What is a functional interface? Features of functional interfaces

Functional Interface is a new concept introduced in java 8. Functional interfaces are particularly used for Lambda Expressions. Following are the characters of function interfaces.

1. A functional interface can have only one abstract method except those methods declared in the Object class. For example the Runable interface has only one method run(), therefore it is a functional interface. Following is another example.

     public interface Qualified {
            public Object getTheObject();
            public String toString();
            public boolean equals(Object obj);
      }
Since the toString and equals methods are methods declared in the Object class, this Qualified interface is also a functional interface.

2. A functional interface can have static and default methods.

      public interface GetIt {
            public Object getIt();

            public static boolean checkIt(Object obj) {
                  if (obj instanceof String) return true;
                  return false;
            }

            public default Integer getSum(Integer one, Integer two) {
                  return one+two;
            }
      }
The GetIt interface has only one abstract method, getIt, so it is a functional interface.

3. The abstract method of a functional interface can be inherited from another interface or overrides a method in another interface.

       //interface Face1 has two abstract methods
       interface Face1 {
          public boolean isTrue();
          public void changeIt();
      }

      //interface Face2 has one abstract method, changeIt, inherited from face1
      //therefore, interface Face2 is a functional interface
      interface Face2 extends Face1 {
          @Override
          default boolean isTrue() {return true;}
      }

      //interface Face3 has one abstract method that overrides the method in its super interface
      //therefore, interface Face3 is a functional interface
      interface Face3 extends Face2 {
            @Override
            public void changeIt();
      }

      //Since Face2 and Face3 are functional interfaces, they can be implemented with Lambda Expression
      public class TestDefautMethod {
            public void testDefault(Face2 theFace2, Face3 theFace3){
                  theFace2.changeIt();
                  theFace3.changeIt();
            }
 
            public static void main(String[] args){
                 TestDefautMethod tester = new TestDefautMethod();
                 tester.testDefault(()->System.out.println("Changed it successfully in Face2!"),
                                           ()->System.out.println("Changed it successfully in Face3!"));
           }
      }
The output of executing TestDefautMethod is the following.
      Changed it successfully in Face2!
      Changed it successfully in Face3!

4. In java 8, a functional interface is normally marked with the annotation @FunctionalInterface. However, as long as an interface has only one abstract method, it is a qualified functional interface regardless whether it has the annotation or not.

However, if an interface has the @FunctionalInterface annotation and contains more than one abstract method, the compiler will identify it as an error.

       @FunctionalInterface
       interface Face4 extends Face3 {
              public Integer sumIt(Integer one, Integer two);
       }
Face4 has two abstract methods, changeIt inherited from super interface and sumIt declared in itself. Compiling this code generates the following error.

error: Unexpected @FunctionalInterface annotation
@FunctionalInterface
  Face4 is not a functional interface
    multiple non-overriding abstract methods found in interface Face4

References:

1. Default Methods

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