8.1 – Inheritance
Inheritance is a fundam ental technique
Inheritance is a fundam ental technique
used to create and organize
used to create and organize reusable
reusable cla sses
classes
Inheritance
Inheritance allows a software developer
allows a softwar e developer
to derive a new class from an existing one
to derive a new class from an existing one
The existing class is c alled
The existing class is c alled
the
the parent c lass,
parent class, or
or super class
superclass
The derived class is ca lled
The derived class is ca lled
the
the child cla ss
child class or
or subc lass
subclass
As the name implies,
As the name implies,
the child
the child in herits
inherits c haracteristics of the parent
characteristics of the parent
(the methods and data defined by the parent class)
(the methods and data defined by the parent class)
Proper inheritance cre ates an
Proper inheritance cre ates an is-a
is-a relat ionship:
relationship:
=> the child
=> the child is a
is a more specif ic version of the parent
more specific version of the parent
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8.1 – Inheritance
A programmer can tail or a derived class as needed
by adding new varia bles or methods,
or by modifying t he inherited ones
By using existing com ponents to create new ones,
we capitalize on all the effort that went
into the design, implem entation, and testing of the existin g
software
extends is used to establish a n is-a (inheritance) relationship
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class Car extends Vehicle
{
// class contents
}
8.1 – Class Diagram for Words
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Inheritance symbol
8.1 – Words.java
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//********************************************************************
// Words.java Java Foundations
//
// Demonstrates the use of an inherited method.
//********************************************************************
public class Words
{
//-----------------------------------------------------------------
// Instantiates a derived class and invokes its inherited and
// local methods.
//-----------------------------------------------------------------
public static void main (String[] args)
{
Dictionary webster = new Dictionary();
System.out.println ("Number of pages: " + webster.getPages()) ;
System.out.println ("Number of definitions: " +
webster.getDefinitions());
System.out.println ("Definitions per page: " +
webster.computeRatio());
}
}
8.1 – Book.java
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//********************************************************************
// Book.java Java Foundations
//
// Represents a book. Used as the parent of a derived class to
// demonstrate inheritance.
//********************************************************************
public class Book
{
protected int pages = 1500;
//----------------------------------------------------------------
// Pages mutator.
//----------------------------------------------------------------
public void setPages (int numPages)
{
pages = numPages;
}
//----------------------------------------------------------------
// Pages accessor.
//----------------------------------------------------------------
public int getPages ()
{
return pages;
}
}
8.1 – Dictionary.java
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//********************************************************************
// Dictionary.java Java Foundations
// Represents a dictionary, which is a book. Used to demonstrate inheritance.
//********************************************************************
public class Dictionary extends Book
{
private int definitions = 52500;
// Computes ration of definitions per page
public double computeRatio () // to demo use of parent vars
{
return definitions/pages;
}
// Setter aka mutator
public void setDefinitions (int numDefinitions)
{
definitions = numDefinitions;
}
// Setter aka. accessor
public int getDefinitions ()
{
return definitions;
}
}
