The Comparator interface in Java is used to define custom sorting orders for objects, especially when you need multiple sorting strategies or cannot modify the original class. Unlike Comparable, which defines a class’s natural ordering, Comparator externalizes the comparison logic, making it more flexible and reusable.

Key Features of Comparator:

• Functional Interface: Can be used with lambda expressions and method references.

• Core Method: int compare(T o1, T o2) returns a negative, zero, or positive integer based on whether o1 is less than, equal to, or greater than o2.

• Static Factory Methods (Java 8+):

  • Comparator.comparing(Function keyExtractor) – creates a comparator based on a property (e.g., User::getAge).

  • Comparator.comparingInt, comparingDouble, comparingLong – optimized for primitive types.

• Chaining Comparators:

  • thenComparing() – applies a secondary sort when primary keys are equal.

  • reversed() – reverses the current comparator’s order.

  • reverseOrder() – returns a comparator that reverses natural ordering.

Example Usage:

List<User> users = List.of(new User("Alice", 30), new User("Bob", 25));
// Sort by age ascending
users.stream().sorted(Comparator.comparing(User::getAge)).forEach(System.out::println);

// Sort by age descending, then by name
users.stream()
     .sorted(Comparator.comparing(User::getAge).reversed()
                      .thenComparing(User::getName))
     .forEach(System.out::println);

Best Practices:

• Use Comparator when sorting objects without modifying their source code.

• Ensure comparators are consistent with equals to avoid unexpected behavior in TreeSet or TreeMap.

• Implement Serializable if used in serializable collections.

Note: Comparator is part of java.util and has been enhanced in Java 8 with functional programming features, making it a cornerstone for modern Java sorting.