final Function<String, Integer> length = str -> str.length();
final var lengthOfFoo = length.apply("foo");

// not correct! But most functional libraries also supply those for convenience
final BiFunction<String, String, Integer> length2 = (str1, str2) -> str1.length() + str2.length();
final var lengthOfFooAndBar = length2.apply("foo", "bar");

// curried form of the above "convenient" form (compact)
final Function<String, Function<String, Integer> length2Curried = str1 -> str2 -> str1.length() + str2.length();
// the above function can also be written more verbosely as:
final Function<String, Function<String, Integer> length2Curried = str1 -> {
  // the return value is Function<String, Integer>
  // the context (str1) has been "closed over" in the returned function <- this is called a closure
  return str2 -> str1.length() + str2.length();
}
final var lengthOfFooAndBar = length2Curried.apply("foo").apply("bar");

// partially applied function
final Function<String, Int> plusLengthFoo = length2Curried.apply("foo");
final var lengthOfFooAndBar = plusLengthFoo.apply("bar");

// partial function: is not defined for divisor == 0
// this is slightly more involved, but I wanted to supply one possible implementation for completeness
// We will get to what Option is and how one could implement this in a slightly more "functional" way
final Function<Integer, Function<Integer, Option<Integer>>> divisionByN = divisor -> dividend -> {
  if (divisor == 0)
    return None();
  return Some(dividend / divisor);
}