Can someone please explain what's wrong with this code?

Question

This program does the following: First input obtains two numbers from the user, second input if the user inputs (+) symbol the program adds the two numbers together, if the user enters (-) symbol it subtracts the 1st number from the 2nd number. But no results are shown the program just runs and terminates.

import javax.swing.JOptionPane;

public class ObtainNumber {
public static void main(String []args) {

    String strNum1 = JOptionPane.showInputDialog("Enter the first number");
    String strNum2 = JOptionPane.showInputDialog("Enter the second number");

    int num1 = Integer.parseInt(strNum1);
    int num2 = Integer.parseInt(strNum2);

    String askForOperation = JOptionPane.showInputDialog("What operation needs to be done?");

    int sum;
    if (askForOperation == "+") {
            sum = num1 + num2;
            JOptionPane.showMessageDialog(null, "The result of the Addition is " + sum);
    }

    double subtract;
    if (askForOperation == "-") {
            subtract = num2 - num1;
            JOptionPane.showMessageDialog(null, "The result of the subtraction is " + subtract);

    }



    }

}

Comparison of String... – Vinay Veluri Feb 25 '14 at 14:15 — Vinay Veluri, Feb 25 '14 at 14:15
use == '+' and == '-' since (+) and (-) are characters – Mahmoud Hashim Feb 25 '14 at 14:16 — Mahmoud Hashim, Feb 25 '14 at 14:16
The problem also is that it is a hacky Swing! – Chiron Feb 25 '14 at 14:17 — Chiron, Feb 25 '14 at 14:17
@MahmoudHashim no, they are strings – fge Feb 25 '14 at 14:17 — fge, Feb 25 '14 at 14:17

Kick · Answer 1 · 2014-02-25T14:30:48.887

6

if (askForOperation == "+")  // Reference is not same

Do the comparition check using equals method.

if ("+".equals(askForOperation))  // Compare content

edited Feb 25 '14 at 14:30

answered Feb 25 '14 at 14:15

Kick

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While this is true, can you explain it little bit more? What is wrong with `==`? – Pshemo Feb 25 '14 at 14:16
1

== only check the reference whereas equals method compare the value. – Kick Feb 25 '14 at 14:19
I know that. Point in answering is not only to give solution, but also explain problem. Don't give fish, give rod and teach how to use it. Let OP learn *why* `==` is wrong here. Put your comment in your answer. – Pshemo Feb 25 '14 at 14:20
Sry cant understand @Pshemo the reference of askForOperation and + is not same – Kick Feb 25 '14 at 14:22
@fge i also say that reference is not same so need to use equals to compare the content – Kick Feb 25 '14 at 14:25

score 1 · Accepted Answer · answered Feb 25 '14 at 14:16

Try this

if (askForOperation.equals("+")) {
        sum = num1 + num2;
        JOptionPane.showMessageDialog(null,
                   "The result of the Addition is " + sum);
}

To avoid any Nullpointer exception, in any case

do this,

final String plus = "+";
if (plus.equals(askForOperation)) {
        sum = num1 + num2;
        JOptionPane.showMessageDialog(null,
                   "The result of the Addition is " + sum);
}

fge · Answer 3 · 2014-02-25T14:26:31.007

1

This test:

askForOperation == "+"

is wrong. What is does is compare the references of askForOperation and (unnamed) string "+", which is not the same at all.

You want to compare the contents; therefore you must use:

"+".equals(askForOperation)

Same for "-".

Sample:

String h1 = "hello";
String h2 = new String("hello");
h1 == h1; // true: same reference
h1 == h2; // false: different reference
h1.equals(h2); // true: same contents

Note that strings are a little special due to string literals -- "+" will "silently" create a String object for you, which is why you can write "strange-looking" code such as "foo".equals(bar)

edited Feb 25 '14 at 14:26

answered Feb 25 '14 at 14:21

fge

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@Pshemo gee, I keep forgetting about this one :/ – fge Feb 25 '14 at 14:26
That (because it works for interned literals) is probably main reason why people are trying to use `==` to compare all Strings :/ – Pshemo Feb 25 '14 at 14:28

score 0 · Answer 4 · edited Feb 25 '14 at 14:27

0

if(askForOperation.equals("+"){
    do smth..
}

Use of == for String is wrong,you must compare content like String1.equals(String2);

edited Feb 25 '14 at 14:27

Kick

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answered Feb 25 '14 at 14:17

Ruben JG

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= for primitive types equals -for objects – Ruben JG Feb 25 '14 at 14:19

saravanakumar · Answer 5 · 2014-02-25T14:43:15.263

0

try

    .equals

method insteed of ==

== compares the object reference and .equals compares value of the object value.

edited Feb 25 '14 at 14:43

answered Feb 25 '14 at 14:18

saravanakumar

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I asked what's wrong with the program if I had known the problem I would have Googled it – Django Feb 25 '14 at 14:37
Ya sure @Korar F. sorry, I removed that. – saravanakumar Feb 25 '14 at 14:42

score 0 · Answer 6 · answered Feb 25 '14 at 14:21

0

I think this will clear your doubt.

http://www.javabeat.net/what-is-difference-between-equals-and/

answered Feb 25 '14 at 14:21

user3213851

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Can someone please explain what's wrong with this code?

6 Answers6