function.java

@FunctionalInterface
public interface Function<T, R> {
  
  R apply(T t);
  
  // default method
  
  default <V> Function<V, R> compose(Function<? super V, ? extends T> before);
  
  default <V> Function<V, T> andThen(Function<? super R, ? extends V> after);
  
  // static method
  
  static <T> Function<T, T> identity() {
    return t -> t;
  }
  
}