Java generic code builds in eclipse, but not in mvn (same VM version)

Question

Can anyone help me to find what's wrong with this piece of code run in jdk1.6.0_45?

public class App {
    public static void main(final String args[]) {
        SortComparator<String, Integer> a = build();
    }

    private static <T, O extends Comparable<O>> SortComparator<T, O> build() {
        return null;
    }

    public class SortComparator<T, O extends Comparable<O>> implement Comparator<T> {
        @Override
        public int compare(final T o1, final T o2) {
            return 1;
        }
    }
}

This is a pure Java language question. Why it does not compile? Why it does in Eclipse?

If I change SortComparator<String, Integer> to Map<String, Integer> it compiles.
If I add the parameter Class< O> arg as an argument of build it compiles.
If I add App. build() to the call it compiles.

Console output:

incompatible types; no instance(s) of type variable(s) T,O exist so that build<T,O> conforms to SortComparator<java.lang.String,java.lang.Integer>

Versions

Apache Maven 3.0.5 (r01de14724cdef164cd33c7c8c2fe155faf9602da; 2013-02-19 14:51:28+0100)
Java version: 1.6.0_45, vendor: Sun Microsystems Inc.
Default locale: es_ES, platform encoding: Cp1252
OS name: "windows 7", version: "6.1", arch: "x86", family: "windows"

The console output is: incompatible types; no instance(s) of type variable(s) T,O exist so that build conforms to SortComparator — Jordi P.S., Dec 18 '13 at 16:18
Note that Eclipse uses its own compiler, which might explain the difference in behaviour. — Oliver Charlesworth, Dec 18 '13 at 16:19
possible duplicate of [Generics compiles and runs in Eclipse, but doesn't compile in javac](http://stackoverflow.com/questions/2858799/generics-compiles-and-runs-in-eclipse-but-doesnt-compile-in-javac) — Oliver Charlesworth, Dec 18 '13 at 16:21
Your example is incomplete, this code compiles well in Java 1.6, I just added dummy implementation of Comparator#compare method in SortComparator — hoaz, Dec 18 '13 at 16:26
@hoaz I have updated the code, I removed the parameter, now it fails (sorry for that) — Jordi P.S., Dec 18 '13 at 16:43

score 0 · Accepted Answer · answered Dec 18 '13 at 16:34

0

Because the return type of SortComparator is not a tuple (or Map.Entry).

// Only takes one argument...
private <O extends Comparable<O>> SortComparator<O> build(final Class<O> clazz) {
  return null;
}

answered Dec 18 '13 at 16:34

Elliott Frisch

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score 0 · Answer 2 · answered Dec 18 '13 at 16:52

I tried following example and it compile fine in both Eclipse and from command line:

public class Test {

    public static class SortComparator<T, O extends Comparable<O>> implements Comparator<T> {
        @Override
        public int compare(T o1, T o2) {
            return 0;
        }
    }

    private static <T, O extends Comparable<O>> SortComparator<T, O> build(
            final Class<O> clazz) {
        // you can return null here too
        return new SortComparator<T, O>();
    }

    public static void main(String[] args) {
        SortComparator<String, Integer> a = build(Integer.class);
    }

}

I think you have static import of `build` method in your class and it makes your App.build() invisible — hoaz, Dec 18 '13 at 18:21

Java generic code builds in eclipse, but not in mvn (same VM version)

2 Answers2