Comparable Interface

Syntax:

 public int compareTo(element e); 

Rules For Using Comparable Interface

• The class has to implement a comparable Interface.
• Import Comparable Interface from Java.Lang package.
• Specify the generic type of which object to be compared.
• Override the compareTo() in such a way it has the logic of sorting. 

Example: Sorting Student Objects by Name

package quipoin;

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

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

	@Override
	public String toString() {
		return id + "\t" + name + "\t" + age;
	}

	@Override
	public int compareTo(Student std) {
		// return this.id - std.id;                          // sorting based on the id
		return this.name.compareTo(std.name);                // sorting based on the name
	}
}

package quipoin;

import java.util.TreeSet;

public class TestStudent {
	public static void main(String[] args) {
		Student s1 = new Student(102, "Charan", 25);
		Student s2 = new Student(103, "Abhi", 15);
		Student s3 = new Student(101, "Bharath", 30);

		TreeSet<Student> ts = new TreeSet<>();
		ts.add(s1);
		ts.add(s2);
		ts.add(s3);

		System.out.println("Student records:");
		System.out.println("--------------------");
		System.out.println("Id\tName\tAge");
		System.out.println("--------------------");

		for (Student std : ts) {
			System.out.println(std);
		}
	}
}

Output:

Student records:
--------------------
Id	Name	Age
--------------------
103	Abhi	15
101	Bharath	30
102	Charan	25

Comparator Interface

Syntax: 

public int compare(element e1, element e2);

Rules For Using Comparator Interface

• The class has to implement a comparator Interface.
• Import Comparator Interface from java.util package.
• Specify the generic type of object that needs to be compared.
• Create an object of that specific class and pass the object reference as a parameter to the constructor.
• Override the compare() method to provide the sorting logic.

Example: Sorting Mobile Objects by Brand and Price

package quipoin;

public class Mobile {

	public String brand;
	public double price;

	public Mobile(String brand, double price) {
		this.brand = brand;
		this.price = price;
	}

	@Override
	public String toString() {
		return brand + "\t" + price;
	}
}

package quipoin;

import java.util.Comparator;

public class SortByBrand implements Comparator<Mobile>{
	
	@Override
	public int compare(Mobile m1, Mobile m2) {
		return m1.brand.compareTo(m2.brand);
	}
}

package quipoin;

import java.util.Comparator;

public class SortByPrice implements Comparator<Mobile> {
	
	@Override
	public int compare(Mobile m1, Mobile m2) {
		return (int) (m1.price - m2.price);
	}
}

package quipoin;

import java.util.Iterator;
import java.util.TreeSet;

public class TestMobile {

	public static void main(String[] args) {
		Mobile m1 = new Mobile("Samsung", 22500.00);
		Mobile m2 = new Mobile("Iphone", 152500.00);
		Mobile m3 = new Mobile("Nokia", 15500.00);
		Mobile m4 = new Mobile("Realme", 18500.00);

		// SortByBrand br=new SortByBrand();
		SortByPrice pr = new SortByPrice();
		TreeSet<Mobile> ts = new TreeSet<>(pr);

		ts.add(m1);
		ts.add(m2);
		ts.add(m3);
		ts.add(m4);

		System.out.println("Mobile details:");
		System.out.println("-----------------");
		System.out.println("Brand\tPrice");
		System.out.println("-----------------");
		Iterator<Mobile> it = ts.iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}
}

Output:

Mobile details:
-----------------
Brand	Price
-----------------
Iphone	152500.0
Nokia	15500.0
Realme	18500.0
Samsung	22500.0

• The mobiles are sorted by price in ascending order.
• If we use sortByBrand, they will be sorted alphabetically by brand.

Difference between Comparable Vs Comparator