Interview question

What is the Stream API in Java and how does it differ from Collections? Java में Stream API क्या है और Collections से कैसे अलग है?

Answer

Aspect	Collection	Stream
Storage	Stores data in memory	No storage - computes elements on demand
Mutability	Can add/remove elements	Doesn't modify the source
Traversal	Can iterate multiple times	Consumed once, single-use
Evaluation	Eager - all operations happen immediately	Lazy - intermediate ops run only when a terminal op is invoked

import java.util.stream.*;

List<String> names = List.of('Charlie', 'Alice', 'Bob', 'Anna');

// A stream pipeline: source -> intermediate operations -> terminal operation
List<String> result = names.stream()                 // create stream from collection
    .filter(name -> name.startsWith('A'))              // intermediate: lazy, returns a stream
    .map(String::toUpperCase)                            // intermediate: lazy, returns a stream
    .sorted()                                             // intermediate: lazy
    .collect(Collectors.toList());                       // terminal: triggers execution

System.out.println(result);  // [ALICE, ANNA]

// Streams are single-use - reusing a consumed stream throws an exception
Stream<String> stream = names.stream();
stream.forEach(System.out::println);
// stream.forEach(System.out::println);  // IllegalStateException: stream has already been operated upon

// Lazy evaluation demonstration
Stream<String> lazyStream = names.stream()
    .filter(name -> {
        System.out.println('Filtering: ' + name);
        return name.length() > 3;
    });
// Nothing printed yet - filter() hasn't actually run because there's
// no terminal operation yet
long count = lazyStream.count();  // NOW the filter actually executes

पहलू	Collection	Stream
Storage	Memory में data store	कोई storage नहीं
Mutability	Add/remove हो सकता है	Source modify नहीं करता
Traversal	कई बार iterate	एक बार use, single-use
Evaluation	Eager	Lazy

import java.util.stream.*;

List<String> names = List.of('Charlie', 'Alice', 'Bob', 'Anna');

List<String> result = names.stream()
    .filter(name -> name.startsWith('A'))
    .map(String::toUpperCase)
    .sorted()
    .collect(Collectors.toList());

System.out.println(result);  // [ALICE, ANNA]

// Streams single-use हैं
Stream<String> stream = names.stream();
stream.forEach(System.out::println);
// stream.forEach(System.out::println);  // IllegalStateException

// Lazy evaluation
Stream<String> lazyStream = names.stream()
    .filter(name -> {
        System.out.println('Filtering: ' + name);
        return name.length() > 3;
    });
// अभी कुछ print नहीं हुआ - terminal operation नहीं है
long count = lazyStream.count();  // अब filter चलता है

Was this answer clear?

Subjects

Related questions