Lambda conversion of method reference

Question

If I have:

class B
{
    public static boolean test1(File f)
    {
        return true;
    }

    public boolean test2(File f)
    {
        return true;
    }    
}

are the following conversions to a full lambda expressions correct?

File dir = new File("C:\\TEST");

// here UNBOUND instance method reference
// converted as?  dir.listFiles((File f) -> f.isFile());
dir.listFiles(File::isFile);        

// here static method reference
// converted as? dir.listFiles((File f) -> B.test1(f));
dir.listFiles(B::test1);  

// here BOUND instance method reference
// converted as?  dir.listFiles((File f) -> b.test2(f));
B b = new B();
dir.listFiles(b::test2);

then, still, one question: if I write: dir.listFiles(B::test2); I have from the compiler:

non-static method test2(File) cannot be referenced from a static context

but why is that error not throws for: dir.listFiles(File::isFile);

I think you've got confused: B::test2 must be an unbound instance method reference to a method in the class B, so the lambda argument must be a B not a File. So you should have got a different compile error telling you that the argument is of the wrong type. — Maurice Naftalin
– Maurice Naftalin, Commented Jan 15, 2014 at 17:34
@Naftalin, thanks, I see the problem!!! However, are the conversions I wrote correct? What do you think about? — xdevel2000
– xdevel2000, Commented Jan 15, 2014 at 18:57

Toby · Accepted Answer · 2014-03-19 19:47:42Z

Short answer: Yes, your conversions are correct.

public void question() {
    File dir = new File("C:\\TEST");

    // here UNBOUND instance method reference
    dir.listFiles((File f) -> f.isFile());
    dir.listFiles(File::isFile);

    // here static method reference
    dir.listFiles((File f) -> B.test1(f));
    dir.listFiles(B::test1);

    // here BOUND instance method reference
    B b = new B();
    dir.listFiles((File f) -> b.test2(f));
    dir.listFiles(b::test2);
}

The compilation problem is because the reference;

dir.listFiles(B::test2);

or (expanded as a lambda);

dir.listFiles((pathname) -> B.test2(pathname));

is trying to access test2 as a static method. It's a static method reference. Whereas

dir.listFiles(File::isFile);

or as a lambda;

dir.listFiles((f) -> f.isFile())

is an instance method reference, it's referring to a (non-static) method on the File class.

So although the File::isFile looks like a static reference, the use of the type in this context is actually referring to an object! Weird huh?

Maurice Naftalin · Accepted Answer · 2014-03-04 02:05:26Z

1

I think you've got confused: B::test2 must be an unbound instance method reference to a method in the class B, so the lambda argument must be a B not a File. So you should have got a different compile error telling you that the argument is of the wrong type.

answered Mar 4, 2014 at 2:05

Maurice Naftalin

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