Java 8 Lambda: Two Interface with method have same number parameter

Question

I have two interfaces:

interface A {
    public void pull(SomeclassA a, SomeclassB b); 
}

interface B {
    public void make(SomeClassM m, SomeclassN n); 
}

In each interface I have a method with same number of parameters. Then I have a class with methods using both the above two interfaces, that is:

public class C {

    public void test(B b) {

    }

    public void test(A a) {

    }

}

I need to use the class with Java 8 lambda expression. How will Java recognize which interface is passed in to a method of this class?

public static void main(String[] args) {
    C c = new C(); 
    c.test(
        (a , b ) ->{

        }
    ); 
}

one of the methods gets selected implicitly, for the other one you should either change to inner class creation or try a cast to the proper interface. But you sould definitely change the method names so that they are unique. — Timothy Truckle
– Timothy Truckle, Commented Oct 19, 2016 at 11:11
@TimothyTruckle: is java 8 lambda expression created without awareness of overloading methods in java.... what if I created API that have overloading method? — Mohammad Fajar
– Mohammad Fajar, Commented Oct 19, 2016 at 11:25
looks like. but on the other hand you should not overload methods when you plan to use labda expressions. This significanly harms the readability of your code. — Timothy Truckle
– Timothy Truckle, Commented Oct 19, 2016 at 11:33
The intended solution is to redesign C. Overloading lambda accepting methods is a bad idea most of the time. — Louis Wasserman
– Louis Wasserman, Commented Oct 19, 2016 at 18:25

Eran · Accepted Answer · 2016-10-19 11:15:11Z

4

You would get a compilation error, since the compiler has no way of choosing between the two overloaded test methods.

To fix that you'll have to specify the types of the lambda arguments.

For example :

public static void main(String[] args) {
    C c = new C(); 
    c.test(
        (SomeClassM a , SomeClassN b) -> {

        }
    ); 
}

will cause test(B b) to be chosen.

answered Oct 19, 2016 at 11:15

Eran

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5 Comments

Mohammad Fajar Over a year ago

There is no compilation error if I created a class with overloading methods (same name and number parameter, but different in type). I think that is written in java 8 specification...

Eran Over a year ago

@MohammadFajar I tried your code and got a compilation error (I did use different class names instead of SomeClassX). The method test(B) is ambiguous for the type C.

user4910279 Over a year ago

Or it may be c.test((A)(a, b) ->{ });

Holger Over a year ago

@Mohammad Fajar: nobody said that you get a compilation error when compiling the class C with the overloads. You’ll get a compilation error at the place where you try to make the ambiguous call with an untyped lambda expression. But note that when you compile C using javac with -Xlint option (strongly recommended), you’ll get a warning: [overloads] test(B) in C is potentially ambiguous with test(A) in C

Stephan Herrmann Over a year ago

Explicitly typed lambda is good. As a general rule of thumb avoiding overloading is always better.

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