String comparison failing when using "=="?

Question

I am an extreme coding noob and I've just started. I designed a rock paper scissors game and can't figure out why it won't work. For some reason the user input isn't working in the if statement. Please take a look.

    package com.youtube.njillatactics;

    import javax.swing.JOptionPane;

    public class RPS {
public static void main(String args[]){
    //start message
    JOptionPane.showMessageDialog(null, "Welcome to Nick's rock paper scissors game!");
    //get user input and convert to lower case
    String userInput = JOptionPane.showInputDialog("Choose rock, paper, or scissors.").toLowerCase();
    //generate random computer input
    double computerInput = Math.random();
    //match user input to converted computer input
    JOptionPane.showMessageDialog(null, match(userInput, convert(computerInput)));
}
//convert random computer input into choice
public static String convert(double x){
    if(x < 0.33){
        return "rock";
    }else if(x < 0.66){
        return "paper";
    }else
        return "scissors";
}
//check to see who wins
public static String match(String x,String y){
    if(x == y){
        return "Tie!";
    }else
    if(x == "rock"){
        if(y == "paper"){
            return "Computer wins!";
        }else
            return "User wins!";
    } 
    if(x == "paper"){
        if(y == "scissors"){
            return "Computer wins!";
        }else
            return "User wins!";
    }
    if(x == "scissors"){
        if(y == "rock"){
            return "Computer wins!";
        }else
            return "User wins!";
    }else
        return x + ", " + y;
}
}

Wow. It was that simple. Like I said I'm new so thanks for helping me out. That was quick! — user3461740, Mar 28 '14 at 01:20
For future reference, it would be helpful if you took the time to identify _what_ specifically wasn't working. You didn't tell us the symptoms, much less a specific snippet of code that did one thing when you thought it would do another. — yshavit, Mar 28 '14 at 01:21

aliteralmind · Answer 1 · 2014-04-04T02:38:04.477

5

You are comparing strings with == when you should be comparing them with .equals(s)

Instead of, for example,

if(y == "rock")  {

change it to

if(y.equals("rock"))

(You should also ensure that y is not null either by testing it, or try-catching for it.)

Comparing strings with == evaluates to true if the same string object is on both sides of the ==.

Comparing strings with equals(s) compares their values, regardless if they're the same object...and obviously, if they happen to be the same object, it will always evaluate to true.

edited Apr 04 '14 at 02:38

answered Mar 28 '14 at 01:18

aliteralmind

19,847
17
77
108

2

Another variation would be `if ("rock".equals(y))` – takendarkk Mar 28 '14 at 01:20
1

"[Yoda conditions](http://blog.codinghorror.com/new-programming-jargon)." – yshavit Mar 28 '14 at 01:23
2

Even though I fully support your answer, the user might not always put exactly "rock", so you could also do y.equalsIgnoreCase("rock"); and it would make it more universal for the person playing the game – user2277872 Mar 28 '14 at 01:24
@yshavit: You think it should be the other way around, as a matter of style. I would (not so strongly) argue that `y.equals("rock")` is preferable from a "fail fast" point of view. Obviously needs to be `try`ed. – aliteralmind Mar 28 '14 at 01:31
@user2277872: I'll leave it as is, but it's a good point. – aliteralmind Mar 28 '14 at 01:36
1

@aliteralmind I wasn't trying to argue one way or another. I just like that name for the pattern. :) – yshavit Mar 28 '14 at 01:38
1

I wasn't saying you SHOULD use it, I was just giving a suggestion on another approach that might have been more of a user friendly input is all :) – user2277872 Mar 28 '14 at 12:09
@user3461740: If you feel this has helped you, please consider up-voting it, and accepting it by clicking on the big green checkmark. Thank you, and best of luck! – aliteralmind Mar 29 '14 at 11:12

score 1 · Answer 2 · answered Mar 28 '14 at 01:19

1

In java, you do not compare two strings by x == y. Instead, you compare by using x.equals(y). This explains why those if statements do not work.

answered Mar 28 '14 at 01:19

Lawrence Choy

6,088
1
20
30

String comparison failing when using "=="?

2 Answers2