Why does my java Code output a wrong number?

Question

I´m trying out eclipse for the first time and I´m trying to programm an Calculator with a numpad. Currently i´m programming the comma function but my code doesnt work.

Here is my code for the decimal places:

System.out.println(first+"");
System.out.println(dec_length(first));
first=((first*Math.pow(10, dec_length(first)))+1) / Math.pow(10, dec_length(first));
textfield.setText(""+format.format(first));

The method dec_length() is:

public static int dec_length(double input) {
    String param=(""+input);
    String vergleich =(".0");
    String sub= param.substring((param.indexOf('.')));
    if (sub.equals(vergleich)) {
        return 1;
    } else
        return sub.length();
}

When I try it it works for the first 2 decimal places but then it stops working.

Here is the console output:

5.0
1
5.1
2
5.109999999999999
16
5.109999999999999
16
5.109999999999999
16

Does anybody know whats wrong with this?

Answer 1

The problem is that you assume that all decimal places can be represented exactly by the double type. But that is not the case.

I suggest you store the number as a string until you use it for calculation:

String first = "0";

And then either add a digit or a comma(you should of course make sure there is only one . in your number):

first += digit; // Add a digit from 0 to 9

first += '.'; // Call this when the comma function is called.

And afterwards, when you want to do the calculation part you can easily parse the string to a double:

double value = Double.parseDouble(first);

Answer 2

I´m trying out eclipse for the first time and I´m trying to programm an Calculator with a numpad. Currently i´m programming the comma function but my code doesnt work.

Here is my code for the decimal places:

System.out.println(first+"");
System.out.println(dec_length(first));
first=((first*Math.pow(10, dec_length(first)))+1) / Math.pow(10, dec_length(first));
textfield.setText(""+format.format(first));

The method dec_length() is:

public static int dec_length(double input) {
    String param=(""+input);
    String vergleich =(".0");
    String sub= param.substring((param.indexOf('.')));
    if (sub.equals(vergleich)) {
        return 1;
    } else
        return sub.length();
}

When I try it it works for the first 2 decimal places but then it stops working.

Here is the console output:

5.0
1
5.1
2
5.109999999999999
16
5.109999999999999
16
5.109999999999999
16

Does anybody know whats wrong with this?