sort list of objects using comparator in java

one of the most important question for interviews of java programmer is how to sort list of objects in java , lets understand the solution using scenario of Patients , we will create a class Patient with two properties patientId and patientName and then we will provide options to sort list of patients by patientId and PatientName.

Step 1 :  create Patient.java class as below :

package com.sorting.sv;
public class Patient {
	private int patientId;
	private String patientName;
	public Patient() {
	}
	public Patient(int patientId, String patientName) {
		this.patientId = patientId;
		this.patientName = patientName;
	}
	public int getPatientId() {
		return patientId;
	}
	public void setPatientId(int patientId) {
		this.patientId = patientId;
	}
	public String getPatientName() {
		return patientName;
	}
	public void setPatientName(String patientName) {
		this.patientName = patientName;
	}
	@Override
	public String toString() {
		return "Patient [patientId=" + patientId + ", patientName=" + patientName + "]";
	}	
}

Step 2: Now In patient class we have two variables , patientId and patientName as we want to sort by both , lets create sorter classes by implementing Comparator interface. for sorting by patientId , create IdSorter.java class as below :

package com.sorting.sv;

import java.util.Comparator;

public class IdSorter implements Comparator<Patient> {

	@Override
	public int compare(Patient p1, Patient p2) {
	if(p1.getPatientId()>p1.getPatientId()) {
		return 1;
	}
	else if(p1.getPatientId()<p2.getPatientId()) {
		return -1;
	}else {
		return 0;
	}
	}

}

for sorting patients by name create NameSorter.java class as below :

package com.sorting.sv;
import java.util.Comparator;
public class NameSorter implements Comparator<Patient>{
 @Override
 public int compare(Patient o1, Patient o2) {	
	return o1.getPatientName().compareTo(o2.getPatientName());
 }
}

finally for testing write main class as below :

package com.sorting.sv;

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class TestSorting {
	public static void main(String[] args) {
		List<Patient> myPatientList = Arrays.asList(new Patient(3, "A"), new Patient(1, "C"), new Patient(2, "B"));
		Scanner scn = new Scanner(System.in);
		int option = 0;
		try {
			System.out.println("original list is :");
			for(Patient patient:myPatientList) {
			System.out.println(patient);
			}
			do {
			System.out.println("Enter 1 for Sort by Id");
			System.out.println("Enter 2 for Sort by Name");
			System.out.println("Enter 0 for EXIT:");
			option = scn.nextInt();
			if (option == 1) {
				Collections.sort(myPatientList, new IdSorter());
				System.out.println("List sorted by patientId");
				for(Patient patient:myPatientList) {
					System.out.println(patient);
				}
			}
			else if (option == 2) {
				System.out.println("List sorted by patientName");
				Collections.sort(myPatientList, new NameSorter());
				for(Patient patient:myPatientList) {
					System.out.println(patient);
				}
			}
			else if(option == 0) {
				System.out.println("Exited successfuly");
			}
			else {
				System.out.println("Please enter 1 or 2 or 3 :");
			}
			}while(option!=0);
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			scn.close();
		}
	}
}

Output :

original list is :
Patient [patientId=3, patientName=A]
Patient [patientId=1, patientName=C]
Patient [patientId=2, patientName=B]
Enter 1 for Sort by Id
Enter 2 for Sort by Name
Enter 0 for EXIT:
1
List sorted by patientId
Patient [patientId=1, patientName=C]
Patient [patientId=2, patientName=B]
Patient [patientId=3, patientName=A]
Enter 1 for Sort by Id
Enter 2 for Sort by Name
Enter 0 for EXIT:
2
List sorted by patientName
Patient [patientId=3, patientName=A]
Patient [patientId=2, patientName=B]
Patient [patientId=1, patientName=C]
Enter 1 for Sort by Id
Enter 2 for Sort by Name
Enter 0 for EXIT:
0
Exited successfuly

Thanks For Reading , Happy Learning.