Inheritance

1
Inheritance

• Inheritance allows the derivation of a new class
  from an existing one, for the purpose of reuse,
  enhancement, adaptation, etc.
• superclass (a.k.a. base class)
• subclass (a.k.a. derived class, extended class)
• Inheritance models the is-a relationship.
• If class E is an extended class of class B, then
  any object of E can act-as an object of B.

2
Example Book.java
class Book protected int pages 1500
public void pageMessage()
System.out.println("Number of pages "
pages)
3
Example Dictionary.java
class Dictionary extends Book private int
definitions 52500 public void
definitionMessage() System.out.println("Numb
er of definitions "
definitions) System.out.println("Definitions
per page "
definitions/pages)
4
Example Words.java
class Words public static void main
(String args) Dictionary webster new
Dictionary() webster.pageMessage()
webster.definitionMessage()
Output
C\Examplesgtjava Words Number of pages
1500 Number of definitions 52500 Definitions per
page 35
5
Extending Classes

• Protected visibility
• Super constructor
• Overriding
• Super reference
• Single vs. multiple inheritance
• A class may have only one superclass

6
Example Book2.java
class Book2 protected int pages public
Book2(int pages) this.pages pages
public void pageMessage()
System.out.println("Number of pages "
pages)
7
Example Dictionary2.java
class Dictionary2 extends Book2 private int
definitions public Dictionary2(int pages, int
definitions) super (pages)
this.definitions definitions public
void definitionMessage ()
System.out.println("Number of definitions "
definitions)
System.out.println("Definitions per page "
definitions/pages)
8
Example Words2.java
class Words2 public static void main
(String args) Dictionary2 webster new
Dictionary2(1500, 52500) webster.pageMessage(
) webster.definitionMessage()
Output
C\Examplesgtjava Words2 Number of pages
1500 Number of definitions 52500 Definitions per
page 35
9
Example Book3.java
class Book3 protected String title
protected int pages public Book3(String
title, int pages) this.title title
this.pages pages public void info()
System.out.println("Title " title)
System.out.println("Number of pages " pages)

10
Example Dictionary3a.java
class Dictionary3a extends Book3 private int
definitions public Dictionary3a(String title,
int pages, int
definitions) super (title, pages)
this.definitions definitions public
void info() System.out.println("Dictionary
" title) System.out.println("Number of
definitions "
definitions) System.out.println("Definitions
per page "
definitions/pages)
11
Example Dictionary3b.java
class Dictionary3b extends Book3 private int
definitions public Dictionary3b(String title,
int pages, int
definitions) super (title, pages)
this.definitions definitions public
void info() super.info()
System.out.println("Number of definitions "
definitions)
System.out.println("Definitions per page "
definitions/pages)
12
Example Books.java
class Books public static void main (String
args) Book3 java new Book3("Introduction
to Java", 350) java.info()
System.out.println() Dictionary3a webster1
new Dictionary3a("Webster English
Dictionary", 1500, 52500)
webster1.info() System.out.println()
Dictionary3b webster2 new
Dictionary3b("Webster English Dictionary",
1500, 52500) webster2.info()
13
Example Books.java
Output
C\Examplesgtjava Books Title Introduction to
Java Number of pages 350 Dictionary Webster
English Dictionary Number of definitions
52500 Definitions per page 35 Title Webster
English Dictionary Number of pages 1500 Number
of definitions 52500 Definitions per page 35
14
Overloading vs. Overriding

• Overloading
• More than one methods have the same name but
  different signatures
• Overriding
• Replacing the implementation of a methods in the
  superclass with one of your own.
• You can only override a method with the same
  signature.
• You can only override instance methods.

15
The Object Class

• The root of Java class hierarchy
• Defines some common methods
• public String toString()
• public boolean equals(Object other)
• If a class does not explicitly declare a
  superclass, its superclass is Object.

16
Abstract Class

• An abstract class is a class with partial
  implementation.
• It implements behaviors that are common to all
  subclasses, but defers to the subclasses to
  implement others (abstract methods).
• An abstract class cannot be instantiated
• The subclass of an abstract class must override
  the abstract methods of the parent, or it too
  will be considered abstract

17
Interface

• Interfaces are classes with no implementation.
• Interfaces represent pure design.
• Abstract classes represent mixed design and
  implementation.
• An interface consists of only abstract methods
  and constants, i.e., static and final.
• All methods and constants are public.
• No static methods.
• An interface cannot be instantiated

18
Inheritance on Interface

• Inheritance relation also applies to interfaces
• superinterface and subinterface
• Effects of Multiple inheritance effected by
• An interface may extend multiple interfaces
• A class my implement multiple interfaces

19
Type ConversionImplicit Conversion

• Numeric variables Any numeric types can be
  converted to another numeric type with larger
  range, e.g.
• char gt int, int gt long,
• int gt float, float gt double.
• Object reference An object reference of class C
  can be converted to a reference of a superclass
  of C.

20
Type ConversionExplicit Conversion (Cast)

• Numeric variables Any numeric types can be
  explicitly cast to any other numeric type. May
  lose bits, precision.
• Object reference Cast an object reference of a
  class to a reference of any other class is
• syntactically allowed but
• runtime checked.

21
Cast Object References
class Student ... class Undergraduate extends
Student ... class Graduate extends Student
... Student student1, student2 student1 new
Undergraduate() // ok student2 new Graduate()
// ok Graduate student3 student3
student2 // compilation error student3
(Graduate) student2 // explicit cast,
ok student3 (Graduate) student1 //
compilation ok, run-time error
22
Polymorphic Reference

• A polymorphic reference is one which can refer to
  different types of objects.

23
Example Books2.java
class Books2 public static void main
(String args) Book3 books new
Book3("Introduction to Java", 350), new
Dictionary3a("Webster English Dictionary",
1500, 52500), new Dictionary3b("Webst
er English Dictionary", 1500,
52500) for (int i 0 i lt books.length
i) Book3 book booksi
book.info() System.out.println()

24
Example Books2.java
Output
C\Examplesgtjava Books2 Title Introduction to
Java Number of pages 350 Dictionary Webster
English Dictionary Number of definitions
52500 Definitions per page 35 Title Webster
English Dictionary Number of pages 1500 Number
of definitions 52500 Definitions per page 35