Strings and While loops

Question

I think I'm having trouble with strings and while loops. When I run this program and I type in an action the program does nothing. It doesn't exit, it just sits there. That's why I think it's a problem with my while loop. But I think it could also be with my Strings right before the while loop. Am I declaring those Strings correctly? Or am I comparing them wrong in the while loop or something? Thanks for the help.

import java.util.Scanner;

public class HW2tester3 {

    public static void main(String args[]) {

        MotorBoat ex3 = new MotorBoat();
        Scanner keyboard = new Scanner(System.in);
        System.out.printf("Input how fast the motorboat was going: ");
        int s = keyboard.nextInt();
        ex3.ChangeSpeed(s);
        System.out.printf("Input the time the motorboat ran: ");
        int t = keyboard.nextInt();
        ex3.OperatingTime(t);

        // Ask the user what action he or she wants to take
        System.out.printf("If you want your distance travelled type:" + 
                          " distance\n");
        System.out.printf("If you want how much fuel you used type: fuel\n");
        System.out.printf("If you want to refuel type: refuel\n");
        System.out.printf("If you are finished type: done\n");
        System.out.printf("What would you like to do? ");

        // Compares the input with the defined strings and preforms the
        //   the action requested
        String a = keyboard.nextLine();
        String b = "done";
        String c = "distance";
        String d = "fuel";
        String e = "refuel";
        if (a != b) {
            while (a != b) {
                a = keyboard.nextLine();
                if (a == c) {
                    ex3.getDistance();
                }
                else if (a == d) {
                    ex3.getFuelUsed();
                }
                else if (a == e) {
                    ex3.Refuel();
                }
            }
        }
        if (a == b) {
            System.exit(0);
        }
    }
}

Solution 1

a == b checks whether a and b are the same object, which isn't always the case for strings. Use string.equals() instead.

Also, use variable names you can differentiate between. a, b, c, d, etc. are not good variable names and will frequently confuse you.

That being said, try this:

String input = "";

do {
  input = keyboard.nextLine();

  if (input.equals("distance")) {
    ex3.getDistance();
  } else if (input.equals("fuel")) {
      ex3.getFuelUsed();
  } else if (input.equals("refuel")) {
      ex3.Refuel();
  }
} while (!input.equals("done"));

System.exit(0);

Solution 2

a == b does not compare the value of two strings, but rather that a and b represent the same objects. Same for !=.

You want to use a.equals(b) not a == b.

Solution 3

In java you can't compare strings with == or !=

Use a.equals(b) and !a.equals(b)

Solution 4

The “==” operator can be used to test primitive values for equality(i.e. int, char, boolean...).

However, when you compare two object reference variables with the "==" operator, you are actually testing whether the two references point to the same object.

Rectangle box1 = new Rectangle(5, 10, 20, 30); 
Rectangle box2 = box1;
Rectangle box3 = new Rectangle(5, 10, 20, 30);

Comparing:

box1 == box2; // true;

Comparing:

box1 == box3; // false;

To compare the contents of the objects use the equals(Object) method, which will return true if two Objects have the same contents.

String a = "distance";
String b = "done";
if(a.equals(b)){
  //code...
}

View more solutions

Comments

[Wade J]

Oh wow, I just learned that, too. That's a little embarrassing haha. I get how the .equals operates kind of like == , but how do I get it to operate like != ?

[Wade J]

Thanks for the do-while loop! I don't know why I didn't think of that! But that gives a "cannot find symbol" error on "input" in the conditions for " while (!input.equals("done")); " I'm assuming because input is declared within the do block?

[Blender]

Probably. Try declaring String input; outside of the do {} while block and just use input = keyboard.nexLine();.