Why does this method reference assignment compile?

Question

I'm struggling to see why the following code compiles:

public class MethodRefs {

    public static void main(String[] args) {
        Function<MethodRefs, String> f;

        f = MethodRefs::getValueStatic;

        f = MethodRefs::getValue;
    }

    public static String getValueStatic(MethodRefs smt) {
        return smt.getValue();
    }

    public String getValue() {
        return "4";
    }

}

I can see why the first assignment is valid - getValueStatic obviously matches the specified Function type (it accepts a MethodRefs object and returns a String), but the second one baffles me - the getValue method accepts no arguments, so why is it still valid to assign it to f?

Possible duplicate of [lambda with non-static methods in Java 8](http://stackoverflow.com/questions/25784337/lambda-with-non-static-methods-in-java-8) — Didier L, Apr 25 '17 at 20:16
Also [:: (double colon) operator in Java 8](http://stackoverflow.com/questions/20001427/double-colon-operator-in-java-8/22245383#22245383) and [Use of double colons - difference between static and non-static method references](http://stackoverflow.com/questions/34982972/use-of-double-colons-difference-between-static-and-non-static-method-reference?noredirect=1&lq=1) — Didier L, Apr 25 '17 at 20:20

Peter Lawrey · Accepted Answer · 2017-04-22T07:19:42.857

50

The second one

f = MethodRefs::getValue;

is the same as

f = (MethodRefs m) -> m.getValue();

For non-static methods there is always an implicit argument which is represented as this in the callee.

NOTE: The implementation is slightly different at the byte code level but it does the same thing.

edited Apr 22 '17 at 07:19

answered Apr 20 '17 at 15:10

Peter Lawrey

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Note that the implicit argument can be made explicit (as of Java 8): `public String getValue(MethodRefs this)` is legal and identical to `public String getValue()`. – Pokechu22 Apr 20 '17 at 17:34
@Pokechu22 I didn't know that. Is there a name for that construct? – user253751 Apr 21 '17 at 01:09
5

@immibis: [the spec](http://docs.oracle.com/javase/specs/jls/se8/html/jls-8.html#jls-8.4.1) (§8.4.1) calls it a "receiver parameter" as far as I can tell. – Pokechu22 Apr 21 '17 at 02:51

Wyzard · Answer 2 · 2017-04-20T15:21:39.030

9

A non-static method essentially takes its this reference as a special kind of argument. Normally that argument is written in a special way (before the method name, instead of within the parentheses after it), but the concept is the same. The getValue method takes a MethodRefs object (its this) and returns a string, so it's compatible with the Function<MethodRefs, String> interface.

edited Apr 20 '17 at 15:21

answered Apr 20 '17 at 15:11

Wyzard

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score 9 · Answer 3 · answered Apr 20 '17 at 15:12

Lets flesh it out a bit:

import java.util.function.Function;

public class MethodRefs {

  public static void main(String[] args) {
    Function<MethodRefs, String> f;


    final MethodRefs ref = new MethodRefs();

    f = MethodRefs::getValueStatic;
    f.apply(ref);
    //is equivalent to 
    MethodRefs.getValueStatic(ref);

    f = MethodRefs::getValue;
    f.apply(ref);
    //is now equivalent to 
    ref.getValue();
  }

  public static String getValueStatic(MethodRefs smt) {
    return smt.getValue();
  }

  public String getValue() {
    return "4";
  }
}

score 6 · Answer 4 · answered Apr 20 '17 at 16:09

In the Java Tutorial it is explained that there are 4 different types of method references:

reference to a static method
reference to an instance method of a particular object
reference to an instance method of an arbitrary object of a particular type
reference to a constructor

Your case is #3, meaning that when you have an instance of MethodRef i.e. ref, calling apply on your function f will be equivalent to String s = ref.getValue().

Carcigenicate · Answer 5 · 2017-04-25T21:20:41.300

5

For non-static methods, the type of this is considered implicitly to be the first argument type. Since it's of type MethodRefs, the types check out.

edited Apr 25 '17 at 21:20

answered Apr 20 '17 at 15:10

Carcigenicate

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Why does this method reference assignment compile?

5 Answers5