Decimal format in Java?

Question

can someone explain to me why the output for this:

    double y = 15/7;
    DecimalFormat first = new DecimalFormat("#.###");
    System.out.println(y); 
    String format_string = first.format(y);
    System.out.println(format_string);

Is this:

   2.0
   2

(Which is wrong)

However, when I change 15/7 to

 15.0/7.0

It gives me the correct answer

 2.142857142857143
 2.143

Explanation please?

Thank you!

Answer 1

Numbers without a dot are Integers so 15/7 is an integer-operation and the result is 2 (divistion without remainder). Afterwards it gets converted to a double but keaps it's value of 2 (conversion after finishing the operation).

Numbers with dots are doubles in the first place so 15.0/7.0 is a double-operation and leads to the result you want to have (floating point division).

Answer 2

Replace

double y = 15/7;

with

double y = 15.0/7;

When dividing two int, you get an int.

See specification :

Integer division rounds toward 0. That is, the quotient produced for operands n and d that are integers after binary numeric promotion (§5.6.2) is an integer value q whose magnitude is as large as possible while satisfying |d · q| ≤ |n|. Moreover, q is positive when |n| ≥ |d| and n and d have the same sign, but q is negative when |n| ≥ |d| and n and d have opposite signs.

You convert it to a double by storing it in a double variable but that's too late : you can't get the lost precision back. The solution is to have one of the operands being a double so that you get a double when dividing.

Answer 3

Division of two integers always results in integer - that's why you get 2 (2.0 after formatting).

Division of at least one double gives you double

In your case 15/7 is a division of two integers and the result is an integer (incorrect result). If you change to 15/7.0 or 15.0/7 or 15.0/7.0 your result will be double (correct result)