Chapter 6 Inheritance

1
Inheritance
Inheritance
Chapter 6
Chapter 6
2
Outline
• Creating subclasses
• Class hierarchies
• The protected modifier
• Overriding methods
• Polymorphism via inheritance
3

Inheritance is the process in which a
new class is derived from an existing one

Inheritance allows a new class to be
written that extends an existing class

The existing class is called the parent
class, or super class, or base class

The derived class is called the child class
or subclass, derived class
A
B
B inherits A
Inheritance
Inheritance
4

Inheritance
Inheritance

The child class inherits characteristics of the parent class

That is, the child class inherits the variables and
methods defined by the parent class

The derivation process should establish a specific kind of
relationship between two classes: an is-a relationship

For example:
•
Car is-a Vehicle
•
Dictionary is-a Book
Book
Dictionary
Vehicle
Car
5
Creating subclasses
Creating subclasses

In Java, we use the reserved word extends to establish
an inheritance relationship
class Car extends Vehicle
{
// class contents
}

class Dictionary extends Book
{
// class contents
}
6
Outline
• Creating subclasses
• Class hierarchies
• The protected modifier
• Overriding methods
• Polymorphism via inheritance
7

A child class of one parent can be the parent of another
child

Furthermore, multiple classes can be derived from a
single parent

Therefore, inheritance relationships often develop into
class hierarchies
Class hierarchies
Class hierarchies
8

Two children of the same parent are called siblings

Common features should be put as high in the hierarchy

A child class inherits from all its ancestor classes

Animal
Snake Lizard
Bird
Parrot
Reptile
Class hierarchies
Class hierarchies
10
Example
Example

Cylinder inherits Circle
Circle
Cylinder
11
Example
Example
Circle
- radius : double
+ Circle()
+ Circle(r : double)
+ getRadius() : double
+ setRadius(r : double) : void
+ Area() : double
Cylinder
- length : double
+ Cylinder()
+ Cylinder(r : double, l : double)
+ getLength() : double
+ setLength(l : double) : void

+ Volume() : double
TestCylinder
+ main (args : String[]) : void
Class diagram for TestCylinder
12
Circle.java
Circle.java
public class Circle
{
private double radius;
public Circle()
{
radius = 1.0;
}
public Circle (double r)
{
radius = r;
}
public double getRadius()
{
return radius;
}
public void setRadius (double r)
{
radius = r;
}
public double Area()
{
return radius*radius*Math.PI;
}

}
Circle
- radius : double
+ Circle()
+ Circle(r : double)
+ getRadius() : double
+ setRadius(r : double) : void
+ Area() : double
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Cylinder.java
Cylinder.java
public class Cylinder extends Circle
{
private double length;
public Cylinder()
{
radius = 1.0; // ??
length = 1.0;
}
public Cylinder (double r, double l)
{
radius = r;
length = l;
}
public double getLength()
{
return length;
}
public void setLength (double l)
{

length = l;
}
public double Volume()
{
return Area() * length;
}
}

Cylinder
- length : double
+ Cylinder()
+ Cylinder(r : double, l : double)
+ getLength() : double
+ setLength(l : double) : void
+ Volume() : double
14
TestCylinder.java
TestCylinder.java
public class TestCylinder
{
public static void main (String[] args)
{
Cylinder c = new Cylinder (5.0, 5.2);
System.out.println(“The radius is “ + c.getRadius());
System.out.println(“The length is “ + c.getLength());
System.out.println(“The area of the circle is “ + c.Area());
System.out.println(“The volume of the cylinder is “ + c.Volume());
}
}
15

The Object class
The Object class

A class called Object is defined in the java.lang
package of the Java standard class library

All classes are derived from the Object class

If a class is not explicitly defined to be the child of an
existing class, it is assumed to be the child of the Object
class

Therefore, the Object class is the ultimate root of all
class hierarchies
16

The Object class contains a few useful methods, which
are inherited by all classes
•
For example, the toString method, the equals method is
defined in the Object class

The toString method in the Object class returns a
string that contains the name of the object’s class along
with some other information
•
We can override toString method in any class to get a more
meaningful string

The equals method in the Object class returns true if

two references are aliases
•
We can override equals method in any class to define equality in
some more appropriate way
The Object class
The Object class
28
Outline
• Creating subclasses
• Class hierarchies
• The protected modifier
• Overriding methods
• Polymorphism via inheritance
29
The protected modifier
The protected modifier

Variables and methods declared with private visibility
cannot be referenced in a child class

They can be referenced in the child class if they are
declared with public visibility - but public variables violate
the principle of encapsulation

There is a third visibility modifier that helps in inheritance
situations: protected
30
The protected modifier
The protected modifier


The protected modifier allows a child class to
reference a variable or method directly in the child class

It provides more encapsulation than public visibility, but
is not as tightly encapsulated as private visibility

A protected variable is visible to any class in the
same package as the parent class

Protected variables and methods can be shown
with a # symbol preceding them in UML diagrams
31
Example 1
Example 1
Circle
# radius : double
+ Circle()
+ Circle(r : double)
+ getRadius() : double
+ setRadius(r : double) : void
+ Area() : double
Cylinder
- length : double
+ Cylinder()
+ Cylinder(r : double, l : double)
+ getLength() : double
+ setLength(l : double) : void
+ Volume() : double
TestCylinder
+ main (args : String[]) : void

Class diagram for TestCylinder
32
Circle.java
Circle.java
public class Circle
{
protected double radius;
public Circle()
{
radius = 1.0;
}
public Circle (double r)
{
radius = r;
}
public double getRadius()
{
return radius;
}
public void setRadius (double r)
{
radius = r;
}
public double Area()
{
return radius*radius*Math.PI;
}
}
Circle
# radius : double

+ Circle()
+ Circle(r : double)
+ getRadius() : double
+ setRadius(r : double) : void
+ Area() : double
33
Cylinder.java
Cylinder.java
public class Cylinder extends Circle
{
private double length;
public Cylinder()
{
radius = 1.0;
length = 1.0;
}
public Cylinder (double r, double l)
{
radius = r;
length = l;
}
public double getLength()
{
return length;
}
public void setLength (double l)
{
length = l;
}
public double Volume()

{
return findArea() * length;
}
}

Cylinder
- length : double
+ Cylinder()
+ Cylinder(r : double, l : double)
+ getLength() : double
+ setLength(l : double) : void
+ Volume() : double
34
Example 2
Example 2
Book
# pages : int
+ Book (numPages : int)
+ setPages (numPages : int) : void
+ getPage() : int
Dictionary
- definitions : int
+ Dictionary (numPages : int, numDefinitions :
int)
+ computeRatio() : double
+ setDefinitions (numDefinitions : int) : void
+ getDefinitions() : int
Words
+ main (args : String[]) : void
Class diagram for Words

Class diagram for Words
35
Book.java
Book.java
public class Book
{
protected int pages;
public Book (int numPages)
{
pages = numPages;
}
public void setPages (int numPages)
{
pages = numPages;
}
public int getPages ()
{
return pages;
}
}
Book
# pages : int
+ Book (numPages : int)
+ setPages (numPages: int) : void
+ getPage() : int
36
Dictionary.java
Dictionary.java
public class Dictionary extends Book
{

private int definitions;
public Dictionary (int numPages, int numDefinitions)
{
pages = numPages;
definitions = numDefinitions;
}
public double computeRatio ()
{
return definitions / pages;
}
public void setDefinitions (int numDefinitions)
{
definitions = numDefinitions;
}
public int getDefinitions ()
{
return definitions;
}
}
Dictionary
- definitions : int
+ Dictionary (numPages : int, numDefinitions : int)
+ computeRatio() : double
+ setDefinitions (numDefinitions : int) : void
+ getDefinitions() : int
37
Words.java
Words.java
public class Words
{

public static void main (String[] args)
{
Dictionary w = new Dictionary (1500, 52500);
System.out.println ("Number of pages: " + w.getPages());
System.out.println ("Number of definitions: " + w.getDefinitions());
System.out.println ("Definitions per page: " + w.computeRatio());
}
}