SampleFunction.java

interface Function<T, R> {
  R apply(T value); 
}

// a function that returns the same type with the input type
Function<Integer, Integer> square = (num) -> {
  return num * num;
}
System.out.println(square.apply(3));  // prints out 9

// a function that converts into another type
Function<Integer, Boolean> mapper = (num) -> {
  return num > 0;
}

// a function that returns nothing but uses the given input
Function<String, Void> printer = (value) -> {
  System.out.println(value);
  return null;
}

// a function that has no input but returns something
Function<Void, Integer> randomInt = (aVoid) -> {
  return new Random().nextInt(); 
}