Basic Java Calcualtor: Only addition working

Question

I am trying to build a simple calculator that operates on Double values and performs:

+

-

*

/

negation

^2

for some reason, whichever operation i specify when running the program executes the addition function. So when I try to do

5 7 *

I get 12.0 and not 35.

Any suggestions?

I would also really like some help implementing a way to quit this program when the word "exit" is entered. I was thinking something like System.exit(0);

but Im not sure how to implement. this is my code so far:

import java.util.Scanner;

public class Calc {

public static void main(String[] args) 
{
    double num1;
    double num2;
    String operation;


    Scanner input = new Scanner(System.in);

    System.out.println ("Enter 'exit' to quit the calculator or 'help' for more options");
    System.out.println("Enter the first number:");
    num1 = input.nextInt();


    System.out.println ("Display:" +  num1);

    System.out.println("Enter the second number:");
    num2 = input.nextInt();

    System.out.println ("Display:" + num2);

    Scanner op = new Scanner(System.in);


    System.out.println("Please enter operation:");
    operation = op.next();

    if (operation == "+");
    {
        System.out.println((num1 + num2));
    }
    if  (operation == "-"){
    {
        System.out.println((num1 - num2));
    }

    if (operation == "/"){
    {
        System.out.println((num1 / num2));
    }
    if (operation == "*"){
    {
        System.out.println((num1 * num2));

    }

    }

    }

`operation == "+"` is not how `String` comparison works in Java — MadProgrammer, Oct 10 '13 at 07:06

score 5 · Answer 1 · edited Oct 10 '13 at 07:20

5

there is a semicolon at if (operation.equals("+")); so, System.out.println((num1 + num2)); will always work

And also replace, == with .equals("+") as == checks for reference, while equals checks for the value of the variable.

edited Oct 10 '13 at 07:20

Matthias

3,582
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answered Oct 10 '13 at 07:11

HimanshuR

1,390
5
16
35

Very helpful, thank you. However now the only operation that returns a value is +. the others no longer return the + operation, but they just do not return anything – Charlie Tidmarsh Oct 10 '13 at 07:18
1

replace == with equals function everywhere when you compare a string – HimanshuR Oct 10 '13 at 07:19
please check your open `{` and close `}` its a little bit messy – ggDeGreat Oct 10 '13 at 07:20
operation.equals("-") – HimanshuR Oct 10 '13 at 07:20
treat each if individually or as elseif.. – HimanshuR Oct 10 '13 at 07:21
if (operation.equals("+")) { System.out.println((num1 + num2)); } if (operation.equals("-")) { System.out.println((num1 - num2)); } if (operation.equals("/")) { System.out.println((num1 / num2)); } if (operation.equals("*")) { System.out.println((num1 * num2)); } – HimanshuR Oct 10 '13 at 07:23

SpringLearner · Answer 2 · 2013-10-10T07:53:40.147

0

try this way Instead of using == use .equals()

import java.util.Scanner;

public class Calc {

public static void main(String[] args) 
{
    double num1;
    double num2;
    String operation;


    Scanner input = new Scanner(System.in);

    System.out.println ("Enter 'exit' to quit the calculator or 'help' for more options");
    System.out.println("Enter the first number:");
    num1 = input.nextInt();


    System.out.println ("Display:" +  num1);

    System.out.println("Enter the second number:");
    num2 = input.nextInt();

    System.out.println ("Display:" + num2);

    Scanner op = new Scanner(System.in);


    System.out.println("Please enter operation:");
    operation = op.next();

    if (operation.equals("+"))
    {
        System.out.println((num1 + num2));
    }
    if  (operation.equals("-"))
    {
        System.out.println((num1 - num2));
    }

    if (operation.equals("/")){
    {
        System.out.println((num1 / num2));
    }
    if (operation.equals( "*")){
    {
        System.out.println((num1 * num2));

    }

    }

    }

edited Oct 10 '13 at 07:53

answered Oct 10 '13 at 07:09

SpringLearner

13,738
20
78
116

With `if (operation.equals("+"));` in there, it will still not work as expected. – Matthias Oct 10 '13 at 07:13
@MadProgrammer It should work – SpringLearner Oct 10 '13 at 07:47
@Matthias Unaware of ; present so fixed now – SpringLearner Oct 10 '13 at 07:47
`if (operation.equals("-")){ {`...? – MadProgrammer Oct 10 '13 at 07:48
@MadProgrammer yes thats true,well OP has mentioned that only + was working So i did not check line by line.Well == shouldnt be replaced with .equals()? – SpringLearner Oct 10 '13 at 07:53
Yes, the `String` comparison is fix, but if you're going to dump code back on the user, you should at least make an effort to make it actually work, letting the poor OP know everything else that's about to blow up in there face - IMHO – MadProgrammer Oct 10 '13 at 07:54
@MadProgrammer Sure I will do it next time . – SpringLearner Oct 10 '13 at 08:01

score 0 · Answer 3 · answered Oct 10 '13 at 07:10

0

please change operation == to operation.equals("your value")

answered Oct 10 '13 at 07:10

ggDeGreat

1,098
1
17
33

I changed all of the == to .equals and the same error persists – Charlie Tidmarsh Oct 10 '13 at 07:11

score 0 · Answer 4 · answered Oct 10 '13 at 07:32

There are at least three main issues with your code...

Firstly, String comparison in Java is done using .equals not ==

Secondly...

if (operation == "+");

Has a semi-colon at the end, which basically means that even if the condition is meet, it will not run anything, but will skip to the next execution block, which is ...

{
    System.out.println((num1 + num2));
}

Thirdly, you have an extra open brace ({) here...

if  (operation == "-"){
{

Here...

if (operation == "/"){
{

And here...

if (operation == "*"){
{

Which is going to completely screw up your logic

You should also make use of else-if blocks as well...

Instead, try something like...

if ("+".equals(operation))//;
{
    System.out.println((num1 + num2));
} 
else if ("-".equals(operation)) //{
{
    System.out.println((num1 - num2));
}
else if ("/".equals(operation)) //{
{
    System.out.println((num1 / num2));
}
else if ("*".equals(operation)) //{
{
    System.out.println((num1 * num2));
}

I've commented out the problem areas so you can see where they exists before hand

Basic Java Calcualtor: Only addition working

4 Answers4