Java Functional Interfaces

 The term Java functional interface was introduced in Java 8. A functional interface in Java is an interface that contains only a single abstract (unimplemented) method. A functional interface can contain default and static methods which do have an implementation, in addition to the single unimplemented method.

Here is a Java functional interface example:

public interface MyFunctionalInterface {
    public void execute();
}

The above counts as a functional interface in Java because it only contains a single method, and that method has no implementation. Normally a Java interface does not contain implementations of the methods it declares, but it can contain implementations in default methods, or in static methods. Below is another example of a Java functional interface, with implementations of some of the methods:

public interface MyFunctionalInterface2{
    public void execute();

    public default void print(String text) {
        System.out.println(text);
    }

    public static void print(String text, PrintWriter writer) throws IOException {
        writer.write(text);
    }
}

The above interface still counts as a functional interface in Java, since it only contains a single non-implemented method.

Functional Interfaces Can Be Implemented by a Lambda Expression

A Java functional interface can be implemented by a Java Lambda Expression. Here is an example that implements the functional interface MyFunctionalInterface defined in the beginning of this Java functional interface tutorial:

MyFunctionalInterface lambda = () -> {
    System.out.println("Executing...");
}

A Java lambda expression implements a single method from a Java interface. In order to know what method the lambda expression implements, the interface can only contain a single unimplemented method. In other words, the interface must be a Java functional interface.