Why can method reference use non-final variables?

by

A method reference is not a lambda expression, although they can be used in the same way. I think that is what is causing the confusion. Below is a simplification of how Java works, it is not how it really works, but it is close enough.

Say we have a lambda expression:

Runnable f = () -> one.bar();

This is the equivalent of an anonymous class that implements Runnable:

Runnable f = new Runnable() {
    public void run() {
       one.bar();
    }
}

Here the same rules apply as for an anonymous class (or method local class). This means that one needs to effectively final for it to work.

On the other hand the method handle:

Runnable f = one::bar;

Is more like:

Runnable f = new MethodHandle(one, one.getClass().getMethod("bar"));

With MethodHandle being:

public class MethodHandle implements Runnable {
    private final Object object;
    private final Method method;

    public MethodHandle(Object object, java.lang.reflect.Method method) {
        this.object = Object;
        this.method = method;
    }

    @Override
    public void run() {
        method.invoke(object);
    }
}

In this case, the object assigned to one is assigned as part of the method handle created, so one itself doesn’t need to be effectively final for this to work.

More Related Contents:

  1. Instance Method Reference and Lambda Parameters
  2. Comparator.reversed() does not compile using lambda
  3. What is the equivalent lambda expression for System.out::println
  4. Java 8: Difference between method reference Bound Receiver and UnBound Receiver
  5. java.lang.NullPointerException is thrown using a method-reference but not a lambda expression
  6. Java 8 chained method reference?
  7. Big execution time difference between java Lambda vs Anonymous class
  8. Difference between method reference Bound Receiver and Unbound Receiver
  9. How will Java lambda functions be compiled?
  10. Using Java 8’s Optional with Stream::flatMap
  11. Is there a way to compare lambdas?
  12. Java 8 Streams: multiple filters vs. complex condition
  13. Lambda ‘special void-compatibility rule’ – statement expression
  14. Optional orElse Optional in Java
  15. Modifying local variable from inside lambda
  16. Why doesn’t java.util.Collection implement the new Stream interface?
  17. What does “an Arbitrary Object of a Particular Type” mean in java 8?
  18. Java 8 lambda expression and first-class values
  19. Comparator.comparing(…) of a nested field
  20. Variable is already defined in method lambda
  21. Java 8 Lambda Expression Within REST Service not working
  22. Unable to deserialize lambda
  23. Can a java lambda have more than 1 parameter?
  24. Annotating the functional interface of a Lambda Expression
  25. Java Lambda expressions [closed]
  26. Are defaults in JDK 8 a form of multiple inheritance in Java?
  27. Using method reference instead of multi argument lambda
  28. Cannot use Java 8 method with lambda arguments without specifying type arguments
  29. Creating String representation of lambda expression [duplicate]
  30. Reference to methods with different parameters in Java8

Leave a Comment