java tutorial - Java Comparable interface - java programming - learn java - java basics - java for beginners

Java Comparable interface is used to order the objects of user-defined class.This interface is found in java.lang package and contains only one method named compareTo(Object).

public int compareTo(Object obj): is used to compare the current object with the specified object.

We can sort the elements of:

Collections class provides static methods for sorting the elements of collections. If collection elements are of Set or Map, we can use TreeSet or TreeMap.

But We cannot sort the elements of List. Collections class provides methods for sorting the elements of List type elements.

Let's see the example of Comparable interface that sorts the list elements on the basis of age.

File: Student.java

public class Student implements Comparable<Student>{  
int rollno;  
String name;  
int age;  
Student(int rollno,String name,int age){  
this.rollno=rollno;  
this.name=name;  
this.age=age;  
}  
  
public int compareTo(Student st){  
if(age==st.age)  
return 0;  
else if(age>st.age)  
return 1;  
else  
return -1;  
}  
}  

File: TestSort3.java

import java.util.*;  
import java.io.*;  
public class TestSort3{  
public static void main(String args[]){  
ArrayList<Student> al=new ArrayList<Student>();  
al.add(new Student(101,"Vijay",23));  
al.add(new Student(106,"Ajay",27));  
al.add(new Student(105,"Jai",21));  
  
Collections.sort(al);  
for(Student st:al){  
System.out.println(st.rollno+" "+st.name+" "+st.age);  
}  
}  
}  

105 Jai 21
101 Vijay 23
106 Ajay 27