What are practical uses of the java.util.function.Function.identity method?

Question

Why should I use Function.identity() when it returns the same thing which it receives without doing anything using the input or modifying the input in some way?

Apple apple = new Apple(10, "green"); Function<Apple, Apple> identity = Function.identity(); identity.apply(apple); 

There must be some practical usage of this which I am not able to figure out.

Answer 1

The intended usage is when you're using a method that accepts a Function to map something, and you need to map the input directly to the output of the function (the 'identity' function).

As a very simple example, mapping a list of persons to a map from name to person:

import static java.util.function.Function.identity  // [...]  List<Person> persons = ... Map<String, Person> = persons.stream()         .collect(Collectors.toMap(Person::name, identity())) 

The identity() function is just for convenience and readability. As Peter indicates in his answer, you could just use t -> t, but personally I think that using identity() communicates intent better as it leaves no room for interpretation like wondering if the original author forgot to do a transformation in that lambda. I admit though that is highly subjective, and assumes the reader knows what identity() does.

Possibly it may have some additional advantages in terms of memory as it reuses a single lambda definition, instead of having a specific lambda definition for this call. I think that impact is probably negligible in most cases.

Answer 2

You can use it for a frequency count for example.

public static <T> Map<T, Long> frequencyCount(Collection<T> words) {     return words.stream()             .collect(Collectors.groupingBy(Function.identity(),                     Collectors.counting()); } 

In this case, you are saying the key to group by is the element in the collection (without transforming it).

Personally, I find this briefer

import static java.util.stream.Collectors.*;  public static Map<String, Long> frequencyCount(Collection<String> words) {     return words.stream()             .collect(groupingBy(t -> t,                     counting()); }