Question

Say you have a method that

• takes a threshold and an input
• raises an exception if the input is less than the threshold
• otherwise returns the input

it would look something like this:

<N extends Number & Comparable<N>, S extends N> S ensureLessThan(N threshold, S input) {
    if (input.compareTo(threshold) >= 0) {
        throw new IllegalArgumentException("Input " + input + " is not less than " + threshold);
    }
    return input;
}

When run, this method throws a NoSuchMethodError:

Exception in thread "main" java.lang.NoSuchMethodError: java.lang.Number.compareTo(Ljava/lang/Object;)I

Adding what looks like a redundant cast makes it work:

...
    if (((N) input).compareTo(threshold) >= 0) {
...

So what’s going on here?

UPDATE: My Java version is

java version "1.6.0_37"
Java(TM) SE Runtime Environment (build 1.6.0_37-b06-434-11M3909)
Java HotSpot(TM) 64-Bit Server VM (build 20.12-b01-434, mixed mode)

And here’s a runnable example: https://gist.github.com/4526536

Answer 1

I suspect it’s a bug in the compiler. It has staticly bound to Number.compareTo(Object) which doesn’t exist, when it should be Comparable.compareTo(Object).