Q: What is the difference between Comparable and Comparator?

Answer:

Both are used for sorting objects, but they serve different purposes.

Comparable — Natural Ordering (Built-In)

The class itself implements Comparable<T> and defines its natural ordering via compareTo(). There is only one natural ordering per class.

public class Employee implements Comparable<Employee> {
    private int id;
    private String name;

    @Override
    public int compareTo(Employee other) {
        return Integer.compare(this.id, other.id); // Natural order: by ID
    }
}

List<Employee> employees = new ArrayList<>();
Collections.sort(employees); // Uses compareTo() — sorts by ID

Comparator — Custom Ordering (External)

A separate functional interface that defines a custom ordering without modifying the class. You can have multiple comparators for different sort criteria.

// Sort by name
Comparator<Employee> byName = Comparator.comparing(Employee::getName);

// Sort by salary descending, then by name ascending
Comparator<Employee> bySalaryDesc = Comparator.comparing(Employee::getSalary).reversed()
                                              .thenComparing(Employee::getName);

employees.sort(byName);
employees.sort(bySalaryDesc);

Key Differences

Modern Java Comparator Utilities

// Null-safe comparisons
Comparator.nullsFirst(Comparator.comparing(Employee::getName))

// Chaining
Comparator.comparing(Employee::getDepartment)
          .thenComparing(Employee::getSalary)
          .reversed()

// Lambda shorthand
employees.sort((a, b) -> a.getName().compareTo(b.getName()));

[!TIP] Rule of thumb: Implement Comparable for the one obvious natural ordering (e.g., alphabetical for String, numeric for Integer). Use Comparator for any alternative orderings (e.g., sort employees by salary, by department, etc.).