Understanding polymorphism

Polymorphism is one of the defining features of object-oriented languages. Java has expanded the concept by including interfaces. Let us begin with a simple hierarchy of classes as shown here:

public class SuperClass {
    
    protected int count;
    
    public void setCount(int count) {
       this.count = count;
    }
    
    public void displayCount() {
       System.out.printf("SuperClass count = %d%n", count);
    }
}

In this trivial class, we have a public method to assign a value to count and a second method that displays the value in count along with the name of the class. In the following code block, we have a class that uses SuperClass:

public class Polymorphism {
     private...