Interview question

What is the difference between Collectors.toList() and Stream.toList() in modern Java? Modern Java में Collectors.toList() और Stream.toList() में क्या अंतर है?

Answer

Method	Available since	Returned list mutability
collect(Collectors.toList())	Java 8	Typically mutable (ArrayList), but not officially guaranteed
.toList()	Java 16+	Unmodifiable - guaranteed immutable

import java.util.stream.*;

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

// Java 8 style - via Collectors
List<String> result1 = names.stream()
    .filter(n -> n.length() > 3)
    .collect(Collectors.toList());
result1.add('Extra');  // usually works - typically returns a mutable ArrayList

// Java 16+ shorthand - more concise
List<String> result2 = names.stream()
    .filter(n -> n.length() > 3)
    .toList();
// result2.add('Extra');  // UnsupportedOperationException - immutable by design

// Also useful in modern Java: Collectors.toUnmodifiableList() (Java 10+)
// gives the SAME guarantee as .toList() but with more explicit intent
List<String> result3 = names.stream()
    .collect(Collectors.toUnmodifiableList());

// Practical implication: if you build a list with .toList() and later
// need to modify it, wrap it explicitly:
List<String> mutableCopy = new ArrayList<>(result2);
mutableCopy.add('Extra');  // now works

// Choose .toList() for concise, safe-by-default code in Java 16+ projects;
// use Collectors.toList() when you specifically need a mutable result
// or need to target compatibility with Java 8-15

Method	कब से उपलब्ध	Mutability
collect(Collectors.toList())	Java 8	आमतौर पर mutable
.toList()	Java 16+	Unmodifiable

import java.util.stream.*;

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

// Java 8 style
List<String> result1 = names.stream()
    .filter(n -> n.length() > 3)
    .collect(Collectors.toList());
result1.add('Extra');  // आमतौर पर काम करता है

// Java 16+ shorthand
List<String> result2 = names.stream()
    .filter(n -> n.length() > 3)
    .toList();
// result2.add('Extra');  // UnsupportedOperationException

List<String> result3 = names.stream()
    .collect(Collectors.toUnmodifiableList());

List<String> mutableCopy = new ArrayList<>(result2);
mutableCopy.add('Extra');  // अब काम करता है

Was this answer clear?

Subjects

Related questions