Collections.sort() for ArrayList of objects using a method reference from object class

Question

I want to write a program such that:

1) A class Student (with name, id, batch)

2) In that class, write a int compareByBatch(Student s1, Student s2) method

3) Use collections.sort() to compare objects

4) Use the compareByBatch method as the second argument to the sort method.

I am able to write the program as follows:

Student.java class file:

public class Student {
    private String id;
    private String name;
    private int Batch;

    //Added a class constructor getters and setters

     public static Comparator<Student> studBatch = new Comparator<Student>() {

            public int compare(Student s1, Employee s2) {

               int batch1 = s1.getBatch();
               int batch2 = s2.getBatch();

               //For ascending order
               return batch1-batch2;
           }};
}

Then, in the Main file added the following:

Created an ArrayList and added records in it:

ArrayList<Student> stud = new ArrayList<>();

stud.add(new Student("A01", "John Doe", 101));

stud.add(new Student("A02", "Mike Gaman", 102));

Collections.sort(stud, stud.studBatch);

The above code works fine and I get the sorted records by Batch.

But, my question is, how to write a code as per my coding requirements which I have mentioned in the beginning i.e. using compareByBatch() method inside Student class and using in with Collections.sort()? I need to pass a method reference from object class as the second argument for the Collections.sort().

Answer 1

I suppose the purpose of this exercise is for you to learn to use method reference syntax.

If you look at Collections.sort, it requires a Comparator<T> as the second parameter. This Comparator<T> can actually be represented by a method of the signature int compare(T obj1, T obj2). This is one of those functional interfaces.

So instead of passing it an implementation of Comparator<T> as an object, you can just pass it a method reference.

Collections.sort(stud, Student::compareByBatch);

Notice how I used ::. That is the syntax you need to use to denote a method reference.

And your compareByBatch method can just be like this:

public static int compareByBatch(Student s1, Student s2) {
    int batch1 = s1.batch;
    int batch2 = s2.batch;
    return batch1-batch2;
}

By the way, your compareByBatch is not a very good implementation. You should probably check for nulls first and use Integer.compare(batch1, batch2) instead of just subtracting them. See here for why.