1. MSIS 670 Object-Oriented Software Engineering
Week 5
Object-Oriented Programming (1):
Inheritance
In this chapter you will learn:
How inheritance promotes software reusability.
The notions of superclasses and subclasses.
To use keyword extends to create a class that inherits attributes and behaviors from another class.
To use access modifier protected to give subclass methods access to superclass members.
To access superclass members with super.
How constructors are used in inheritance hierarchies.
The methods of class Object, the direct or indirect superclass of all classes in Java.
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5.1

2. Object-Oriented Programming
Inheritance
Software reusability
Classes are created from existing ones
Polymorphism
Enables developers to write programs in general fashion – handle variety of existing and yet-to-be specified classes
Helps add new capabilities to a system
Inheritance
Software reusability
Create new class from existing class
Absorb existing class’s data and behaviors
Enhance with new capabilities
Subclass extends superclass
Subclass
More specialized group of objects
Behaviors inherited from superclass
Can customize
Additional behaviors
Class hierarchy
Direct superclass
Inherited explicitly (one level up hierarchy)
Indirect superclass
Inherited two or more levels up hierarchy
Single inheritance
Inherits from one superclass
Multiple inheritance
Inherits from multiple superclasses
Java does not support multiple inheritance
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3. Superclasses and Subclasses
“Is a” Relationship
Object “is an” object of another class
Rectangle “is a” quadrilateral
Class Rectangle inherits from class Quadrilateral
Form tree-like hierarchical structures
Object of one class “is an” object of another class
Superclass typically represents larger set of objects than subclasses
Example:
superclass: Vehicle (includes cars, trucks, boats, bicycles, …)
subclass: Car (smaller, more-specific subset of vehicles)
Examples of Superclasses-Subclasses
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4. Constructors in Subclasses
Superclass constructor
Initializes superclass instance variables of subclass
Not inherited by subclass (only variables and methods)
Called by subclass
Implicitly or explicitly with super reference
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The Java compiler sets the superclass of a class to Object when the class declaration does not explicitly extend a superclass.
Java ensures that even if a constructor does not assign a value to an instance variable, the variable is still initialized to its default value (e.g., 0 for primitive numeric types, false for booleans, null for references).

5. Software Engineering with Inheritance
Create class (subclass) from existing one (superclass)
Subclass creation does not affect superclass
New class inherits attributes and behaviors
Software reuse
In object-oriented system, classes are often related. “Factor out” common attributes and behaviors and place these in a superclass. Use inheritance to form subclasses without having to repeat common attributes and behaviors.
Changes in subclasses does not affect the superclass, and thus, no other subclasses either. Changes in superclass may not affect all the subclasses unless the modification is done in the public interfaces to the superclass.
Why do we extend a class? For code reuse; the structure and behavior already implemented by one class may be used to define other classes. If someone else has already written a class that satisfies 85% of your need, you can just extend the class and save time by implementing only the remaining 15% of functionality.
Copying and pasting code from one class to another can spread errors across multiple source code files. To avoid duplicating code (and possibly errors), use inheritance, rather than the “copy-and-paste” approach, in situations where you want one class to “absorb” the instance variables and methods of another class.
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6. Composition vs. Inheritance
“Has a” relationship
Employee has a TelephoneNumber
Inheritance
“Is a” relationship
Teacher is an Employee
Composition is implemented by having instance variables of that type (composite class).
Inheritance is implemented by subclass extending the superclass to inherit the attributes and behaviors of superclass.
There may be multiple components in a class; there is only one (direct) superclass for a given class in Java (multi-inheritance is possible in some other languages).
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10. Composition
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11. Lab activities (Week 5)
Rewrite the Point and Circle program of the examples as a Point and Rectangle program and test them. Let the Point coordinators as one corner of the Rectangle and set a length and a width instead of a radius.
If you have time, use composition instead of inheritance; will the test program work as is?
For this assignment, use the sample classes.
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