The Object Class, Abstract Classes, Interfaces, Polymorphism

• The topics covered here are related to inheritance.

• Every class in Java is descended from the java.lang.Object class.
  • If no inheritance is specified when a class is defined, the superclass of the class is Object by default.
    • Every class, whether superclass or subclass, inherits from the Object class in Java.
    • Two notable methods that every class inherits from Object are toString and equals.
      • A third method that every class inherits from Object is clone (Link)

• An abstract class is used solely for creating subclasses.
  • You can't instantiate an object of an abstract class using new.
  • So, an abstract class is usually a highly general-sounding class, like GeometricObject or Student.

• See the Student ← CompSciStudent abstract class inheritance example.

• If a class contains an abstract method (like the Student class does), the class itself has to be declared abstract.
  • Abstract methods are declared, but not defined, e.g. the Student class' getRemainingHours() method.
  • An abstract class' subclasses MUST include and override the abstract methods in the abstract superclass.
    • So, the CompSciStudent subclass MUST contain a definition of the getRemainingHours() method or else an error will occur.
    • In this way, an abstract superclass can impose certain restrictions (or requirements) on what its subclasses can do.

• An interface is similar to an abstract class in that a class that implements an interface is REQUIRED to define certain methods.
• An interface must only contain constants and abstract methods, no variables.
  • The intent of interfaces is to ensure the behavior of objects.

• See the GradedActivity ← FinalExam3 Relatable interface example

• GUI Example: DogYearsListen.java

          // The ActionListener interface contains only one abstract
          // method called actionPerformed().
          // Because the class ConvertBtnListener implements the
          // ActionListener interface, it is required to define the
          // actionPerformed() method.
          class ConvertBtnListener implements ActionListener {
                  public void actionPerformed(ActionEvent e) {

• One of the foundations of object-oriented programming (along with encapsulation and inheritance).

• Related to inheritance
  • A subclass object is an object of its superclass, but not vise versa.
  • In the GeometricObject1 <- Triangle example, when you construct the Triangle object tri, tri is a Triangle. At the same time, tri is a GeometricObject1.
    • If you construct a GeometricObject1 object geo, geo is a GeometricObject1, but geo is not a Triangle.

• Polymorphism means that an object of a subclass can be used wherever its superclass object is used.

• Example: (main class only)

          // Assume that the GeometricObject1, Circle4 and
          // Rectangle1 classes are defined already:

          public class PolymorphismDemo {

            public static void main(String[] args) {

                // Display Circle and rectangle properties
                displayObject(new Circle4(1, "red", false));
                displayObject(new Rectangle1(1, 1, "black", true));
            }



            /** Method to display geometric object properties */
            // (POLYMORPHISM BEING EMPLOYED HERE:)
            public static void displayObject(GeometricObject1 object) {
                System.out.println("Created on " + object.getDateCreated() +
                                   ". Color is " + object.getColor());
            }

          }

• The above example demonstrates that you can always pass an instance of a subclass (Circle4 or Rectangle1) to a parameter of its superclass (GeometricObject1).

• Example:

        public class DynamicBindingDemo {

            public static void main(String[] args) {
    
                m(new GraduateStudent());
                m(new Student());
                m(new Person());
                m(new Object());
            }



            // (POLYMORPHISM BEING EMPLOYED HERE:)
            public static void m(Object x) {
                System.out.println(x.toString());
            }
        }


        class GraduateStudent extends Student {
        }


        class Student extends Person {
            public String toString() {
                return "Student";
            }
        }


        class Person extends Object {
            public String toString() {
                return "Person";
            }
        }

• See also the methods in the Relatable interface above.
  • The equals, isGreater and isLess methods take one GradedActivity object as an input parameter, but any subclass of GradedActivity such as FinalExam3 can use the three methods because of the property of polymorphism.

        public interface Relatable
        {
           public abstract boolean equals(GradedActivity g);
           public abstract boolean isGreater(GradedActivity g);
           public abstract boolean isLess(GradedActivity g);
        }

• Polymorphism can help to reduce the number of listeners that are required for GUI components.
  • Example: FractionGUI