Download Chapter 6 Inheritance and Abstract Classes

Chapter 6 Inheritance
Inheritance Hierarchies
Modeling Specialization and Generalization
Real world: Hierarchies describe
general/specific relationships
 General concept at root of tree
 More specific concepts are children
Programming: Inheritance hierarchy
 General superclass at root of tree
 More specific subclasses are children
When designing systems, look for instances
of generalization and specialization
Example
Hierarchy of Employee Classes
The Substitution Principle (1)
 Formulated by Barbara Liskov
 You can use a subclass object whenever a
superclass object is expected
example:
Employee e;
e = new Manager(“Barnie Smith”);
...
System.out.println("salary=" + e.getSalary());
 Can set e to Manager reference
 Polymorphism: Correct getSalary method is
invoked
The Substitution Principle (2)
 In Java, the type rules always allow a subclass
object to be used when a superclass object is
expected.
 However, the question is whether the subclass
object can replace the superclass one,
conceptually!
 Don't use inheritance if substitution principle is
violated
Invoking Superclass Methods (1)
 Can't access private fields of superclass
(If salary is a private fields in Employee)
public class Manager extends Employee
{
public double getSalary()
{
return salary + bonus;
// ERROR--private field
}
...
}
Invoking Superclass Methods (2)
(1st Trial to Solve)
Be careful when calling superclass method
public double getSalary()
{
return getSalary() + bonus;
// ERROR--recursive call
}
Invoking Superclass Methods (3)
(Solution) Use super keyword
public double getSalary()
{
return super.getSalary() + bonus;
}
Can you do super.super?
 No, super is not a reference
Invoking Superclass Methods (4)
(Another Trial to Solve)
public class Manager extends Employee
{
...
private double salary;
//ERROR-replicated field
}
Invoking Superclass Constructors
 Use super keyword in subclass constructor:
public Manager(String aName)
{
super(aName); // calls superclass constructor
bonus = 0;
}
 Call to super must be first statement in subclass
constructor
 If a subclass constructor does not call a superclass
constructor, then the superclass constructor with no
parameters is called automatically.
Hierarchy of Swing Components (1)
Base of hierarchy: Component
Most important subclass: Container
Hierarchy of Swing Components (2)
Graphic Programming with Inheritance
 Chapter 4: Create drawings by implementing Icon
interface type
 Now: Form subclass of JComponent
public class MyComponent extends JComponent
{
public void paintComponent(Graphics g)
{
drawing instructions go here
}
...
}
 Advantage: Inherit behavior from JComponent
 Example: Can attach mouse listener to JComponent
Graphic Programming with Inheritance
Overriding paintComponent (1)
 Draw a car:
public class CarComponent extends JComponent
{
public void paintComponent(Graphics g)
{
Graphics2D g2 = (Graphics2D)g;
car.draw(g2);
}
...
private CarShape car;
}
Mouse Listeners
To Complete the car drawing program (1)
 Attach mouse listener to component
 Can listen to mouse events (clicks) or mouse
motion events
public interface MouseListener
{
void mouseClicked(MouseEvent event);
void mousePressed(MouseEvent event);
void mouseReleased(MouseEvent event);
void mouseEntered(MouseEvent event);
void mouseExited(MouseEvent event);
}
Mouse Listeners
To Complete the car drawing program (2)
public interface MouseMotionListener
{
void mouseMoved(MouseEvent event);
void mouseDragged(MouseEvent event);
}
Mouse Adapter
To Complete the car drawing program (3)
 What if you just want to listen to mousePressed?
Listener interface types with many methods
have corresponding adapter classes with donothing methods. Extend the adapter rather than
implementing the listener.
Mouse Adapter
To Complete the car drawing program (4)
public class MouseAdapter implements MouseListener
{
public void mouseClicked(MouseEvent event) {}
public void mousePressed(MouseEvent event) {}
public void mouseReleased(MouseEvent event) {}
public void mouseEntered(MouseEvent event) {}
public void mouseExited(MouseEvent event) {}
}
Mouse Adapter
To Complete the car drawing program (5)
 Extend MouseAdapter
 Component constructor adds listener:
addMouseListener(new
MouseAdapter()
{
public void mousePressed(MouseEvent event)
{
mouse action goes here
}
});
Car Drawing Program
ch6/car/CarShape.java
ch6/car/CarComponent.java
ch6/car/CarMover.java
Abstract Classes (1)
An abstract method is undefined and must
be defined in a subclass.
A class with one or more abstract class
methods must be declared as an abstract
class.
public abstract class SelectableShape
implements SceneShape
{…}
Abstract Classes (2)
 You can not construct an
object of an abstract
class.
 SelectableShape shape =
new SelectableShape();
//Error
 SelectableShape shape =
new HouseShape();
//OK
Abstract Classes (3)
SceneShape.java
SelectableShape.java
HouseShape.java
CarShape.java
ScenePanel.java
SceneEditor.java