Collections vs Collectors.md

Collections vs Collectors in Java

Java provides two similar-sounding utility classes: Collections and Collectors. They serve different purposes in the ecosystem of Java Collections and Streams.

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📌 1. Collections (java.util.Collections)

• A utility class in java.util.
• Provides static methods to manipulate existing collections (List, Set, Map).
• Works with traditional collections API (not streams).

Common Methods

• Collections.sort(list) → Sort a List
• Collections.reverse(list) → Reverse a List
• Collections.shuffle(list) → Random shuffle
• Collections.binarySearch(list, key) → Search in a sorted list
• Collections.max(list) → Find max element
• Collections.min(list) → Find min element
• Collections.fill(list, value) → Replace all elements with a value
• Collections.copy(dest, src) → Copy one list into another
• Collections.swap(list, i, j) → Swap elements
• Collections.replaceAll(list, oldVal, newVal) → Replace values
• Collections.unmodifiableList(list) → Read-only List
• Collections.synchronizedList(list) → Thread-safe List wrapper

Example

import java.util.*;

public class CollectionsExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(Arrays.asList(4, 2, 9, 1, 5));

        Collections.sort(numbers);    // [1, 2, 4, 5, 9]
        Collections.reverse(numbers); // [9, 5, 4, 2, 1]

        Collections.swap(numbers, 0, 4); // swap first and last
        System.out.println("After swap: " + numbers);

        System.out.println("Max: " + Collections.max(numbers)); // 9
        System.out.println("Min: " + Collections.min(numbers)); // 1
    }
}

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📌 2. Collectors (java.util.stream.Collectors)

• A utility class in java.util.stream.
• Provides static factory methods for collectors.
• Collectors are used with Streams API → stream.collect(...).
• Helps gather stream results into List, Set, Map, String, numbers, grouping, partitioning etc.

Common Collectors

• Collectors.toList() → Collect into a List
• Collectors.toSet() → Collect into a Set
• Collectors.toMap(keyMapper, valueMapper) → Collect into a Map
• Collectors.joining(", ") → Join into a String
• Collectors.counting() → Count elements
• Collectors.summingInt(...) / summingLong(...) / summingDouble(...) → Sum numbers
• Collectors.averagingInt(...) / averagingDouble(...) / averagingLong(...) → Average
• Collectors.groupingBy(...) → Group elements
• Collectors.partitioningBy(predicate) → Partition into two groups
• Collectors.mapping(...) → Apply mapping before collecting
• Collectors.reducing(...) → Custom reduction

Example

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

public class CollectorsExample {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("John", "Jane", "Jack", "Jill", "Bob");

        // Collect into List
        List<String> upper = names.stream()
                .map(String::toUpperCase)
                .collect(Collectors.toList());
        System.out.println(upper); // [JOHN, JANE, JACK, JILL, BOB]

        // Count
        long count = names.stream().collect(Collectors.counting());
        System.out.println("Count: " + count); // 5

        // Join into a string
        String joined = names.stream().collect(Collectors.joining(", "));
        System.out.println("Joined: " + joined); // John, Jane, Jack, Jill, Bob

        // Group by length
        Map<Integer, List<String>> grouped = names.stream()
                .collect(Collectors.groupingBy(String::length));
        System.out.println(grouped); // e.g. {3=[Bob], 4=[John, Jane, Jack, Jill]}

        // Partitioning: names starting with J vs others
        Map<Boolean, List<String>> partitioned = names.stream()
                .collect(Collectors.partitioningBy(name -> name.startsWith("J")));
        System.out.println(partitioned); // {false=[Bob], true=[John, Jane, Jack, Jill]}
    }
}

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🔑 Key Differences

Feature	Collections (java.util)	Collectors (java.util.stream)
Package	java.util	java.util.stream
Type	Utility class for collections	Utility class for collectors (Streams)
Works with	List, Set, Map	Stream API
Purpose	Manipulate existing collections	Collect stream results
Example	Collections.sort(list)	stream.collect(Collectors.toList())

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✅ Summary

• Collections → Works with traditional collections (sorting, searching, synchronization, etc.).
• Collectors → Works with streams to gather results into a collection, string, or aggregated form.