public interface Fcn<T, R> { 
    public R x(T t); 
} 

abstract class Executioner<R, T> { 
    public abstract R calls(T arglist); 
} 

public class Nil { 
    private Nil( ) { }
    public final static Nil INSTANCE = new Nil();
}   

Example 2: (a) Fcn interface; (b) abstract class Executioner; (c) class Nil.