新人问一个关于字符串是否相等的问题。谢谢！

            Scanner in = new Scanner(System.in);
            String d = in.next();
            String a = "byes";
            String b = "byes";
            String c = new String("byes");
            System.out.println(a==b);
            System.out.println(a==c);
            System.out.println(a=="byes");
            System.out.println(a.equals(b));
            System.out.println(a==d);

为什么 a==b ,而a!=c;a!=d;d得到的也是"byes"字符串。