String cannot be converted to double

Question

I am trying to make a calculator with Java, and everything worked normally until this error popped up: imcompatible types:

Invalid value type 'String' for format specifier '%.0f', parameter 1

This is the script:

private void btnResultActionPerformed(java.awt.event.ActionEvent evt) {                                     
    String answer;

    second = Double.parseDouble(display.getText()); // The second number to count
    if (operation == "+") {
        answer = String.valueOf(first + second);
        result = String.format("%.0f",answer);
        display.setText(String.valueOf(result));
    } else if (operation == "-") {
        answer = String.valueOf(first - second);
        result = String.format("%.0f",answer);
        display.setText(String.valueOf(result));
    } else if (operation == "*") {
        answer = String.valueOf(first * second);
        result = String.format("%.0f",answer);
        display.setText(String.valueOf(result));
    } else if (operation == "/") {
        answer = String.valueOf(first / second);
        result = String.format("%.0f",answer);
        display.setText(String.valueOf(result));
    }

}

Variables "first", "second" and "result" are doubles, operation is String. I did not have this error before, but after a while it started to error those so I added String.valueOf(first + second); , before it was answer = first + second; but adding valueOf fixed that. Now the problem still exists on "result = String.format("%.0f", answer);"

Possible duplicate : http://stackoverflow.com/questions/5769669/convert-string-to-double-in-java — Bob Swager, May 16 '17 at 10:13

score 1 · Accepted Answer · edited May 23 '17 at 12:18

1

You'll want to make answer a double as well, since you're doing calculations. It's only at display/formatting phase where you should convert to a String.

You should put only the calculation part inside the if (i.e. answer = first + second, with answer being a double), and afterwards convert it to a String to avoid duplicate code.

Finally use equals() to compare Strings, so operation.equals("+") and not ==.

edited May 23 '17 at 12:18

Community

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answered May 16 '17 at 10:14

Kayaman

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I changed that operation == "+" to operation.equals("+") and it works the same way as == would. I removed answer and added your result = String.format(...); but it still says, that String cannot be converted to double. Might there be an error with my NetBeans? Because I followed a video guide to make this, it works for him, so did for me but until a moment it popped all those errors. – Timppa May 16 '17 at 10:20
Read the linked post, do not use `==` to compare Strings. It doesn't matter if it happens to work by chance this time. Anyway, follow the advice in my answer and it should work nicely. – Kayaman May 16 '17 at 10:24
Yeah, thank you for that, will remember in future! However, the error exists on result = String.format(...) AND display.setText(result); but I changed this to display.setText(String.valueOf(result)); which I had before and the error disappeared, the only thing that ain't working is the String.format. – Timppa May 16 '17 at 10:39
It's still the same basic thing. Do your calculations using `doubles`. Then convert the result to `String` with `String.format()`. – Kayaman May 16 '17 at 11:02
answer = first + second; display.setText(String.format("%.0f", answer)); <- This fixed all errors, thank you! =) – Timppa May 16 '17 at 11:07

Prim · Answer 2 · 2017-05-16T10:43:37.393

1

With %.0f as argument for String formatter, you need to pass a number (a float), but in your snippet answer is currently a String.

You can use %s in formatter for a String, or (better) pass as second argument the result of the operation as a float instead of a String

To resume :

String answer = "1";
System.out.println( String.format("%.0f",answer) ); // Throws an exception


double answer = 1.d;
System.out.println( String.format("%.0f",answer) ); // Prints 1

edited May 16 '17 at 10:43

answered May 16 '17 at 10:21

Prim

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I changed my answer from string to double right now. The problem still exists. – Timppa May 16 '17 at 10:37
@Timppa Please update your post with your current code and error – Prim May 16 '17 at 10:40
double answer; second = Double.parseDouble(display.getText()); if (operation.equals("+")) { answer = first + second; result = String.format("%.0f", answer); ruutu.setText(String.valueOf(result)); <- This is the most important part of it, rest of the code are basically the same. And error: String cannot be converted to double for the result = String.format(); line – Timppa May 16 '17 at 10:55
Where is the declaration of result ? Are you sure is declared as String ? – Prim May 16 '17 at 11:38
Check the correct answer, I fixed it in there by removing one line. – Timppa May 16 '17 at 12:22
Yes, sure. So probably i had right : your variable `result` was not declared as a String, and you tried to store a String on it. Impossible for us to confirm, the declaration of result doesn't appear on your post ;) – Prim May 16 '17 at 12:26

score 1 · Answer 3 · answered May 16 '17 at 10:25

1

If result is of double data type

double result = first + second;

display.setText(String.valueOf(result));

answered May 16 '17 at 10:25

Cork Kochi

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Yeah, that fixed one of the problems but String.format is still popping an error message. – Timppa May 16 '17 at 10:39
http://stackoverflow.com/questions/703396/how-to-nicely-format-floating-numbers-to-string-without-unnecessary-decimal-0 – Cork Kochi May 16 '17 at 15:09

String cannot be converted to double

3 Answers3