Java Iterator 报错

            static char[] chars;
            TreeSet set = new TreeSet<Character>();
            set = [Ins(a,d), dE(a,c), ~E(a,b)]; 
            chars = new char[set.size()];
            System.out.println(chars);     //打印出来
            System.out.println("aaaaaaaaa");  //可以打印
            Iterator iterator = set.iterator();
            int index = 0;
//从下面开始报错
            while (iterator.hasNext()) {
                chars[index++] = (Character) iterator.next();
                System.out.println(chars);
            }

• 看下这篇博客，也许你就懂了，链接：Java集合框架遍历之迭代器(Iterator)