Polymorphism

Polymorphism is a term from biology, in which an organism can take many forms or stages.

In Java, polymorphism allows us to use superclass references to subclass objects.

Recall the Person class hierarchy, to which we will add SoftwareDeveloper and DatabaseAdmin classes.

We can assign an Employee or DataAnalyst object to a Person reference.

Person p1 = new Employee();
Person p2 = new DataAnalyst();

This creates two objects on the heap.

Practice Exercise

Polymorphism is based on the "IS-A" relationship. We can assign DataAnalyst objects to Person references because DataAnalyst IS-A Person (via Employee).

When Is a Superclass Reference Useful?

Superclass references to subclass objects are useful for collections and methods.

Collections

If we have an array of superclass references, we can store subclass objects. * This allows us to create an Employee array to store all employees in an organization, for example.

Employee[] emps = new Employee[20];

emps[0] = new DataAnalyst();
emps[1] = new SoftwareDeveloper();

Methods

We know we can assign a SoftwareDeveloper or DataAnalyst object to an Employee reference, because each subclass is-a Employee.

Employee persEmp = new SoftwareDeveloper();
Employee persData = new DataAnalyst();

Imagine we wanted to print the name of anyone that is-a Employee. * Instead of a separate method for every subclass of Employee...

public void printEmpName(DataAnalyst a) {
  // ...
}

public void printEmpName(SoftwareDeveloper s) {
  // ...
}

public void printEmpName(DatabaseAdmin a) {
  // ...
}

... we can have one method with an Employee reference parameter.

public void printEmpName(Employee e) {
  System.out.println(e.getName());
}

The method does not care if the reference points to an Employee object or subtype of Employee, just that the object is-a Employee.

Drill

Polymorphism/com.example.polymorphism.drills.employee.EmployeePrintingApp

• private void run()
• Create an array of type Employee with space for 3 Employees
• Create a DataAnalyst, SoftwareDeveloper, and DatabaseAdmin using the multi-arg constructors for each, and add each object to the array.
• Call processEmployees and pass the Employee array reference.
• Add a method printEmployeeNameAndTitle that takes an Employee reference and prints name - title to the screen.
• public void processEmployees(Employee[] employees)
• Loop through the Employee array and call printEmployeeNameAndTitle for each Employee.

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