Inheritance

Report
Inheritance
Polymorphism
Briana B. Morrison
CSE 1302C
Spring 2010
Topics
• Inheritance Concepts
• Inheritance Design
– Inherited Members of a Class
– Subclass Constructors
– Adding Specialization to the Subclass
– Overriding Inherited Methods
• The protected Access Modifier
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Inheritance Concepts
• A common form of reuse of classes is
inheritance.
• We can organize classes into hierarchies of
functionality.
• The class at the top of the hierarchy
(superclass) defines instance variables and
methods common to all classes in the
hierarchy.
• We derive a subclass, which inherits
behavior and fields from the superclass.
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Inheritance
• Inheritance allows a software developer to
derive a new class from an existing one
• The existing class is called the parent class,
or superclass, or base class
• The derived class is called the child class,
subclass, or derived class
• As the name implies, the child inherits
characteristics of the parent
• That is, the child class inherits the methods
and data defined by the parent class
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A Sample Vehicle Hierarchy
• This hierarchy is
depicted using a
Unified Modeling
Language (UML)
diagram.
• In UML
diagrams, arrows
point from the
subclass to the
superclass.
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Superclasses and Subclasses
• A superclass can have multiple
subclasses.
• Subclasses can be superclasses of
other subclasses.
• A subclass can inherit directly from only
one superclass.
• All classes inherit from the Object class.
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Superclasses and Subclasses
• A big advantage of inheritance is that we
can write common code once and reuse
it in subclasses.
• A subclass can define new methods and
instance variables, some of which may
override (hide) those of a superclass.
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Specifying Inheritance
• The syntax for defining a subclass is to use the :
symbol in the class header, as in
accessModifier class SubclassName: SuperclassName
{
// class definition
}
• The superclass name specified after the : is called
the direct superclass.
• As mentioned, a subclass can have many
superclasses, but only one direct superclass.
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Class Hierarchies
• A child class of one parent can be the
parent of another child, forming a class
hierarchy
Business
RetailBusiness
KMart
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Macys
ServiceBusiness
Kinkos
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Class Hierarchies
• Two children of the same parent are called
siblings
• Common features should be put as high in
the hierarchy as is reasonable
• An inherited member is passed continually
down the line
• Therefore, a child class inherits from all its
ancestor classes
• There is no single class hierarchy that is
appropriate for all situations
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The Object Class
• A class called Object exists
• 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
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The Object Class
The Object class contains a few useful methods, which are
inherited by all classes:
• Constructor
• Equals
• GetHashCode
• GetType
• ReferenceEquals
• ToString (virtual)
• Finalize (overridden, destructor)
• MemberwiseClone (shallow copy of object)
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namespace ConsoleApplication1
public class Temp
{
{
private int a;
public class ObjectExample
private char ch;
{
}
static void Main(string[] args)
{
Object o1 = new Object();
Console.WriteLine("This is an object " + o1);
Temp t = new Temp();
Console.WriteLine("This is a temp object " + t);
}
}
}
Example
Output
This is an object System.Object
This is a temp object ConsoleApplication1.Temp
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The Bank Account Hierarchy
• The BankAccount class is
the superclass.
– Instance variables:
• balance (double)
– Methods:
•
•
•
•
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Default and overloaded constructors
deposit and withdraw methods
balance accessor
toString
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ScreenManager Hierarchy
• //**pull from A2**//
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private Members
• Superclass members declared as
private are part of the subclass, but not
accessible by the subclass
• /** rework based on a2 **/
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Subclass Constructors
• Constructors are not inherited.
• The first task of a constructor is to
call a base class constructor.
• Use this syntax:
base( argument list );
• Or:
public constructor(arg1 list):base(
argument list );
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The base Reference
• A child’s constructor is responsible for
calling the parent’s constructor
• Normally the first line of a child’s
constructor should use the base
reference to call the parent’s constructor
• The base reference can also be used to
reference other variables and methods
defined in the parent’s class
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Adding Specialization
• A subclass can define new fields and
methods.
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Software Engineering
Tip
The superclasses in a class hierarchy
should contain fields and methods
common to all subclasses. The
subclasses should add specialized fields
and methods.
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Overriding Inherited Methods
• A subclass can override (or replace) an inherited method by
providing a new version of the method.
• Base class must include virtual
• Descendant must include override
• The API of the new version must match the inherited method.
• When the client calls the method, it will call the overridden
version.
• The overridden (original) method is invisible to the client of the
subclass, but the subclass methods can still call the overridden
(original) method using this syntax:
base.methodName( argument list )
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Overloading vs. Overriding
• Overloading deals with multiple methods with the
same name in the same class, but with different
signatures
• Overriding deals with two methods, one in a parent
class and one in a child class, that have the same
signature
• Overloading lets you define a similar operation in
different ways for different parameters
• Overriding lets you define a similar operation in
different ways for different object types
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Common Error Trap
• Do not confuse overriding a method with
overloading a method.
• Overriding a method:
– A subclass provides a new version of that method
(same signature), which hides the superclass version
from the client.
• Overloading a method:
– A class provides a version of the method, which
varies in the number and/or type of parameters
(different signature). A client of the class can call any
of the public versions of overloaded methods.
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protected Members
• protected members are accessible by
subclasses (like public members), while still
being hidden from client classes (like private
members).
• Also, any class in the same package as the
superclass can directly access a protected
field, even if that class is not a subclass.
• Disadvantage:
– Because more than one class can directly access a
protected field, protected access compromises
encapsulation and complicates maintenance of a program.
– For that reason, try to use private, rather than protected, for
instance variables.
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The protected Access Modifier
• Declaring fields as private preserves
encapsulation.
– Subclass methods call superclass methods to set
the values of the fields, and the superclass
methods enforce the validation rules for the data.
– But calling methods incurs processing overhead.
• Declaring fields as protected allows
them to be accessed directly by
subclass methods.
– Classes outside the hierarchy and package must
use accessors and mutators for protected fields.
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protected fields: Tradeoffs
• Advantage:
– protected fields can be accessed directly by
subclasses, so there is no method-invocation
overhead.
• Disadvantage:
– Maintenance is complicated because the subclass
also needs to enforce validation rules.
• Recommendation:
– Define protected fields only when high
performance is necessary.
– Avoid directly setting the values of protected fields
in the subclass.
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Inheritance Rules
Superclass
Members
Inherited by
subclass?
Directly Accessible
by Subclass?
Directly Accessible by
Client of Subclass?
public fields
yes
yes, by using field
name
yes
public methods
yes
yes, by calling method
from subclass methods
yes
protected fields
yes
yes, by using field
name
no, must call accessors
and mutators
protected
methods
yes
yes, by calling method
from subclass methods
no
private fields
yes* - no
access
no, must call accessors no, must call accessors
and mutators
and mutators
private methods yes* - no
access
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no
no
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Questions?
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