Mastering Abstract Classes and Interfaces in Java

1. Lecture 17 • Abstract classes • Interfaces • The Comparable interface • Event listeners • All in chapter 10: please read it

2. Abstract classes and abstract methods • Abstract methods define a method signature but nothing else • They must be implemented in subclasses • If a class has an abstract method, it must also be abstract

3. Example from chapter 10 [public] abstract class GeometricObject{ private String name; private String dateCreated; private Color color; [public] abstract double getArea(); // don’t know how to compute it until we know what kind of object it is [public] abstract double getPerimeter(); // same story }

4. Implementation is in subclass [public] class Circle extends GeometricObject{ private double radius; … public double getArea(){ return radius*radius*Math.PI; } }

5. Points to note • You cannot construct an object of an abstract class • Instead, you construct an object of one of the subclasses that implements the abstract methods • Do not use the keyword implements • It isn’t needed because the subclass must implement the abstract methods of the superclass– unless it is also abstract • An abstract class can still have constructors, which can then be invoked using super in the subclasses (constructor chaining)

6. Points to note, continued • An abstract class can be used as a datatype, for example: • GeometricObject[] geoObjs = new GeometricObject[10]; • Here it is not the geometric object that is being constructed with “new”, but the array of references to possible future GeometricObjects

7. Interfaces • An interface is like an abstract class, but different. • It contains only constants and abstract methods • This is different from an abstract class which can have data fields and concrete (non-abstract) methods • You cannot construct an interface object. Instead you construct objects of classes that implement the interface • These classes are not subclasses of the interface, so the keyword implements is used • You cannot construct an interface object • However, you can declare variables to have a type interface, and such a variable can reference any instance of a class that implement the interface

8. The Comparable interface • Here is the declaration in java.lang: public interface Comparable{ public int compareTo(Object o); } • The documentation explains that compareTo should return -1 if the implicit parameter object is “less than” the explicit parameter object, +1 if it is “greater than”, and 0 if it is “equals” • It also “strongly recommends” that it should return 0 only in the case that the equals method would returns true for the same pair of objects • Actually any int value can be returned; only the sign is relevant.

9. Implementing Comparable • in GeometricObject • in Date (instead of “later”)

10. A Max Function //returns the “bigger” of 2 objects publicstatic Comparable maxObj(Comparable o1, Comparable o2){ if(o1.compareTo(o2)>= 0){ return o1; } return o2; }

11. The ActionListener Interface • In our GUI program, we can add a new inner class which implements ActionListener, providing the actionPerformed method class YesListener implements ActionListener{ public void actionPerformed(ActionEvent e){ // take some action, such as print msg } } • After we construct such a listener we can add it to our “yes” button (for example)