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Using comparable and comparator in java collection


   Comparable

   Comparator

1 this interface allows the object to compare itself with another object while sorting the list of such type objects. This interface provide the provision to compare two different objects.
2 The objects those implements Comparable interface are sorted automatically without specifying any other comparator. Comparator is passed as arguments to the method who need to sort some objects.
3 less flexible more flexible
4 uses compareTo(object2) method it has compare(object1,object2) method
5 this interface is not a part of collection API. It belongs to java.lang Packages this interface belongs to java.util package.

Using Comparable
package collection.examples;

import java.util.ArrayList;
import java.util.Collections;

class Contact implements Comparable<Contact>{
    String name;
    String email;
    int phone;
    public Contact(String name, String email, int phone) {
        this.name = name;
        this.email = email;
        this.phone = phone;
    }
    public int compareTo(Contact otherContact) {
        return this.name.compareTo(otherContact.name);
    }

    @Override
    public String toString() {
        return "Contact{" + "name=" + name + ", email=" + email + ", phone=" + phone + '}';
    }
}

 public class UsingComparable {
    public static void main(String[] args) {
        Contact c1=new Contact("rbc","mail1@gmail.com",2424342);
        Contact c2=new Contact("tbc","mail2@gmail.com",2424342);
        Contact c3=new Contact("bcc","mail3@gmail.com",2424342);
        ArrayList<Contact> al=new ArrayList<Contact>();
        al.add(c1);
        al.add(c2);
        al.add(c3);
        Collections.sort(al);
        for (Contact contact : al) {
            System.out.println(contact);
        }
    }
}

Using Comparator 
 package collection.examples;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

class Contact1 {
    String name;
    String email;
    int phone;
    public Contact1(String name, String email, int phone) {
        this.name = name;
        this.email = email;
        this.phone = phone;
    }
 
    @Override
    public String toString() {
        return "Contact1{" + "name=" + name + ", email=" + email + ", phone=" + phone + '}';
    }
    
}
class MyComparator implements Comparator<Contact1>{

    public int compare(Contact1 o1, Contact1 o2) {
        //comparing name from both objects
        return o1.name.compareTo(o2.name);
    }   
}
public class UsingComparator {
    public static void main(String[] args) {
        Contact1 c1=new Contact1("zzz","mail1@gmail.com",2424342);
        Contact1 c2=new Contact1("bbb","mail2@gmail.com",2424342);
        Contact1 c3=new Contact1("ccc","mail3@gmail.com",2424342);
        ArrayList<Contact1> al=new ArrayList<Contact1>();
        al.add(c1);
        al.add(c2);
        al.add(c3);
        Collections.sort(al, new MyComparator());
        for (Contact1 contact : al) {
            System.out.println(contact);
        }
    }
}

Comments

while using comparable interface in Java and overriding compareTo method its worth noting that compareTo must be compatible with s equals method in Java i.e. if two objects are equal via equals method compareTo method must return "0" for them, failing this may result in some subtle bug when you store those objects in collection class like TreeSet and TreeMap.

Source: How to use Comparator and Comparable in Java

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