Java 8 method reference usage example

Question

I am going through an example which pulls an Array of hidden files from current directory related to method reference which is as mentioned below

• using Anonymous inner class implementation

    File[] hiddenFiles = new File(".").listFiles(new FileFilter() {
      public boolean accept(File file) {
        return file.isHidden();
      }
    });

• using Method reference implementation

    File[] hiddenFiles = new File(".").listFiles(File::isHidden);

My question is FileFilter interface has only one abstract method ( boolean accept(File pathname) ) while implementing accept method using method reference how it is valid to using boolean isHidden() in File class which has no parameters. I learnt that we can apply method reference only when parameters match with abstract method but here accept method has a parameter of type File but isHidden has no parameters. Could you please explain how it is valid.

Answer 1

It's Lambda expression + method reference. What you mentioned about accept method is about Lambda expression, and what you mentioned about File::isHidden is method reference.

They are 2 different things.

• https://docs.oracle.com/javase/tutorial/java/javaOO/methodreferences.html
• https://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html

Your original one:

File[] hiddenFiles = new File(".").listFiles(new FileFilter() {
      public boolean accept(File file) {
        return file.isHidden();
      }
    });

Can be turned into: (Lamda expression)

File[] hiddenFiles = new File(".").listFiles(file ->  file.isHidden());

Then it can be turned into: (method reference)

File[] hiddenFiles = new File(".").listFiles(File::isHidden);