Comparable interface in java

-> Comparable is an interface of comparing the objects with other objects os the same type. This is also called "natural ordering"

-> by using compare interface, we can sort the elements based on single data member only.

-> It provides single method name as compareTo(object) and provides single sorting sequence only.

-> The interface found in java.lang.package


compareTo(Object obj) method

The method used to compare the current object with specified object. Please find the below return values

positive integer = if the current object is greater than the specified object.
negative integer = if the current object is less than the specified object.
zero = if the current object is equal to the specified object.



Example 1: Sort by Integer Value (Ascending Order) by using Comparable

import java.util.*; 

// User - Defined Class

class Employee implements Comparable<Employee>{ 
int empid; 
String empname; 
int empage; 
Employee(int empid,String empname,int empage){ 
this.empid=empid; 
this.empname=empname; 
this.empage=empage; 
} 
 
public int compareTo(Employee st){ 
if(empage==st.empage) 
return 0; 
else if(empage>st.empage) 
return 1; 
else 
return -1; 
} 
}



// Main Class



public class TestSort1{ 
public static void main(String args[]){ 
ArrayList<Employee> al=new ArrayList<Employee>(); 
al.add(new Employee(1,"Ajith",23)); 
al.add(new Employee(6,"Vijay",27)); 
al.add(new Employee(4,"Surya",21)); 
 
Collections.sort(al); 
for(Employee st:al){ 
System.out.println(st.empid+" "+st.empname+" "+st.empage); 
} 
} 
} 




Output

4 Surya 21
1 Ajith 23
6 Vijay 27


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Example 2: Sort by Integer Value (Decending Order) by using Comparable


import java.util.*; 

// User - Defined Class

class Employee implements Comparable<Employee>{ 
int empid; 
String empname; 
int empage; 
Employee(int empid,String empname,int empage){ 
this.empid=empid; 
this.empname=empname; 
this.empage=empage; 
} 
 
public int compareTo(Employee st){ 
if(empage==st.empage) 
return 0; 
else if(empage<st.empage) 
return 1; 
else 
return -1; 
} 
}



// Main Class



public class TestSort1{ 
public static void main(String args[]){ 
ArrayList<Employee> al=new ArrayList<Employee>(); 
al.add(new Employee(1,"Ajith",23)); 
al.add(new Employee(6,"Vijay",27)); 
al.add(new Employee(4,"Surya",21)); 
 
Collections.sort(al); 
for(Employee st:al){ 
System.out.println(st.empid+" "+st.empname+" "+st.empage); 
} 
} 
} 




Output


6 Vijay 27
1 Ajith 23
4 Surya 21