Java - ternary operator weird behaviour

Question

I was trying to remove the fractional part from a double in case it is whole using:

(d % 1) == 0 ? d.intValue() : d 

And encountered the following behavior which i don't understand:

public static void main(String[] args) {     Double d = 5D;     System.out.println((d % 1) == 0);                               // true     System.out.println((d % 1) == 0 ? d.intValue() : "not whole");  // 5     System.out.println((d % 1) == 0 ? d.intValue() : d);            // 5.0 } 

As you can see on the third line, the operator chooses the else value - 5.0 even though the condition (d % 1) == 0 is met.

What's going on here?

Answer 1

The return type of the ternary conditional operator must be such that both the 2nd and 3rd operands can be assigned to it.

Therefore, in your second case, the return type of the operator is Object (since both d.intValue() and "not whole" must be assignable to it) while in the third case it is Double (since both d.intValue() and d must be assignable to it).

Printing an Object whose runtime type is Integer gives you 5 while printing a Double gives you 5.0.

Answer 2

The type of an expression a ? b : c is always the same as c or the closest common parent of b and c.

System.out.println((d % 1) == 0 ? d.intValue() : "not whole");  // Comparable a parent of Integer and String System.out.println((d % 1) == 0 ? d.intValue() : d);            // Double is a widened int 

BTW d % 1 will only check it is a whole not, not that it's small enough to fit in anint value. A safer check is to see if the value is the same when cast to an int or long

double d = 5.0; if ((long) d == d)     System.out.println((long) d); else     System.out.println(d); 

or you can prevent it widening the long back to a double with

double d = 5.0; System.out.println((long) d == d ? Long.toString((long) d) : Double.toString(d));