Subclasses

COMP122 Lectures 2021-03-08

Inheritance

This is a mechanism of creating a new subclass based on an existing superclass, retaining a similar implementation.

This allows the following two:

• Abstraction - It allows to express an “is-a” relationship. Every instance of the subclass is also and instance of the superclass.
• Code re-use - A subclass inherits methods/attributes and only needs to implement what’s new.

Shapes Example

Our existing example was left with the following classes:

classDiagram
    class Circle{
    +colour String
    +radius double
    +area() double
    }
    class Square{
    +colour String
    +corners int
    +side double
    +area() double
    }
    class Triangle{
    +colour String
    +corners int
    +side double
    +area() double
    }

We can make this into a super-class of Shape buy reassigning redundant variables:

classDiagram
    Shape <|-- Circle
    Shape <|-- Square
    Shape <|-- Triangle
    Shape: +colour String
    class Circle{
    +radius double
    +area() double
    }
    class Square{
    +corners int
    +side double
    +area() double
    }
    class Triangle{
    +corners int
    +side double
    +area() double
    }

You could also make an additional intermediate class:

classDiagram
    Shape <|-- Circle
    Shape <|-- Polygon
    Shape: +colour String
    Polygon: +corners int
    Polygon: +side double
    Polygon: +area() double
    Polygon <|-- Square
    Polygon <|-- Triangle
    class Circle{
    +radius double
    +area() double
    }

This tree is called a class hierarchy

Always exclude all attributes and methods from the superclass in subclasses.

classDiagram
classA --|> classB : Inheritance
classC --* classD : Composition
classE --o classF : Aggregation
classG --> classH : Association
classI -- classJ : Link(Solid)
classK ..> classL : Dependency
classM ..|> classN : Realization
classO .. classP : Link(Dashed)

Java Syntax for Subclass Definitions

The Java keyword extends is used to specify the subclass/superclass relationship in class definitions.

public class Triangle extends Polygon {
	// public int colour;	// inherited from Shape
	// public int corners;	// inherited from Polygon
	public double side;	// only in Triangle
	public double area; () {...}
}

Single vs. Multiple Inheritance

classDiagram
    Shape <|-- Triangle
    MusicalInstrument <|-- Triangle
    Shape: +colour String
    MusicalInstrument: +pitch int
    class Triangle{
    +corners int
    +side double
    +area() double
    }

In Java this is not possible, every class has exactly one superclass, with the exception of Object.

The exception is because Object is the root class, meaning every class is a sub-class of Object.

What is Inherited?

A subclass inherits all public or protected members of its superclass. This includes:

• Attributes
• Methods

This does not include constructor methods as they are not members of the super-class.

protected

This is similar to private fields or methods but they cannot be accessed from outside an instance directly.

A protected field or method is inherited whereas a private identifier is only accessible in the class where it is defined.

In UML diagrams, # depicts protected class members.

Uses of Inheritance

• Inheritance can introduce more abstraction in the code.
• It enhances core re-use.
• It improves the code readability.
• Properly applied, inheritance can reduce software maintenance costs.