1. Lecture 10: Inheritance Subclasses and superclasses
The inheritance chain
Access control
The Object cosmic superclass
The super keyword
Overriding methods
Overriding vs overloading

2. Read Chapter 10
Chapter 7, 10 and 11 are the most important chapters in the book for OOP.
Make sure to read and understand well.

3. Subclasses and Superclasses
Examples
Circle extends GeometricObject
Rectangle extends GeometricObject
GeometricObject extends Object
Example
Time extends Date
Date extends Object
JOptionPane extends JComponent
JComponent extends Container
Container extends Component
Component extends Object
In each case, a new subclass extends the superclass
The subclass typically has additional data fields (instance variables) and/or additional methods

4. class Time extends Date
Additional data fields (instance variables):
hour (int between 0 and 23)
minute (int between 0 and 59)
second (int between 0 and 59)
Additional method
public boolean morning(){
return (hour < 12) }

5. Sub or Super? The Time class So why is it called sub, not super ?
extends the Date class
has additional data fields and methods
has more capabilities than the Date class
So why is it called sub, not super ?
The set of objects of class Time is a subset of the set of objects of class Date
The set of objects of class Date is a superset of the set of objects of class Time

6. Why Inheritance? Code re-use
By extending an exisiting class, we can inherit its methods as well as add new capabilities

7. Subclass Methods
Subclass can inherit methods of the superclass. This happens by default.
Example: Time inherits dayOfWeek from Date
Subclass can override methods of the superclass by defining a method with the same signature (reminder: what is this?)
Example: Time should override Date’s toString
Subclass can provide additional methods not in the superclass
Example: Time provides morning

8. Subclass Data Fields
Subclass can inherit data fields of the superclass. This happens by default, unless they are private, which is typically a good idea – then they are only accessed by methods of the superclass.
Example: year, month, day
Subclass can shadow data fields of the superclass. This happens when new data fields with the same name are used. Typically not advisable.
Subclass can define new data fields.
Example: hour, minute, second.

9. Inheritance Chains Suppose C extends B and B extends A
Then we say A is part of C’s inheritance chain, and that C is descended from A
We also say (somewhat confusingly) that C is a subclass of A
We can be more specific by saying that C is an immediate subclass of B and B is an immediate subclass of A
Likewise A is the immediate superclass of B and B is the immediate superclass of A
Notice that a class only has one immediate superclass, but it could have many immediate subclasses.

10. Access Control public: available to all methods in all classes
package (default): available to methods in classes that are in the same package
protected: available to methods in the same class and all of its subclasses
private: available only to methods in the same class. Usually the right choice for all data fields and perhaps for some methods.

11. The Object Cosmic Superclass
Every class is descended from the Object class
Thus, every class can call or override the methods in the Object class (except any private ones)
Date overrides Object’s toString
Time overrides Date’s toString

12. The Super Keyword Similar usage to this
Used in constructor for the subclass to call the constructor for the (immediate) superclass.
Must be the first statement in the constructor.
If omitted, there is an understood constructor call super()
If there isn’t any “no-arg” constructor in the superclass, this will give an error.
Constructor chaining: constructor for C calls constructor for immediate superclass B which calls constructor for immediate superclass A, etc.
super.super.p() does not work!

13. The Super Keyword, continued
Super can also be used to call methods from the superclass: for example, Time.toString might call Date.toString via super.toString()
If forget to write super, what happens?
If write Date.toString(), what happens?
Super can also be used to access data fields in the (immediate) superclass, as long as they are not private. But this is usually not necessary, unless the data field is shadowed.

14. Overriding vs. Overloading
When we have several methods with different signatures, this is called overloading
When a method in one class has the same signature as the method in a superclass, this is called overriding
Class A might have 2 methods (maybe constructors) with the same name and different signatures, so the method is overloaded, but its subclass B might override only one of them.