javac compilation error when passing method reference to method accepting interface.

Tomasz Kowalczewski tomasz.kowalczewski at
Mon Nov 3 11:15:30 UTC 2014


please excuse my ignorance if this is a known bug, was discussed here or is
plain silly.

I have a curious problem when calling a method with lambda or method
reference as an argument. This happens on several versions of Java 8
including latest (u25). Test case is:

package test;

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.Callable;

public class CompilationProblem {

    public void shouldCompile() {
        toMap(() -> new LinkedHashMap()); // Compilation error
        toMap(LinkedHashMap::new); // Compilation error

    public <K, V> Map<K, V> toMap(Func0<? extends Map<K, V>> mapFactory) {
        return null;

    interface Func0<R> extends Callable<R> {

        public R call();

The errors are:

Error:(10, 17) java: method toMap in class
pl.codewise.voluum.server.db.CompilationProblem cannot be applied to given
  required: pl.codewise.voluum.server.db.CompilationProblem.Func0<? extends
  found: ()->new Li[...]Map()
  reason: cannot infer type-variable(s) K,V
    (argument mismatch; bad return type in lambda expression
      java.util.LinkedHashMap cannot be converted to ? extends

Error:(11, 22) java: incompatible types: no instance(s) of type variable(s)
K,V exist so that java.util.LinkedHashMap<K,V> conforms to ? extends

This test case compiles if Func0 does not extend Callable. How does
Callable influence type inference here? What does it change?

Tomasz Kowalczewski
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