Collections Utility Class

Direct Answer

Collections is a java.util utility class of static helpers such as sort, reverse, shuffle, binarySearch, max, min, frequency, unmodifiable views, and synchronized wrappers. Collections.unmodifiableList(list) is a read-only view whose contents can still change through the backing list. List.of(...) creates an unmodifiable list, but mutable elements inside it can still change.

Code Example

The wrappers and copy factories have different relationships to the source list.

import java.util.*;

public class CollectionsDemo {
    public static void main(String[] args) {
        List<Integer> nums = new ArrayList<>(List.of(3, 1, 2, 1));
        Collections.sort(nums);
        Collections.reverse(nums);
        int max = Collections.max(nums);
        int count = Collections.frequency(nums, 1);

        List<Integer> view = Collections.unmodifiableList(nums);
        List<Integer> snapshot = List.copyOf(nums);

        nums.add(9);
        System.out.println(view);     // includes 9: it is backed by nums
        System.out.println(snapshot); // does not include the later change
        // view.add(10);              // UnsupportedOperationException
    }
}

Both results are shallow: if an element itself is mutable, these APIs do not freeze that object's fields.

Common Mistakes & Pitfalls

• Collections vs Collection. Collections is the utility class; Collection is the interface — don't use Collections as a type.
• Thinking unmodifiableList copies. It's a view over the original; mutate the backing list and the view changes too. Use List.of / List.copyOf for an immutable copy.
• Reinventing the wheel. Writing manual loops for sort/max/min instead of calling the helpers.
• Relying on synchronizedList for compound actions. Iteration still needs external synchronization; prefer concurrent collections.