Strange things in java

Question

Found some strange things in java.

Code:

System.out.println(System.getProperty("java.version"));
System.out.println((true) ? (int)2.5 : 3.5);
System.out.println((true) ? (int)2.5 : 3);
System.out.println((true) ? (int)2.5 + "" : 3.5);

Result:

1.8.0_40
2.0
2
2

What is it? Why integer value returns only if value for false is not a double or if string value added to value for true? Why in second line rounding works by (int) cast, but double value returns yet? Is it a bug?

As far as the compiler is concerned, there is only one type for the whole `?:` expression, and it figures that out by finding the narrowest type in common between both sides. — Louis Wasserman, Oct 10 '16 at 20:30
(I confess I'm surprised that the last line compiles, though.) — Louis Wasserman, Oct 10 '16 at 20:32
Take note of [this answer](http://stackoverflow.com/a/25230247/1743880) in the linked question about the same behaviour, with reference to the specification. — Tunaki, Oct 10 '16 at 20:50

score 4 · Answer 1 · answered Oct 10 '16 at 20:34

https://docs.oracle.com/javase/specs/jls/se8/html/jls-15.html#jls-15.25 specifies all these rules, which are behaving exactly consistently with your observed output.

There is only one type for the entire ternary expression, and that's what gets returned and that's what System.out.println is being called on. If you look it up in the table in that specification, you'll find the types in the lines you've mentioned are going to be double, int, and Object respectively.

score 3 · Answer 2 · answered Oct 10 '16 at 20:32

3

In a schematic:

(true) ? (int)2.5 : 3.5

         int        double
           \        /
             double

The double literal 2.5 is downsampled to int 2, then promoted back to double 2.0 because that's the type of the conditional expression.

answered Oct 10 '16 at 20:32

Marko Topolnik

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score 2 · Answer 3 · answered Oct 10 '16 at 20:44

The ternary operator has a constant return type.

From the JLS, section 15.25:

Otherwise, binary numeric promotion (§5.6.2) is applied to the operand types, and the type of the conditional expression is the promoted type of the second and third operands.

This means that in (true) ? (int)2.5 : 3.5, 2.5 is converted to an int, (rounded down) then widened to a double.

In (true) ? (int)2.5 : 3, the int does not need to be widened to a double, because the other side is also an int.

Finally, in (true) ? (int)2.5 + "" : 3.5);, it can't be widened, because it's already been converted to a String.

Strange things in java

3 Answers3