How is method reference syntax working in Java 8

Question

I am having some confusion of how method references work in Java 8. I wrote the following code segment for filtering hidden files in a folder. They are producing correct result. I am not understanding -> how method signature of listFiles method is working for option 2 of this code segment.

This is what I found in Java 8 documentation

1. File[] listFiles()
2. File[] listFiles(FileFilter filter)
3. File[] listFiles(FilenameFilter filter)

File[] hidden = f.listFiles((p)-> p.isHidden()); //Option 1 - function signature matching (based on my understanding)
for (int i = 0; i < hidden.length; i++) {
    System.out.println(hidden[i]);
}
System.out.println("================================");
File[] hidden1 = f.listFiles(File::isHidden); //Option 2 - how this method call is working
for (int i = 0; i < hidden1.length; i++) {
    System.out.println(hidden1[i]);
}

Answer 1

What is a method refernce

You can see a method reference as a lambda expression, that call an existing method.

Kinds of method references

There are four kinds of method references:

• Reference to a static method: ContainingClass::staticMethodName
• Reference to an instance method of a particular object: containingObject::instanceMethodName
• Reference to an instance method of an arbitrary object of a particular type: ContainingType::methodName
• Reference to a constructor: ClassName::new

How to understand it?

The following expression that lists all hidden files: f.listFiles(p -> p.isHidden());

This expression is composed by the instance method listFiles(FileFilter) and your lambda expression p -> p.isHidden() that is not a anonymous method but an existing instance method of the class File.

Note that FileFilter is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference. Hence, you can write your expression f.listFiles(File::isHidden);

Side notes

1. You don't need the parentheses surrounding the p. For a better readibility, I would suggest to replace (p) with simply a p. Hence, your lambda expression will become p-> p.isHidden().
2. Your for loop can be replaced by an enhanced for loop:

for (File value : hidden) {
   System.out.println(value);
}

Documentation:

Method reference

FileFilter