Creating a functional interface

In this recipe, you will learn how to create and use a custom functional interface when none of the standard interfaces in the java.util.function package satisfies the requirements.

Getting ready

Creating a functional interface is easy. One just has to make sure there is only one abstract method in the interface, including methods inherited from other interfaces:

@FunctionalInterface
interface A{
    void m1();
}

@FunctionalInterface
interface B extends A{
    default void m2(){};
}

//@FunctionalInterface
interface C extends B{
    void m3();
}

In the preceding example, interface C is not a functional interface because it has two abstract methods—m1(), inherited from interface A, and its own method...