What is a Java Functional Interface

A functional interface is an interface with a single abstract method. Also know as SAM (Single Abstract Method) types.

Examples:

public interface Comparator<T> {

    int compare(T o1, T o2);

}

public interface Callable<V> {

    V call() throws Exception;

}

public interface ActionListener extends EventListener {

    public void actionPerformed(ActionEvent e);

}

public interface Runnable {

    public void run();

}