Sorting Custom Object by Implementing Comparable Interface in Java

Last Updated : 28 Dec, 2020

Java provides two interfaces to sort objects using data members of the class which are Comparable and Comparator. In this article, we will focus on a Comparable interface. A Comparable object is capable of comparing itself with another object by natural ordering. The class must implement the java.lang.Comparable interface to compare its instances.

Using Comparable interface we can sort:

String objects
Wrapper class objects
User-defined objects

Syntax

public interface Comparable<T> 
{
    public int compareTo(T o);
}

Where T is the type of Object to be sorted.

compareTo() method

For any class to support sorting, it should implement the Comparable interface and override it’s compareTo() method. It returns a negative integer if an object is less than the specified object, returns zero if an object is equal, and returns a positive integer if an object is greater than the specified object.

Example 1: Sort the given data according to the student marks.

Java

// Java program to sort student 
// data according to their marks 
  
import java.util.*; 
  
// implementing comparable interface 
class StudentData implements Comparable<StudentData> { 
  
    String name; 
    int marks; 
  
    // Constructor 
    StudentData(String name, int marks) 
    { 
        this.name = name; 
        this.marks = marks; 
    } 
  
    // overriding method to sort 
    // the student data 
    public int compareTo(StudentData sd) 
    { 
        return this.marks - sd.marks; 
    } 
} 
  
// Driver class 
class GFG { 
    public static void main(String[] args) 
    { 
  
        ArrayList<StudentData> list 
            = new ArrayList<StudentData>(); 
  
        // Inserting data 
        list.add(new StudentData("Ram", 98)); 
        list.add(new StudentData("Shyam", 84)); 
        list.add(new StudentData("Lokesh", 90)); 
  
        Collections.sort(list); 
  
        // Displaying data 
        for (StudentData sd : list) 
            System.out.println(sd.name + " " + sd.marks); 
    } 
}

Output

Shyam 84
Lokesh 90
Ram 98

Example 2: Sort the given data according to the names.

Java

// Java program to sort student 
// data according to their names 
  
import java.util.*; 
  
// implementing comparable interface 
class StudentData implements Comparable<StudentData> { 
  
    int roll; 
    String name; 
    int marks; 
  
    // Constructor 
    StudentData(int roll, String name, int marks) 
    { 
        this.roll = roll; 
        this.name = name; 
        this.marks = marks; 
    } 
  
    // overriding method to sort 
    // the student data 
    public int compareTo(StudentData sd) 
    { 
  
        // compareTo is a string method 
        return this.name.compareTo(sd.name); 
    } 
} 
  
// Driver class 
class GFG { 
    public static void main(String[] args) 
    { 
  
        ArrayList<StudentData> list 
            = new ArrayList<StudentData>(); 
  
        // Inserting data 
        list.add(new StudentData(1, "Ram", 98)); 
        list.add(new StudentData(2, "Shyam", 84)); 
        list.add(new StudentData(3, "Lokesh", 90)); 
  
        Collections.sort(list); 
  
        // Displaying data 
        for (StudentData sd : list) 
            System.out.println(sd.roll + " " + sd.name + " "
                               + sd.marks); 
    } 
}