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{
@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 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.