What are practical usage of java.util.function.Function#identity method?

up vote
8
down vote

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Why should I use Function.identity() when it returns the same thing which it receives without any scope of 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 for sure.

asked 2 hours ago

arunveersingh

413

New contributor

add a commentÂ |Â

up vote
8
down vote

favorite

Why should I use Function.identity() when it returns the same thing which it receives without any scope of 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 for sure.

asked 2 hours ago

arunveersingh

413

New contributor

add a commentÂ |Â

up vote
8
down vote

favorite

Why should I use Function.identity() when it returns the same thing which it receives without any scope of 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 for sure.

asked 2 hours ago

arunveersingh

413

New contributor

Why should I use Function.identity() when it returns the same thing which it receives without any scope of 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 for sure.

java java-8

asked 2 hours ago

arunveersingh

413

New contributor

asked 2 hours ago

arunveersingh

413

New contributor

asked 2 hours ago

arunveersingh

413

New contributor

asked 2 hours ago

arunveersingh

413

asked 2 hours ago

arunveersingh

413

New contributor

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3 Answers
3

active

oldest

votes

up vote
6
down vote

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());

answered 2 hours ago

Peter Lawrey

429k54539916

@Eugene The implementation of identity() is return t -> t
â€“Â Mark Rotteveel
1 hour ago

2

@MarkRotteveel right. what I meant is this
â€“Â Eugene
1 hour ago

If I had to implement a compiler for lambdas, I would make the lambda a synthetic, private static field (using an anonymous class) of the class declaring it unless it references variables which value will only be known at runtime. I'm not sure the answer you linked is 100% correct for cases where the lambda is self-contained.
â€“Â Dici
1 hour ago

@Dici that was btw a legal strategy that they initially wanted to go with (there are talks from Brian Goetz about this) - they choose not to. they choose to go with invokedynamic and generate the classes at runtime
â€“Â Eugene
1 hour ago

@Dici it is 100% correct, under the current implementation at least, very easy to prove...
â€“Â Eugene
1 hour ago

add a commentÂ |Â

up vote
1
down vote

Suppose you have a List<String> strings = List.of("abc", "de") and you want to generate a Map where Key is value form that List and Value is it's length:

 Map<String, Integer> map = strings.stream() 
 .collect(Collectors.toMap(Function.identity(), String::length))

Generally some people see Function.identity() a little less readable than t -> t for example, but as explained here this is a bit different.

answered 1 hour ago

Eugene

61.1k986143

add a commentÂ |Â

up vote
1
down vote

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.

edited 1 hour ago

answered 2 hours ago

Mark Rotteveel

56.6k1472111

add a commentÂ |Â

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

up vote
6
down vote

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());

answered 2 hours ago

Peter Lawrey

429k54539916

@Eugene The implementation of identity() is return t -> t
â€“Â Mark Rotteveel
1 hour ago

2

@MarkRotteveel right. what I meant is this
â€“Â Eugene
1 hour ago

If I had to implement a compiler for lambdas, I would make the lambda a synthetic, private static field (using an anonymous class) of the class declaring it unless it references variables which value will only be known at runtime. I'm not sure the answer you linked is 100% correct for cases where the lambda is self-contained.
â€“Â Dici
1 hour ago

@Dici that was btw a legal strategy that they initially wanted to go with (there are talks from Brian Goetz about this) - they choose not to. they choose to go with invokedynamic and generate the classes at runtime
â€“Â Eugene
1 hour ago

@Dici it is 100% correct, under the current implementation at least, very easy to prove...
â€“Â Eugene
1 hour ago

add a commentÂ |Â

up vote
6
down vote

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());

answered 2 hours ago

Peter Lawrey

429k54539916

@Eugene The implementation of identity() is return t -> t
â€“Â Mark Rotteveel
1 hour ago

2

@MarkRotteveel right. what I meant is this
â€“Â Eugene
1 hour ago

If I had to implement a compiler for lambdas, I would make the lambda a synthetic, private static field (using an anonymous class) of the class declaring it unless it references variables which value will only be known at runtime. I'm not sure the answer you linked is 100% correct for cases where the lambda is self-contained.
â€“Â Dici
1 hour ago

@Dici that was btw a legal strategy that they initially wanted to go with (there are talks from Brian Goetz about this) - they choose not to. they choose to go with invokedynamic and generate the classes at runtime
â€“Â Eugene
1 hour ago

@Dici it is 100% correct, under the current implementation at least, very easy to prove...
â€“Â Eugene
1 hour ago

add a commentÂ |Â

up vote
6
down vote

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());

answered 2 hours ago

Peter Lawrey

429k54539916

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());

answered 2 hours ago

Peter Lawrey

429k54539916

answered 2 hours ago

Peter Lawrey

429k54539916

answered 2 hours ago

Peter Lawrey

429k54539916

answered 2 hours ago

Peter Lawrey

429k54539916

@Eugene The implementation of identity() is return t -> t
â€“Â Mark Rotteveel
1 hour ago

2

@MarkRotteveel right. what I meant is this
â€“Â Eugene
1 hour ago

If I had to implement a compiler for lambdas, I would make the lambda a synthetic, private static field (using an anonymous class) of the class declaring it unless it references variables which value will only be known at runtime. I'm not sure the answer you linked is 100% correct for cases where the lambda is self-contained.
â€“Â Dici
1 hour ago

@Dici that was btw a legal strategy that they initially wanted to go with (there are talks from Brian Goetz about this) - they choose not to. they choose to go with invokedynamic and generate the classes at runtime
â€“Â Eugene
1 hour ago

@Dici it is 100% correct, under the current implementation at least, very easy to prove...
â€“Â Eugene
1 hour ago

add a commentÂ |Â

@Eugene The implementation of identity() is return t -> t
â€“Â Mark Rotteveel
1 hour ago

2

@MarkRotteveel right. what I meant is this
â€“Â Eugene
1 hour ago

If I had to implement a compiler for lambdas, I would make the lambda a synthetic, private static field (using an anonymous class) of the class declaring it unless it references variables which value will only be known at runtime. I'm not sure the answer you linked is 100% correct for cases where the lambda is self-contained.
â€“Â Dici
1 hour ago

@Dici that was btw a legal strategy that they initially wanted to go with (there are talks from Brian Goetz about this) - they choose not to. they choose to go with invokedynamic and generate the classes at runtime
â€“Â Eugene
1 hour ago

@Dici it is 100% correct, under the current implementation at least, very easy to prove...
â€“Â Eugene
1 hour ago

@Eugene The implementation of identity() is return t -> t
â€“Â Mark Rotteveel
1 hour ago

@MarkRotteveel right. what I meant is this
â€“Â Eugene
1 hour ago

If I had to implement a compiler for lambdas, I would make the lambda a synthetic, private static field (using an anonymous class) of the class declaring it unless it references variables which value will only be known at runtime. I'm not sure the answer you linked is 100% correct for cases where the lambda is self-contained.
â€“Â Dici
1 hour ago

@Dici that was btw a legal strategy that they initially wanted to go with (there are talks from Brian Goetz about this) - they choose not to. they choose to go with invokedynamic and generate the classes at runtime
â€“Â Eugene
1 hour ago

@Dici it is 100% correct, under the current implementation at least, very easy to prove...
â€“Â Eugene
1 hour ago

add a commentÂ |Â

up vote
1
down vote

Suppose you have a List<String> strings = List.of("abc", "de") and you want to generate a Map where Key is value form that List and Value is it's length:

 Map<String, Integer> map = strings.stream() 
 .collect(Collectors.toMap(Function.identity(), String::length))

Generally some people see Function.identity() a little less readable than t -> t for example, but as explained here this is a bit different.

answered 1 hour ago

Eugene

61.1k986143

add a commentÂ |Â

up vote
1
down vote

Suppose you have a List<String> strings = List.of("abc", "de") and you want to generate a Map where Key is value form that List and Value is it's length:

 Map<String, Integer> map = strings.stream() 
 .collect(Collectors.toMap(Function.identity(), String::length))

Generally some people see Function.identity() a little less readable than t -> t for example, but as explained here this is a bit different.

answered 1 hour ago

Eugene

61.1k986143

add a commentÂ |Â

up vote
1
down vote

Suppose you have a List<String> strings = List.of("abc", "de") and you want to generate a Map where Key is value form that List and Value is it's length:

 Map<String, Integer> map = strings.stream() 
 .collect(Collectors.toMap(Function.identity(), String::length))

Generally some people see Function.identity() a little less readable than t -> t for example, but as explained here this is a bit different.

answered 1 hour ago

Eugene

61.1k986143

Suppose you have a List<String> strings = List.of("abc", "de") and you want to generate a Map where Key is value form that List and Value is it's length:

 Map<String, Integer> map = strings.stream() 
 .collect(Collectors.toMap(Function.identity(), String::length))

Generally some people see Function.identity() a little less readable than t -> t for example, but as explained here this is a bit different.

answered 1 hour ago

Eugene

61.1k986143

answered 1 hour ago

Eugene

61.1k986143

answered 1 hour ago

Eugene

61.1k986143

answered 1 hour ago

Eugene

61.1k986143

add a commentÂ |Â

up vote
1
down vote

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()))

edited 1 hour ago

answered 2 hours ago

Mark Rotteveel

56.6k1472111

add a commentÂ |Â

up vote
1
down vote

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()))

edited 1 hour ago

answered 2 hours ago

Mark Rotteveel

56.6k1472111

add a commentÂ |Â

up vote
1
down vote

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()))

edited 1 hour ago

answered 2 hours ago

Mark Rotteveel

56.6k1472111

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()))

edited 1 hour ago

answered 2 hours ago

Mark Rotteveel

56.6k1472111

edited 1 hour ago

answered 2 hours ago

Mark Rotteveel

56.6k1472111

answered 2 hours ago

Mark Rotteveel

56.6k1472111

answered 2 hours ago

Mark Rotteveel

56.6k1472111

add a commentÂ |Â

arunveersingh is a new contributor. Be nice, and check out our Code of Conduct.

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