Comparable vs Comparator in Java Collections

1. Comparable (Natural Ordering)

Definition

Comparable is an interface in java.lang used to define the natural ordering of objects.

Method

int compareTo(T o);

Example

class Employee implements Comparable<Employee> {

    int id;
    String name;

    Employee(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public int compareTo(Employee other) {
        return Integer.compare(this.id, other.id);
    }
}

Sorting

Collections.sort(employeeList);

Key Points

• Sorting logic is inside the class
• Only one natural ordering allowed
• Used automatically by TreeSet and TreeMap

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2. Comparator (Custom Ordering)

Definition

Comparator is an interface in java.util used to define custom sorting logic outside the class.

Method

int compare(T o1, T o2);

Example

Comparator<Employee> nameComparator =
    Comparator.comparing(Employee::getName);

employeeList.sort(nameComparator);

Multiple Sorting Criteria

Comparator<Employee> comparator =
    Comparator.comparing(Employee::getName)
              .thenComparing(Employee::getId);

employeeList.sort(comparator);

Key Points

• Sorting logic is external
• Multiple sorting strategies supported
• Preferred in modern Java (with lambdas)

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Comparison Table

Feature Comparable Comparator

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Package java.lang java.util Method compareTo() compare() Sorting Logic Inside class Outside class Multiple sorting No Yes Used by TreeSet default Yes If provided

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Important Interview Insights

1. Avoid Subtraction in compareTo

return this.salary - other.salary; // Risk of overflow

Use instead:

return Integer.compare(this.salary, other.salary);

2. Consistency with equals()

If compareTo() returns 0, ideally equals() should also return true to avoid issues in sorted collections.

3. Sorting Complexity

• Collections.sort() → O(n log n)
• TreeSet insertion → O(log n)

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Final Summary

Comparable → Natural ordering → compareTo()
Comparator → Custom ordering → compare()

Use Comparable for default identity ordering.
Use Comparator for flexible business-based sorting.