Comparable vs Comparator in Java

When you call Collections.sort() on a list of integers, Java knows how to sort them — numbers have a natural order. But what about a list of Student objects? Java does not know whether to sort by name, marks, or roll number. You have to tell it.

Two interfaces handle this: Comparable (built into the class) and Comparator (external sorting logic).

Comparable — Natural Order Built Into the Class

Implement Comparable<T> in the class itself to define its natural order. You write one compareTo() method.

Run this example to see its output.

Line by line:

• implements Comparable<Student> — this class can be naturally sorted.
• compareTo(Student other) — return negative if this should come before other, zero if equal, positive if other comes first.
• this.marks - other.marks — ascending order. For descending, flip to other.marks - this.marks.
• Collections.sort(students) — works because Student implements Comparable.

Comparator — External Sorting Logic

Comparator is a separate object that defines a sort order. Use it when:

• You need multiple different sort orders
• You cannot modify the class (third-party library)
• You want to keep sorting logic separate

Run this example to see its output.

Line by line:

• Comparator.comparingInt(s -> s.marks) — creates a Comparator that extracts marks for comparison.
• .reversed() — flips the order.
• .thenComparing(s -> s.name) — secondary sort when marks are equal.
• students.sort(comparator) — sorts using the given Comparator, not compareTo.

Comparable vs Comparator — Summary

compareTo Return Value Rule

This applies to both compareTo and compare:

Common Mistake 1: Integer Overflow in compareTo

Common Mistake 2: Breaking the Contract

compareTo must be consistent: if a.compareTo(b) < 0 then b.compareTo(a) must be > 0. Violating this breaks sorting and TreeSet/TreeMap.

Practice Task

Task: Create a class Product with fields name (String), price (double), and rating (double). Implement Comparable<Product> to sort by price ascending. Create three separate Comparators:

1. By rating descending
2. By name alphabetically
3. By price descending, then rating descending

Create a list of 4 products and test all three Comparators.

Solution:

Run this example to see its output.

What to observe: products.sort(null) uses the class's own compareTo (natural order by price). Each Comparator is independent — you can switch sorting strategy on the fly. Try adding thenComparing(p -> p.name) to break ties alphabetically.