Wednesday, May 16, 2007

Java Goofery

Let's call this post "Auto-boxing and Silent Type Coercion Considered Harmful".

Two good Java questions courtesy of a good friend:

#1.

java.util.Set<Short> set = new java.util.Set<Short>();

for ( short i = 0; i <= 100; i++ ) {
set.add( i );
set.remove( i - 1 );
}

System.out.println( set.size() );


What is the output?

The answer is 100. The reason is that i - 1 gets cast to a java.lang.Integer, and all the calls to the created Shorts' .equals() methods will return false when passed these Integers, so nothing gets removed from the Set . Fun!


#2.

Number n = args.length == 0 ? new Integer( 3 ) : new Float( 2.0f );
System.out.println( n );


What is the output?

This one bothers me even more. The answer is 3.0. Java 5+ decided that the entire ternary, despite that it only needs to return a java.lang.Number for the assignment, should actually return a java.lang.Float since it could return an Integer or Float and you can shove an Integer into a Float without losing precision but not vice-versa. This is documented but ... yick! gross!

Update: I'm told proper attribution for these examples goes to Neal Gafter and William Pugh. Also, my statement that "you can shove an Integer into a Float without losing precision" isn't accurate. Some of the float's bits go to the mantissa (exponent), so you can technically lose some precision. Going the other way (float -> int) you always lose precision though.

1 comment:

Almann said...

Technically, converting float to int does not always lose precision, if the floating point number is exactly representable as a 32-bit signed integer (e.g. 3.0f converts to 3 without loss of precision).

What can be also slightly surprising is that Java has implicit conversion from int to float which is lossy (e.g. float f = 268435455; yields 268435456.0f).