FileFilter Java 8的方法参考

Question

I have the following code for the FileFilter :

final FileFilter directoryFilter = new FileFilter()
    @Override
    public boolean accept(final File pathname)
    {
        return pathname.isDirectory();
    }
};

I want to write this with a method reference. This was my first try:

final File test;
final FileFilter directoryFilter = test::isDirectory;

This gives me an error:

incompatible Types: invalid method reference.

It works if I try this with an lambda expression like:

final FileFilter directoryFilter = pathname -> pathname.isDirectory()

How must I change my code to get the method reference to work?

Answer 1

You need to use

final FileFilter directoryFilter = File::isDirectory;

This method reference is exactly the same as the lambda expression pathname -> pathname.isDirectory() that you wrote.

Method references with the syntax ContainingType::methodName are used to reference to an instance method of an arbitrary object of the ContainingType type.

The method reference test::isDirectory would actually refer to the instance method isDirectory on the specific test instance (not for an arbitrary File instance).