Java Collections Class

In this page:

Sorting Collections
Reversing and Shuffling
Searching Collections
Modifying Lists
Unmodifiable Collections

Sorting Collections

Collections.sort() reorders a List in place according to natural ordering or a supplied Comparator, saving you from implementing a sorting algorithm yourself for everyday use cases.

Example: Sorting Collections

java

import java.util.Collections;
import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<Integer> nums = new ArrayList<>(java.util.List.of(3, 1, 2));
		Collections.sort(nums);
		System.out.println(nums);
	}
}

Reversing and Shuffling

Collections.reverse() flips a list's order in place, and Collections.shuffle() randomizes it — both operate directly on the list you pass in rather than returning a new one.

Example: Reversing and Shuffling

java

import java.util.Collections;
import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<Integer> nums = new ArrayList<>(java.util.List.of(1, 2, 3));
		Collections.reverse(nums);
		System.out.println(nums);
	}
}

Searching Collections

Collections.binarySearch() finds an element's index in a *sorted* list in O(log n) time — calling it on an unsorted list gives undefined, unreliable results, so always sort first.

Example: Searching Collections

java

import java.util.Collections;
import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<Integer> nums = new ArrayList<>(java.util.List.of(1, 3, 5, 7));
		int index = Collections.binarySearch(nums, 5); // list must already be sorted
		System.out.println(index);
	}
}

Modifying Lists

Methods like Collections.max(), min(), frequency(), and swap() let you query or rearrange list contents without writing manual loops for these very common operations.

Example: Modifying Lists

java

import java.util.Collections;
import java.util.ArrayList;
public class Main {
	public static void main(String[] args) {
		ArrayList<Integer> nums = new ArrayList<>(java.util.List.of(3, 1, 4, 1));
		System.out.println(Collections.max(nums));
		System.out.println(Collections.frequency(nums, 1));
		Collections.swap(nums, 0, 1);
		System.out.println(nums);
	}
}

Unmodifiable Collections

Collections.unmodifiableList() (and its Set/Map equivalents) wraps a collection so any attempted modification throws UnsupportedOperationException, letting you safely hand out a read-only view of internal data.

Example: Unmodifiable Collections

java

import java.util.Collections;
import java.util.ArrayList;
import java.util.List;
public class Main {
	public static void main(String[] args) {
		List<Integer> readOnly = Collections.unmodifiableList(new ArrayList<>(java.util.List.of(1, 2)));
		try {
			readOnly.add(3);
		} catch (UnsupportedOperationException e) {
			System.out.println("Cannot modify: " + e.getClass().getSimpleName());
		}
	}
}

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Java Collections Class

Sorting Collections

Reversing and Shuffling

Searching Collections

Modifying Lists

Unmodifiable Collections

Chapter Quiz — Complete all 17 topics to unlock

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