Collection c = new ArrayList();
    c.add("hello");
    c.add("world");
    c.add("java");
    System.out.println(c); //[hello, world, java]

        public String toString() {
            Iterator<E> it = iterator(); // 当前集合本身调用迭代器方法，得到集合迭代器
            if (! it.hasNext())
                return "[]";
    
            StringBuilder sb = new StringBuilder();
            sb.append('[');
            for (;;) {
                E e = it.next(); // e = hello,world,java
                sb.append(e == this ? "(this Collection)" : e);
                if (! it.hasNext())
                    // [hello, world, java]
                    return sb.append(']').toString();
                sb.append(',').append(' ');
            }
        }