Why does this generic code produce a NoSuchMethodError?

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).

Answer 2

if (threshold.compareTo(input) < 0) {

will work.

It certainly appears to be a compiler error. The reason this works is that the compiler generates a checked cast to Comparable. Whereas the other way around it does not.

ensureLessThan(Ljava/lang/Number;Ljava/lang/Number;)Ljava/lang/Number;
  L0
        LINENUMBER 11 L0
        ALOAD 1
        CHECKCAST java/lang/Comparable
        ALOAD 2
        INVOKEINTERFACE java/lang/Comparable.compareTo (Ljava/lang/Object;)I
        IFGE L1

versus

ensureLessThan(Ljava/lang/Number;Ljava/lang/Number;)Ljava/lang/Number;
        LINENUMBER 11 L0
        ALOAD 1
        ALOAD 2
        INVOKEVIRTUAL java/lang/Number.compareTo (Ljava/lang/Object;)I
        IFGE L1

Answer 3

That's strange, the following code compiles and runs from my side:

/**
 * @author Buhake Sindi
 * @since 14 January 2013
 *
 */
public class Test {

    public static <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;
    }

    public static void main(String[] args) {
        ensureLessThan(10, 5);
    }
}

Tested in Eclipse Juno (Java EE), with the following JDK:

1. JDK 1.6.0_21.
2. JDK 1.7.0_09-b05