如何使我的while循环与用户提示一起工作？

Question

Over here, I have a robot application which consists of mainly three user prompts. Right after the third prompt, and it has to loop back to the first one and this goes on until when the user types QUIT which will then display all the details from the vector elements. Right now, I'm having problem with the application as when I put the first prompt in the while-loop and when the application starts, nothing happens unless I put it out of the loop but this will not be looping back. So could someone give me some guidance here as I'm a newbie in programming. Thanks so much!

class KillerRobot {

private String name;
private String mainWeapon;
private int numberOfKills;

KillerRobot()
{
}

public String getName()
{
    return name;
}

public String getMainWeapon()
{
    return mainWeapon;
}

public int getNumberOfKills()
{
    return numberOfKills;
}

public String toString()
{
    return name + " used a " + mainWeapon + " to destroy " + numberOfKills + " enemies ";
}

public void setName(String a)
{
    name = a;
}

public void setMainWeapon(String b)
{
    mainWeapon = b;
}

public void setNumberOfKills(int c)
{
    numberOfKills = c;
}
}

Main method class:

import java.util.Scanner;
import java.util.Vector;

class TestVector2 {

public static void main(String [] args)
{
    String prompt = "Please enter the name of the Killer Robot or QUIT to finish";
    String prompt2 = "Please enter the main weapon for the robot";
    String prompt3 = "Please enter the number of kills for this robot";

    System.out.println(prompt);                     //The first prompt and has to loop back unless QUIT from user
    Scanner userInput = new Scanner(System.in);

    Vector <KillerRobot> robotDetails = new Vector <KillerRobot>();     //adding object to Vector elements

    do {
        System.out.println(prompt);                 //The first prompt and has to loop back unless QUIT from user
        robot = new KillerRobot();

        String a = userInput.next();
        robot.setName(a);

        System.out.println(prompt2);
        String b  = userInput.next();
        robot.setMainWeapon(b);

        System.out.println(prompt3);
        int c = userInput.nextInt();
        robot.setNumberOfKills(c);

        robotDetails.add(robot);
        System.out.println(robot);
    } while (!userInput.nextLine().equals("QUIT"));
}           
}

Answer 1

The logic of your program is a bit off, and this is giving you problems. You want to keep looping over all three prompts until the user types 'QUIT'. This means that the first prompt must be inside the loop. So the trick is to make your while -condition different. Instead of checking whether there is any input (with hasNext() ), check whether the last input was 'QUIT'.

One way to do that is to set a simple boolean flag outside the loop and then use that as your while -condition, like so:

 boolean quit = false;

 while(!quit) {
   System.out.println(prompt);
   String a = userInput.next(); 
   quit = "QUIT".equals(a); 
   ...
 }

This is actually still not ideal, since this will continue showing all three prompts, even when the user types 'QUIT' on the first one. I assume you want to jump out of the loop immediately when that happens, so you might consider making an infinite loop (using while(true) , for example), and just immediately jumping out of the loop (with a break ) when the user enters the quit command.

Answer 2

This should in line with what you want. There are a few changes - one your code has the prompt displayed twice, second you want them to be able to QUIT before having any robots so a do-while loop would be a little inconvenient, and lastly at the end of the loop I use prompt1 again and if they quit, print all the robots if there are any instead of printing them out in each iteration of the loop (because at that point if you just print it to the console you aren't using the vector at all.

public static void main(String[] args) {
            String prompt1 = "Please enter the name of the Killer Robot or enter QUIT to exit.";
            String prompt2 = "Please enter the main weapon for the robot";
            String prompt3 = "Please enter the number of kills for this robot";
            Scanner userInput = new Scanner(System.in);

            Vector <KillerRobot> robotDetails = new Vector <KillerRobot>();
            KillerRobot robot;


            //prime the loop
            System.out.println(prompt1); 
            String userEntry = userInput.next();

            while(!userEntry.equals("QUIT")){
                robot = new KillerRobot();
                robot.setName(userEntry);

                System.out.println(prompt2);
                String b  = userInput.next();
                robot.setMainWeapon(b);

                System.out.println(prompt3);
                int c = userInput.nextInt();
                robot.setNumberOfKills(c);

                robotDetails.add(robot);

                //verify repeat
                System.out.println(prompt1); 
                userEntry = userInput.next();
            }

            if(robotDetails.size() < 1){
                System.out.println("No robots");
            } else {
                for(KillerRobot i : robotDetails){
                    System.out.println(i);
                }
            }

            System.out.println("done");
        }