 对于不可变集合，其迭代器的remove方法中会抛出UnsupportedOperationException
     */
    public boolean remove(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext()) {
                if (it.next()==null) {
                    it.remove();
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (o.equals(it.next())) {
                    it.remove();
                    return true;
                }
            }
        }
        return false;
    }
    /*
     * 迭代器循环遍历集合，调用自身的contains方法
     */
    public boolean containsAll(Collection<?> c) {
        for (Object e : c)
            if (!contains(e))
                return false;
        return true;
    }
    /**
     * foreach循环，调用自身add方法
     * 添加成功返回true,失败返回false
     */
    public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }
    /**
     * 调用迭代器遍历移除
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            if (c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
    /**
     * 下面的方法类类似，都是通过迭代器操作，与迭代器的具体实现无关
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            if (!c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
    public void clear() {
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            it.next();
            it.remove();
        }
    }
    /**
     * 重写了toString,使得集合可以直接打印出可读性很好的结果
     */
    public String toString() {
        Iterator<E> it = iterator();
        if (! it.hasNext())
            return "[]";

        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (;;) {
            E e = it.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (! it.hasNext())
                return sb.append(']').toString();
            sb.append(',').append(' ');
        }
    }