Functional interface

Description

A functional interface is an interface with one method and used as the type of a lambda expression.

public interface ActionListener extends EventListener {
    public void actionPerformed(ActionEvent event);
}

ActionListener has only one method, actionPerformed. It is a functional interface. It doesn't matter what the single method is called, the Java compiler will match it up to your lambda expression as long as it has a compatible method signature.

A lambda expression represents an instance of a functional interface.

The type of a lambda expression is a functional interface type.

(String str) -> str.length() takes a String parameter and returns its length.

Its type can be any functional interface type with an abstract method that takes a String as a parameter and returns an int.

The following is an example of such a functional interface:

@FunctionalInterface
interface Processor  {
    int  getStringLength(String str);
}

We can assign lambda expression to its functional interface instance.

Example

In the following code we assign a lambda expression to its functional interface. Then we execute the lambda expression by calling the method defined in the functional interface and pass in a parameter.

public class Main {
  public static void main(String[] argv) {
    Processor stringProcessor = (String str) -> str.length();
    String name = "Java Lambda";
    int length = stringProcessor.getStringLength(name);
    System.out.println(length);/*from   ww w . j  a  v a 2  s.c  o m*/

  }
}

@FunctionalInterface
interface Processor {
  int getStringLength(String str);
}

The code above generates the following result.

Note

A lambda expression by itself cannot be used as a standalone expression.

The type of a lambda expression is inferred by the compiler.