循环的前哨问题，以允许进入Java中的退出程序

Question

I'm having trouble at the bottom part where i am trying to set the sentinel exit key to quit.. im not sure exactly how i am supposed to do it.

int number;

    do
    {

            Scanner Input = new Scanner(System.in);
    System.out.print("Enter a positive integer or q to quit program:");
        number = Input.nextInt();
    }
    while(number >= 0);

    do
    {

            Scanner Input = new Scanner(System.in);
    System.out.print("Input should be positve:");
        number = Input.nextInt();
    }
    while(number < 0);

            do 
            {

             Scanner quit = new Scanner(System.in);   
             System.out.print("Enter a positive integer or quit to end program");
             input = quit.nextstring();   


            }
             while (!input.equals(quit))//sentinel value allows user to end program
            {
            quit = reader.next()  

Answer 1

A couple tips:

• Don't initialize a new Scanner every iteration, that will be really expensive.
• Notice that as you wrote it, if a user enters a negative number, they can't get back "up" into the first while loop to keep entering positive numbers.
• I assume you want this all in one while loop, not three, or these actions would have to be performed in sequence to break through all 3 sentinel conditions.
• System.out.print() will not add a new line, which will look awkward.

Working on these assumptions, here's a version that has one sentinel variable endLoop that gets reset if the quit condition is met, ie the user enters 'quit'. If they enter a negative number, the "Input should be positive" message will print, then the loop will start over, and if they enter a positive number then nothing will happen (I marked where to add whatever action goes there) and the loop will start over. We only cast the input (which is a String) to an int after checking that it isn't 'quit', because if we try to cast a String (like 'quit') to an int, the program will crash.

Scanner input = new Scanner(System.in);
boolean endLoop = false;
String line;
while (!endLoop) {
  System.out.print("Enter a positive integer or 'quit' to quit program: ");
  line = input.nextLine();
  if (line.equals("quit")) {
    endloop = true;
  } else if (Integer.parseInt(line) < 0) {
    System.out.println("Input should be positive.");
  } else {
    int number = Integer.parseInt(line);
    //do something with the number
  }
}

Edited to use 'quit' instead of 0 for termination condition. Note that this program will crash if the user enters anything other than a number or 'quit'.