Method Reference. Cannot make a static reference to the non-static method

Question

Can someone explain to me,
why passing a non-static method-reference to method File::isHidden is ok,
but passing method reference to a non-static method MyCass::mymethod - gives me a "Cannot make a static reference to the non-static method" ?

public static void main(String[] args) {
    File[] files = new File("C:").listFiles(File::isHidden); // OK
    test(MyCass::mymethod); // Cannot make a static reference to the non-static method
}

static interface FunctionalInterface{
    boolean function(String file);
}

class MyCass{
    boolean mymethod(String input){
        return true;
    }
}

// HELPER
public static void test(FunctionalInterface functionalInterface){}

Peter Walser · Accepted Answer · 2020-05-17T09:14:39.770

11

Method references to non-static methods require an instance to operate on.

In the case of the listFiles method, the argument is a FileFilter with accept(File file). As you operate on an instance (the argument), you can refer to its instance methods:

listFiles(File::isHidden)

which is shorthand for

listFiles(f -> f.isHidden())

Now why can't you use test(MyCass::mymethod)? Because you simply don't have an instance of MyCass to operate on.

You can however create an instance, and then pass a method reference to your instance method:

MyCass myCass = new MyCass(); // the instance
test(myCass::mymethod); // pass a non-static method reference

or

test(new MyCass()::mymethod);

Edit: MyCass would need to be declared static (static class MyCass) in order to be accessible from the main method.

edited May 17 '20 at 09:14

answered Sep 16 '15 at 22:31

Peter Walser

15,208
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2

You cannot instantiate `MyClass` from within a `static` method (such as `main`) because is is a _non-static_ inner class. You would have to declare the class as static. I am assuming the code in the original example is wrapped inside a class. – Marco Torchiano Sep 17 '15 at 10:06
@Peter As per the documentation, Method References is absolutely not a static call. It works on both static and non- static methods. If going by your explanation then who exactly creates the instance in the case of String class private Function f1 = String::toUpperCase; – shrishti May 16 '20 at 19:05

Marco Torchiano · Answer 2 · 2015-09-17T13:08:31.913

As peter-walser pointed out, since MyCass::mymethod is an instance method it requires an instance to be converted to a Function instance.

The static in front of your interface declaration just makes it a static interface, it does not turn each method into a static one.

A possible solution would be to declare the method inside the class as static:

class MyCass{
   static boolean mymethod(String input){
       return true;
   }
}

To understand better how it works, you can consider the code equivalente to the method reference MyCass::mymethod that is (assuming the above modified declaration of MyClass):

new FunctionalInterface{
  boolean function(String file){
    return MyClass.mymethod(file);
  }
}

Your original code would attempt to sort-of translate into:

new FunctionalInterface{
  boolean function(String file){
    return _missing_object_.mymethod(); # mymethod is not static
  }
}

Another possibility is using a BiFunction instead of your FunctionalInterface. In that case the first argument of apply would be the object and the second would be the argument to mymethod.

Your first example has to be `MyClass.mymethod(file)`. The OP's code would also be translated into that. That's where the "Cannot make a static reference ..." error comes from. There is no static reference in your second example. — a better oliver, Sep 17 '15 at 12:05

score 0 · Answer 3 · answered May 24 '19 at 16:21

0

Short answer:

You're trying to access a static method through the class.

test(MyCass::mymethod); // Cannot make a static reference to the non-static method

Is the same as

test(v -> MyCass.mymethod(v)); // static access

Solution

Make the method static

class MyCass {
  static boolean mymethod(String input) {
    return true;
  }
}

Or use an object as reference

public static void main(String[] args) {
  MyCass myCass = new MyCass();
  test(myCass::mymethod);
}

answered May 24 '19 at 16:21

AMagyar

5,108
3
13
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1

Incase of String class, even though toUppercase is non-static then why this does not give error Function f1 = String::toUpperCase; – shrishti May 16 '20 at 19:17

Method Reference. Cannot make a static reference to the non-static method

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