Interview question

What are method references in Java, and what are their different types? Java में method references क्या हैं, और इनके अलग-अलग types क्या हैं?

Answer

Method references are a shorthand for lambda expressions that simply call an existing method, using the :: syntax for extra readability.

Type	Syntax	Example
Static method	ClassName::staticMethod	Integer::parseInt
Instance method of a particular object	instance::instanceMethod	str::toUpperCase (where str is a variable)
Instance method of an arbitrary object	ClassName::instanceMethod	String::toUpperCase
Constructor	ClassName::new	ArrayList::new

import java.util.function.*;
import java.util.stream.*;

// Static method reference
Function<String, Integer> parser1 = s -> Integer.parseInt(s);  // lambda
Function<String, Integer> parser2 = Integer::parseInt;           // method reference - same effect
System.out.println(parser2.apply('42'));  // 42

// Instance method reference on a PARTICULAR object
String greeting = 'Hello World';
Supplier<String> upper1 = () -> greeting.toUpperCase();  // lambda
Supplier<String> upper2 = greeting::toUpperCase;          // method reference
System.out.println(upper2.get());  // HELLO WORLD

// Instance method reference on an ARBITRARY object of a type
// (the object itself becomes the first argument)
List<String> names = List.of('charlie', 'alice', 'bob');
List<String> upperNames1 = names.stream()
    .map(s -> s.toUpperCase())        // lambda
    .collect(Collectors.toList());
List<String> upperNames2 = names.stream()
    .map(String::toUpperCase)          // method reference - equivalent
    .collect(Collectors.toList());

// Constructor reference
Supplier<ArrayList<String>> listCreator1 = () -> new ArrayList<>();  // lambda
Supplier<ArrayList<String>> listCreator2 = ArrayList::new;             // method reference

Function<String, StringBuilder> sbCreator = StringBuilder::new;
StringBuilder sb = sbCreator.apply('Hello');

// Method references with sort/forEach
names.forEach(System.out::println);  // instead of s -> System.out.println(s)

Method references, existing method को simply call करने वाले lambda expressions का shorthand हैं, :: syntax use करते हुए readability के लिए।

Type	Syntax	उदाहरण
Static method	ClassName::staticMethod	Integer::parseInt
किसी particular object का instance method	instance::instanceMethod	str::toUpperCase
Arbitrary object का instance method	ClassName::instanceMethod	String::toUpperCase
Constructor	ClassName::new	ArrayList::new

import java.util.function.*;
import java.util.stream.*;

Function<String, Integer> parser1 = s -> Integer.parseInt(s);
Function<String, Integer> parser2 = Integer::parseInt;
System.out.println(parser2.apply('42'));  // 42

String greeting = 'Hello World';
Supplier<String> upper1 = () -> greeting.toUpperCase();
Supplier<String> upper2 = greeting::toUpperCase;
System.out.println(upper2.get());  // HELLO WORLD

List<String> names = List.of('charlie', 'alice', 'bob');
List<String> upperNames2 = names.stream()
    .map(String::toUpperCase)
    .collect(Collectors.toList());

Supplier<ArrayList<String>> listCreator2 = ArrayList::new;

Function<String, StringBuilder> sbCreator = StringBuilder::new;
StringBuilder sb = sbCreator.apply('Hello');

names.forEach(System.out::println);

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