An Easier Way of Comparing Multiple Fields

Suppose that we have a Squirrel class and assume that the species name will never be null. We could write a constructor to enforce that if we wanted to:

public class Squirrel {
private int weight;
private String species;

public Squirrel(String theSpecies) {
if (theSpecies == null)
throw new IllegalArgumentException();
species = theSpecies;
}

public int getWeight() {
return weight;
}

public void setWeight(int weight) {
this.weight = weight;
}

public String getSpecies() {
return species;
}
}

We want to write a Comparator to sort by species name. If two squirrels are of the species, we want to sort the one that weighs the least first.
With the introduction of static and default methods on interfaces within Java 8, there are now some new helper methods on Comparator. The code could be written as this:

import java.util.Comparator;

public class ChainingComparator implements Comparator<Squirrel> {
@Override
public int compare(Squirrel s1, Squirrel s2) {
Comparator<Squirrel> c = Comparator.comparing(s -> s.getSpecies());
c = c.thenComparingInt(s -> s.getWeight());
return c.compare(s1, s2);
}
}

How to sort an array using Comparable and Comparator interfaces

In order to sort an array in Java using Comparable and Comparator interfaces, first, you may create a Student class as follows :

class Student implements Comparable<Student> {

String id;

String name;

Double cgpa;

public Student(String studentId, String studentName, double studentCGPA) {

id = studentId;

name = studentName;

cgpa = studentCGPA;

}

public String toString() {

return " \n " + id + "  \t  " + name + "  \t  " + cgpa;

}

public int compareTo(Student that) {

return this.id.compareTo(that.id);

}

}

Then, you may add another class entitled ComparatorTest2 as follows :

import java.util.Arrays;

import java.util.Comparator;

class CGPAComparator implements Comparator<Student> {

    public int compare(Student s1, Student s2) {

        return (s1.cgpa.compareTo(s2.cgpa));     

    }    

}

class ComparatorTest2 {

    public static void main(String []args) {

        Student []students = {  new Student("cs011", "Lennon  ", 3.1), 

new Student("cs021", "McCartney", 3.4),  

new Student("cs012", "Harrison ", 2.7),    

new Student("cs022", "Starr ", 3.7) };

        System.out.println("Before sorting by CGPA ");

        System.out.println("Student-ID \t  Name \t  CGPA (for 4.0) ");

        System.out.println(Arrays.toString(students));

        

        Arrays.sort(students, new CGPAComparator());

        

        System.out.println("After sorting by CGPA");

        System.out.println("Student-ID \t  Name \t  CGPA (for 4.0) ");

        System.out.println(Arrays.toString(students));

    }    

}

If you run this program you would get the following output :

Before sorting by CGPA 

Student-ID  Name  CGPA (for 4.0) 

[ 

 cs011    Lennon      3.1,  

 cs021    McCartney    3.4,  

 cs012    Harrison    2.7,  

 cs022    Starr    3.7]

After sorting by CGPA

Student-ID  Name  CGPA (for 4.0) 

[ 

 cs012    Harrison    2.7,  

 cs011    Lennon      3.1,  

 cs021    McCartney    3.4,  

 cs022    Starr    3.7]

That's it, as you might have noticed, the students array was sorted according to cgpa field.