How to sort list of objects using multiple fields?

We can sort list using multiple fields with Comparator. Here we are going to see two ways to do that.

Let’s say we have a Student class as below.

public class Student {
private int rollNum;
private String firstName;
private String stream;
public Student(int rollNum, String firstName, String stream) {
super();
this.rollNum = rollNum;
this.firstName = firstName;
this.stream = stream;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public int getRollNum() {
return rollNum;
}
public void setRollNum(int rollNum) {
this.rollNum = rollNum;
}
public String getStream() {
return stream;
}
public void setStream(String stream) {
this.stream = stream;
}
@Override
public String toString() {
return stream +" " +firstName +" "+ rollNum;
}
}

And a list of Students.

private List<Student> getStudentList() {
return Arrays.asList(new Student(1, "Bindu", "IT"), 
new Student(2, "Sam", "CS"), 
new Student(3, "David", "IT"),
new Student(4, "Anjali", "CS"));}

We can compare the list using multiple fields using two methods as below:

1. Anonymous Class

We can to make use of compare method in Comparator interface to compare using multiple fields. Comparator is a functional interface, which mean it has only one abstract method compare. We are going to override this method in anonymous class.

private List<Student> sortWithAnymClass(List<Student> list) {
Collections.sort(list, new Comparator<Student>() {
// compare using stream and then rollNumber;
@Override
public int compare(Student s1, Student s2) {
if (s1.getStream().equals(s2.getStream())) {
if (s1.getRollNum() == s2.getRollNum())
return 0;
if (s1.getRollNum() > s2.getRollNum())
return 1;
return -1;
}
return s1.getStream().compareTo(s2.getStream());
}
});
return list;
}

2. Using lambda expression

private List<Student> sortWithLambdaExp(List<Student> list) {
// sort using stream and firstName
list.sort(Comparator.comparing((Student s) -> s.getStream()).thenComparing((Student s) -> s.getFirstName()));
return list;
}

Originally published at https://www.thecodecity.com on May 10, 2022.

https://github.com/bindushrestha/comparator-sample