What does identity method do, Generic Singleton java

Question

While reading effective java Chapter 5, Item 27

It talks about generic singleton pattern :

Now suppose that you want to provide an identity function. It would be wasteful to create a new one each time it’s required, as it’s stateless. If generics were reified, you would need one identity function per type, but since they’re erased you need only a generic singleton. Here’s how it looks:

public class GenericSingleton<T> {
    private static UnaryFunction<Object> IDENTIFY_FUNCTION = new UnaryFunction<Object>() {
        @Override
        public Object apply(Object args) {
            return args;
        }
    };

    @SuppressWarnings("unchecked")
    public static <T> UnaryFunction<T> identityFunction() {
        return (UnaryFunction<T>) IDENTITY_FUNCTION;
    }

    public static void main(String[] args) {
        String[] strings = {"jute", "hemp", "nylon"};
        UnaryFunction<String> sameString = identityFunction();
        for (String s : strings) {
            System.out.println(sameString.apply(s));
        }
        Number[] numbers = {1, 2.0, 3L};
        UnaryFunction<Number> sameNumber = identityFunction();
        for (Number n : numbers) {
            System.out.println(sameNumber.apply(n));
        }
    }
}

I can't understand what does apply method actually do!
It's like getting an object and returning itself. why? some useless sample?

Would someone tell me the use case please ?

This is an *identity function*, not an *identify* function. Look up the mathematical definition of an identity function. — chrylis -cautiouslyoptimistic-, Mar 11 '19 at 04:33
@chrylis Thank I read it now and I edited my question, but I have the same question yet. does it have any use case in real programs ? — alizeyn, Mar 11 '19 at 04:36
See https://stackoverflow.com/questions/25987085/what-do-people-use-the-identity-function-for — chrylis -cautiouslyoptimistic-, Mar 11 '19 at 04:46
@chrylis all examples are in `haskell` so I guess it's better to keep my question for java examples — alizeyn, Mar 11 '19 at 04:58
That's why I didn't close it as a duplicate, but the concepts are the same. — chrylis -cautiouslyoptimistic-, Mar 11 '19 at 14:28

score 4 · Accepted Answer · edited Mar 11 '19 at 14:28

A use case is the Collectors.toMap().

Say you have a list of items identified by a unique key and you want a Map from that unique key to the object itself.

Collectors.toMap() needs two functions:

one to extract the key from the object
another one to extract the value from the object

Since the value should be the object itself, you need a function that takes an object and returns the same object - which is your identity function.

What does identity method do, Generic Singleton java

1 Answers1