Java > Sorting Collections

Sorting Collections in Java

Introduction

Sorting is an essential operation in Java collections, allowing us to organize data for better accessibility and efficiency. Java provides two primary ways to sort collections:

1. Using the Comparable Interface
2. Using the Comparator Interface

1. Using the Comparable Interface

The Comparable interface is used for defining the natural ordering of objects. It contains a single method, compareTo(), which must be implemented to define how objects of a class should be compared.

Example: Sorting a List of Students by Name

import java.util.*;

class Student implements Comparable<Student> {
    String name;
    int age;

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public int compareTo(Student other) {
        return this.name.compareTo(other.name);
    }

    @Override
    public String toString() {
        return name + " - " + age;
    }
}

public class ComparableExample {
    public static void main(String[] args) {
        List<Student> students = Arrays.asList(
            new Student("Ajay", 22),
            new Student("Balu", 20),
            new Student("Chaitanya", 25),
            new Student("Dinesh", 21)
        );

        Collections.sort(students);
        System.out.println(students);
    }
}

Significance of compareTo()

• The compareTo() method returns:
  • A negative number if this object is less than the other object.
  • Zero if both objects are equal.
  • A positive number if this object is greater than the other object.
• It defines the natural ordering of objects.

2. Using the Comparator Interface

The Comparator interface is used to define multiple sorting criteria by passing custom comparator objects.

Example: Sorting Students by Age

import java.util.*;

class Student {
    String name;
    int age;

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return name + " - " + age;
    }
}

public class ComparatorExample {
    public static void main(String[] args) {
        List<Student> students = Arrays.asList(
            new Student("Ajay", 22),
            new Student("Balu", 20),
            new Student("Chaitanya", 25),
            new Student("Dinesh", 21)
        );

        students.sort(Comparator.comparingInt(s -> s.age));
        System.out.println(students);
    }
}

Key Differences Between Comparable and Comparator

Feature	Comparable	Comparator
Purpose	Defines natural ordering of objects	Defines custom sorting logic
Method	compareTo(T obj)	compare(T obj1, T obj2)
Implementation	Implemented in the class itself	Implemented separately as a lambda or a class
Sorting Criteria	Only one	Multiple, as required

Conclusion

• Use Comparable when an object has a natural ordering.
• Use Comparator when multiple sorting criteria are needed.
• Both interfaces help in achieving efficient sorting operations in Java Collections.

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• Collections
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