Function.java

public interface Function<T, R> {
  R apply(T t);

  static Function<?, String> toStringFunction() {
    return value -> value.toString();
  }

  static <E> Function<E, E> identity() {
    return value -> value;
  }

  static <R> Function<?, R> constant(R constantValue) {
    return value -> constantValue;
  }
}