CS141: Topics for Week 11: Nov 6, 8 & 10, 2017

Computer Science Headlines: ACM Tech News

Reading:

• Chap 9: Inheritance
• Chp 10 Interfaces

Lab and Other Activities

• Lab 9: Inheritance
• We will look at a much simpler version of the Shape class used in the lab.

Lecture Discussion Topics:

• What is inheritance and why use it?
• Subclasses vs superclasses - See Hierarchy
• A class can have only one direct superclass. A class can have multiple subclasses.
• Don't overuse subclasses.
• Subclasses:
  • How are they defined (using extends)
  • Overriding methods
  • What variables and methods in the super class can be accessed by the subclass?
    • public vs protected: Protected modifier allows access by all subclasses and all classes in the same package (folder).
    • private variables in the superclass are not accessible from the subclass except using setter/getters.
    • Using super() in the first line of constructor
    • Using super.method_name() anywhere in a subclass method.
• Polymorphism:

      Employee[] employees = new Employee[3];
      employees[0] = new Employee();
      employees[1] = new HourlyEmployee();
      employees[2] = new SalariedEmployee();
      for (int i=0; i < employees.length; i++) {
  	     System.out.println("The employee is " + employees[i].getName() + 
  		               " and their pay is " + employees[i].calcPaycheck());
       }
  

  • Which calcPaycheck() method is called? Which toString() is called? The one in superclass (Employee) or in the subclass (HourlyEmployee, SalariedEmployee)?
• Abstract class:
  • Declare as

    public abstract class Employee { 
    	    public abstract double calcPaycheck(); 
    	    ... 
    

    Must define method in the subclass. Can no longer create an Employee object; can only create a subclass.
• Using the keyword final so that one can't override a method in the subclass.
• The universal super class: Object
  All Java classes are subclasses of the Object class and inherit the methods: toString(), equals(), hashCode(). These methods should generally be overriden.
• instanceof operator can be used to check what type of object you have. For example, if you need to cast an object. Should be used sparingly. Don't use as a way to implement different behavior, i.e. to bypass polymorphism.
• Interface - using the implements keyword.
  • An interface (typically) has no variables, only methods which are not implemented. Any variables must be static and final.
  • An interface is used to force a class to define a set of methods so as to force consistency across a set a classes.
  • Examples: The Comparable interface
  • A class can have only one superclass but can implement multiple interfaces.

Concepts and Vocabulary