CODEWITHSUNDEEP
javagenericslambdatype-inferencemethod-reference
Jin Kwon
Jin Kwon

Reputation: 21978

Why method reference doesn't work with this?

I have a method look like this.

static <R> R applyOthers(Some some, Function<List<Other>, R> function) {
    List<Other> others = ...;
    return function.appply(others);
}

Now when I try this,

withSome(some -> {
   List<Other> others1 = applyOthers(some, v -> v); // works
   List<Other> others2 = applyOthers(some, Function::identity); // doesn't
});

An error, I get.

incompatible types: unexpected static method <T>identity() found in unbound lookup
  where T is a type variable:
    T extends Object declared in method <T>identity()

Why ::identity doesn't work?

Upvotes: 1

Views: 849

Answers (2)

Jitendra
Jitendra

Reputation: 109

shouldn't we say that Function::identity is supplier of Function like -

Supplier<Function<?,?>> a = Function::identity;

and that's why its not working ? If we replace '?' with correct type a.get() works.

Upvotes: 1

sanit
sanit

Reputation: 1764

Function<Object, Object> f1 = v1 -> v1;
Supplier<Object> s1 = Function::identity;
Supplier<Object> s2 = () -> Function.identity();

See this piece of code, i guess you just misused the method reference here. When we say SomeObject::method, this method reference should match with some functional interface. In the above example, Function::identity is kind of supplier instance not java.util.function.Function instance.

Upvotes: 2

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