Interview question

What is the difference between map() and flatMap() in Java Streams? Java Streams में map() और flatMap() में क्या अंतर है?

Answer

Method	Transforms to	Use case
map()	Stream<R> - one output per input, may be nested	Simple 1-to-1 transformation
flatMap()	Stream<R> - flattens nested streams into one	When each input produces MULTIPLE outputs (or a stream of them)

import java.util.stream.*;

// map() - simple 1-to-1 transformation
List<String> words = List.of('hello', 'world');
List<Integer> lengths = words.stream()
    .map(String::length)
    .collect(Collectors.toList());
System.out.println(lengths);  // [5, 5]

// PROBLEM: map() with a function returning a Stream creates NESTED streams
List<List<Integer>> nested = words.stream()
    .map(word -> word.chars().boxed().collect(Collectors.toList()))
    .collect(Collectors.toList());
// Result: List<List<Integer>> - a list of lists, not flattened

// flatMap() - flattens the nested structure into a single stream
List<String> sentences = List.of('Hello World', 'How are you');
List<String> allWords = sentences.stream()
    .flatMap(sentence -> Arrays.stream(sentence.split(' ')))  // each sentence -> multiple words
    .collect(Collectors.toList());
System.out.println(allWords);  // [Hello, World, How, are, you]

// Practical example: flattening a list of lists
List<List<Integer>> listOfLists = List.of(
    List.of(1, 2, 3),
    List.of(4, 5),
    List.of(6)
);
List<Integer> flatList = listOfLists.stream()
    .flatMap(List::stream)  // each inner list becomes a stream, all merged into one
    .collect(Collectors.toList());
System.out.println(flatList);  // [1, 2, 3, 4, 5, 6]

// Rule of thumb: use flatMap() whenever your mapping function
// itself returns a Stream/Collection that needs to be merged, not nested

Method	क्या transform करता है	Use case
map()	एक input, एक output	Simple 1-to-1 transformation
flatMap()	Nested streams को flatten करता है	हर input से multiple outputs

import java.util.stream.*;

List<String> words = List.of('hello', 'world');
List<Integer> lengths = words.stream()
    .map(String::length)
    .collect(Collectors.toList());
System.out.println(lengths);  // [5, 5]

// Problem: map() से nested streams बनते हैं
List<List<Integer>> nested = words.stream()
    .map(word -> word.chars().boxed().collect(Collectors.toList()))
    .collect(Collectors.toList());

// flatMap() - nested structure को flatten करता है
List<String> sentences = List.of('Hello World', 'How are you');
List<String> allWords = sentences.stream()
    .flatMap(sentence -> Arrays.stream(sentence.split(' ')))
    .collect(Collectors.toList());
System.out.println(allWords);  // [Hello, World, How, are, you]

List<List<Integer>> listOfLists = List.of(
    List.of(1, 2, 3),
    List.of(4, 5),
    List.of(6)
);
List<Integer> flatList = listOfLists.stream()
    .flatMap(List::stream)
    .collect(Collectors.toList());
System.out.println(flatList);  // [1, 2, 3, 4, 5, 6]

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