Question

Why is the compiler unable to infer the correct type for the result from Collections.emptySet() in the following example?

import java.util.*; import java.io.*;  public class Test {     public interface Option<A> {         public <B> B option(B b, F<A,B> f);     }      public interface F<A,B> {         public B f(A a);     }      public Collection<String> getColl() {         Option<Integer> iopt = null;          return iopt.option(Collections.emptySet(), new F<Integer, Collection<String>>() {             public Collection<String> f(Integer i) {                 return Collections.singleton(i.toString());             }         });     } } 

Here’s the compiler error message:

knuttycombe@knuttycombe-ubuntu:~/tmp/java$ javac Test.java  Test.java:16: <B>option(B,Test.F<java.lang.Integer,B>) in  Test.Option<java.lang.Integer> cannot be applied to (java.util.Set<java.lang.Object>, <anonymous Test.F<java.lang.Integer,java.util.Collection<java.lang.String>>>)             return iopt.option(Collections.emptySet(), new F<Integer, Collection<String>>() {                    ^ 1 error 

Now, the following implementation of getColl() works, of course:

    public Collection<String> getColl() {         Option<Integer> iopt = null;          Collection<String> empty = Collections.emptySet();         return iopt.option(empty, new F<Integer, Collection<String>>() {             public Collection<String> f(Integer i) {                 return Collections.singleton(i.toString());             }         });     } 

and the whole intent of the typesafe methods on Collections is to avoid this sort of issue with the singleton collections (as opposed to using the static variables.) So is the compiler simply unable to perform inference across multiple levels of generics? What’s going on?

Answer 1

Java needs a lot of hand holding with its inference. The type system could infer better in a lot of cases but in your case the following will work:

print('Collections.<String>emptySet();');