Creating a method reference on a null-reference does not throw an exception

Question

Is there a reason why it is possible to create method references on a null reference in Java? Doing this is probably never correct but can result in errors which are hard to find later:

public class Test {
    void m() {
    }

    public static void main(String[] args) {
        Test test = null;
        Runnable fn = test::m; // no exception
        System.out.println(fn); // prints Test$$Lambda$1/791452441@1c20c684
        fn.run(); // throws a null pointer exception
    }
}

Are you testing with Eclipse? I tested with JDK 1.8.0_51 and it throws the NPE where expected. This probably answers the question then stackoverflow.com/questions/37413106/…. Because it should throw an NPE at Runnable fn = test::m. — Tunaki
– Tunaki, Commented Jun 7, 2016 at 14:12
@Michael Yes, it seems there is a bug in Eclipse regarding this (same as the linked question). — Tunaki
– Tunaki, Commented Jun 7, 2016 at 14:23
It makes a kind of sense, since for this type of method reference, the left side of the :: is an expression that returns a reference. While this is more explicitly a null reference, this would have the same effect: functionThatReturnsNull()::method — Hank D
– Hank D, Commented Jun 7, 2016 at 14:38

4 revs · Accepted Answer · 2018-02-26 07:39:56Z

12

Is there a reason why it is possible to create method references on a null reference in Java?

It isn't, but apparently there's a bug in Eclipse in this regard (edit: which has since been fixed). According to the specification, and when you use the JDK's tools, it fails with an NPE on the Runnable fn = test::m; line.

Proof: http://ideone.com/APWXna (or compile and run it locally with javac and java rather than Eclipse)

Theory: From JLS §15.13.3:

First, if the method reference expression begins with an ExpressionName or a Primary, this subexpression is evaluated. If the subexpression evaluates to null, a NullPointerException is raised, and the method reference expression completes abruptly.

(My emphasis.)

edited Feb 26, 2018 at 7:39

community wiki

4 revs
T.J. Crowder

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3 Comments

Didier L Over a year ago

Was this bug reported to Eclipse?

T.J. Crowder Over a year ago

@DidierL: I've no idea, I don't use Eclipse.

Daniel Hári Over a year ago

As a workaround: Lambda, or static method reference does not throw in the evaluation phase, if you want to use nullable object's method anyway. [Runnable fn = () -> test.m()] or [Consumer<Test > c2 = Test ::m]

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