如果在while循环内，则不能突破while循环

Question

I recently started studying Java and came across a problem while testing out something. This might be a very easy question, but I can't seem to solve it. This is my code:

    int firstj = 1;

    if (firstj == 1) {
        String choice = "Type a number between 1 and 4";
        System.out.println(choice);
        while (true) {

            if (firstj == 1) {
                Scanner third = new Scanner(System.in);
                String thirdch = third.nextLine();

                while (true) {

                    if (thirdch.equals("1")) {
                        System.out.println("Show choice and accept input again ");
                        System.out.println(choice);
                        break;
                    } else if (thirdch.equals("2")) {
                        System.out.println("Show choice and accept input again ");
                        System.out.println(choice);
                        break;
                    } else if (thirdch.equals("3")) {
                        System.out.println("Show choice and accept input again ");
                        System.out.println(choice);
                        break;
                    }

                    else if (thirdch.equals("4")) {
                        // I need this to break the loop and move on to the
                        // "Done." string
                        break;
                    }

                    else {
                        System.out.println("Type a number between 1 and 4");
                        thirdch = third.nextLine();
                    }
                }

            }
        }
    }
    String done = "Done";
    System.out.println(done);

I want to make it so that when you type 1, 2 or 3, you get the string telling you to type a number again and accept user input, while when you type 4 the loop breaks and goes to the String done. I would be grateful if you could help me solve this problem with an easy code, since I don't know any of the more advanced things.

Answer 1

You can label loops, and then use the label in your break statement to specify which loop you want to break out of, eg

outer: while (true) {
  while (true) {
    break outer;
  }
}

Answer 2

The most scalable way of breaking out of a nested loop is to put the whole thing in a function and use return .

Break to a label in Java is the other option, but it can make code brittle: you are at the mercy of some recalcitrant refactorer who might feel inclined to move the "break to label" blissfully unaware of the consequences; a compiler is unable to warn of such shenanigans.

Answer 3

You could use a lock to stop, like that:

public static void main(String[] args) throws Exception {
    int firstj = 1;
    if (firstj == 1) {
        boolean lock = true;
        while (lock) {

            if (firstj == 1) {
                Scanner third = new Scanner(System.in);

                while (lock) {

                    System.out.println("Type a number between 1 and 4");
                    String thirdch = third.nextLine();
                    switch (thirdch) {
                    case "1":
                        System.out
                                .println("Show choice and accept input again ");
                        break;
                    case "2":
                        System.out
                                .println("Show choice and accept input again ");
                        break;
                    case "3":
                        System.out
                                .println("Show choice and accept input again ");
                        break;
                    case "4":
                        lock = false;
                        break;
                    }

                }
                third.close();

            }
        }
    }
    String done = "Done";
    System.out.println(done);
}

I hope it helps.

Answer 4

从描述中不太清楚，但是continue将让你跳到循环结束，继续循环的其余部分，你可以使用标志来控制你是否要退出多个级别。