Generic method to perform a map-reduce operation. (Java-8)

Question

How to overload a Function with generic parameter in Java 8?

public class Test {

    List list = new ArrayList<>();

    public int          


        

Answer 1

@srborlongan 's solution won't work very well :)

See a similar example - Comparator methods - comparingDouble(ToDoubleFunction), comparingInt(ToIntFunction), etc. The methods have different names, because overloading is not a good idea here.

The reason is, when you do sum(t->{...}), the compiler is unable to infer which method to call; actually it needs to resolve method overloading first, to pick one method, before inferring the type of the implicit lambda expression (based on that method's signature)

This is disappointing. In the earlier stage, Java8 had a more sophisticated inference engine, and Comparator had overloaded comparing() methods; and sum(t->{...}) would be correctly inferred too. Unfortunately, they decided to simply it :( And here we are now.

Rule of thumb for overloading methods with functional arguments: the arities of the functional interfaces must be different, unless both are 0.

// OK, different arity
m1( X->Y )
m1( (X1, X2)->Y )

// not OK, both are arity 1
m2( X->Y )
m2( A->B )

    m2( t->{...} ); // fail; type of `t` cannot be inferred 

// OK! both are arity 0
m3( ()->Y )
m3( ()->B )

The reason why overloading with arity 0 is OK is that the lambda expressions won't be implicit - all argument types are known (because there's no argument!), we don't need contextual information for inferring the lambda type

m3( ()-> return new Y() );   // lambda type is ()->Y
m3( ()-> return new B() );   // lambda type is ()->B