2009 first semister Instructor: Abdalrahman Obidat 1 Revision 3 Instructor: Abdalrahman Obidat

2009 first semister Instructor: Abdalrahman Obidat 1

Revision 3

Instructor: Abdalrahman Obidat


Classes and Inheritance• Classes have proven very useful

– They help us write programs the way we view the world: interface vs implementation

• C# supports object oriented programming, i.e.,– Inheritance (sub classing)– Dynamic dispatch (polymorphism)

• Two very powerful programming tools!– They help us write less code


Composition: has-a relationship

• We have used many times several classes together, e.g.,– Use an instance of a class as an instance field of another

class

public class Landscape

{

private Mountain m = new Mountain();

}• This a "has-a" relationship

– A Landscape "has-a" Mountain• Also called aggregation


Organization hierarchy• Often, we classify things according to a hierarchy

(from general to specific), e.g. part of the

organization of a university UniversityMember

Staff Faculty

LecturerProfessor

Student

Freshman Sophomore


Inheritance: is-a relationship

• Objects have also an "is-a" relationship

– A Freshman "is-a" Student , a Lecturer "is-a"

UniversityMember

• C# gives us the tools to implement an "is-a"

relation. We can map our view of the world on the

computer


Inheritance in C#• Inheritance is a way to encode the "is-a" relation in OO

languages– Freshman declares that it "is-a" Student by inheriting

from it– A derived class inherits from a base class by writing

": BaseClassName" in the derived class declaration

public class Freshman : Student { /* Freshman-specific stuff here */}

derived class(or subclass)

base class(or superclass)


What is inherited? (1)• The subclass inherits all of the members (data +methods) of its

superclass exept private members.• Members declared public or protected within the superclass can be used

by the subclass as if they were declared within the subclass itself. • protected member of a class: visible within the class itself and the

subclasses (even if they are in a different assembly). More on protected later.


What is inherited? (2)• private members of the superclass are unavailable outside the superclass

scope.• assembly level (=declared internal) members of the superclass are unavailable

outside the assembly scope. If the superclass and subclass are in the same assembly, the subclass can access any assembly level members of the superclass

• A subclass instance is also a superclass instance (is-a relation) but superclass instant is not a subclass instant.– All methods of the superclass can be used on a subclass instance.


Scope example• Base.cs

public class Base {public int i; //visible everywhereinternal int j;//visible within the assembly

protected int k; //visible within //any derived classprivate int l;// visible within Base only

}• Derived.cs

public class Derived: Base{public void update(){

i=4; // OK j=5; // OK if in the same assembly // Error if not in the same assembly k=6; // OK l=7; /*Error (not in Base)*/}}


Example: Person, Student• A person has a name, an age and can speak• A student is a person with a gpa

Person

name: String

age: int

speak: void Student

gpa: double

derived from


Person classpublic class Person{

public String name; // any name is OK private int age; // 0<=age<=130 //Call the other constructor public Person():this("someone",20) {} public Person(String s, int a) { name = s; Age = a; } public int Age{ // Age property set {if (value>=0 && value<=130) age=value;} get {return age;} } public void speak() { Console.WriteLine("Hi, I am "+name);}}


Student classpublic class Student: Person{

private double gpa; //between 0 and 4.0public Student()//Person() automatically called{ gpa = 3.5;} public Student(String s,int a,double g):base(s,a)//Call the constructor Person(s,a) { gpa = g; }public double GPA { // GPA property set{ if (value>=0 && value<=4.0) gpa=value;} get{ return gpa;}}}


Using Person and Student

Person p = new Person("Jane",25);

Student s = new Student("Robert",28,3.8);

p.speak(); // a person can speak

s.speak(); // a student can speak, since // a student is a person

Person p1 = new Person()

Student s2 = new Student()


Method overriding• How can we specialize speak for the Student class?• Define speak as virtual in the Person class

• public virtual void speak(){ /*code*/ }• Redefine speak within the Student classpublic override void speak(){

base.speak();//let the Person speak // Add what the student say Console.WriteLine("My gpa is "+gpa);}

• For any Student instance s, s.speak() invokes the speak method of the Student class. This is called method overriding.


this keyword revisited• this: two uses

– a reference to the current object when within one of the object instance methods

– Use this(argument list) when calling another constructor from a constructor (within the same class). The call is the first executed statement in the constructor.

public Person(): this(name,20) {

/* more code here if necessary */• }


base keyword• base: two uses

– a reference to the base object part of the current derived object within one of the derived object instance method.• base.speak();//In Student.speak()

– Use base(argument list) when calling a base constructor from a derived constructor. The call is the first executed statement in the constructor.• public Student(string s, int a, double g): base(s,a) { gpa = g; }

– Can't chain the base calls• base.base.method();//Error


Constructor calls• When calling a constructor, C#

– invokes the base class constructor (if necessary via an implicit call to base()),

– then, initializes the instance variables of the class,– then, executes the statements in the constructor.

public class Derived: Base{ private int i=3; public Derived(){i++;}}

// And in some method in some other class Derived d = new Derived();// call Base(), initialize any instance field// in class Base, execute Base(), // set i to 3 in class Derived,// increment i as instructed in Derived()

// What happens if Base is derived from// some other class?


Overloading and Overriding• Method overloading: same name but a different number or type of

arguments.– A subclass can define some overloaded methods that augment

the inherited overloaded methods provided by a base class.• Method overriding: define a method in the subclass with the same

signature (return type and arguments) as the inherited method.– The method in the subclass shadows the method of the

superclass.– Powerful when dealing with a hierarchy of objects

(polymorphism: next slide)


Polymorphism example• Consider

Person[] group=new Person[2];group[0]=new Person("Jane",25);

group[1]=new Student("Bob",26,3.9);

//OK, a Student is a Person

for(int i=0; i<2; i++)

group[i].speak();

// What is printed?

• Note: Properties are just syntactic sugar for get and set methods. Overriding works the same way for instance properties as for instance methods.


Polymorphism

• If there are several implementations of a virtual method in the inheritance hierarchy of an object, the one in the most derived class always overrides the others.

• This is the case even if we refer to the object by way of a less derived type.

• group[1].speak();• //invokes speak of Student• //even though group is an• // array of type Person[]


Dynamic binding and Polymorphism

• Polymorphism uses dynamic binding– The selection of the method is done at run time. The CLR

looks at the actual type of the object referred to.

• To turn on overriding, the C# programmer must mark the base instance method as virtual and the derived instance method as override (it is a compiler error to do one without the other).

• To prevent overriding of a method, just don't use virtual. However, it is possible to shadow the method in a derived class by marking it as new (see next slide).


new keyword on a method// in Personpublic void speak(){Consol.WriteLine(“person”)}//can't override since not virtual// in Studentpublic new void speak(){Consol.WriteLine(“Student”}// shadows speak in Person

// in some method of some other classStudent s = new Student();s.speak(); // Student.speak ,print StudentPerson p = s;p.speak(); // Person.speak (static binding) ,Print person// With overriding, Student.speak would// have been called.


Static binding• When the overriding feature is turned off (no virtual

keywork), the compiler uses the static type of the variable to select the instance method.

• Static binding is slightly more efficient than dynamic binding.


Static methods (1)• Reminder: static methods are not attached to any

instance of a class. They belong to the class.• A static method is invoked by using the class name.

They are bound at compile time. (That's why they are called static)

• Math.Sqrt(2);• A class inherits all of the public and protected static

methods of a base class. But there is no overriding mechanism with static methods.– However, you can shadow a base class static method

in the subclass (using the keyword new as for a non virtual instance method).


Static methods (2) public class Base{ public static void foo(){

Console.WriteLine("foo of Base");}

}

public class Derived: Base{

public static void foo(){// No

Console.WriteLine("foo of Derived");}

public new static void foo(){ // OK Console.WriteLine("foo of Derived");}

}


sealed keyword• The class can't be inherited.

• public sealed class ThisIsIt{• /* class code */ }• public class MoreOfIt: ThisIsIt {}

// Error

• A class that is declared sealed cannot be a base class (i.e., a class cannot extend a sealed class).

• All methods in a sealed class are implicitly sealed. (ex. Class string is a sealed class. This class cannot be extended )


• A method declared sealed in a base class cannot be overridden in a derived class.

• Methods that are declared private are implicitly sealed, because it is impossible to override them in a derived class

• Methods that are declared static also are implicitly sealed, because static methods cannot be overridden either


protected internal keyword

• protected members are visible to any subclass even if the subclass is not in the same assembly

• internal members are visible to any class within the same assembly

• C# offers the combination protected internal to mean visible to any subclass anywhere and to any class within the same assembly


Visibility summary for class members

Modifier Visibilityprivate (or

none)Class only

internal All classes within the same assembly

protected Subclasses anywhere

protected internal

Subclasses anywhere and all classes within the same assembly

public All classes


Abstract classes• Some classes are so abstract that instances of

them shouldn't even exist– What does it mean to have an instance of Animal?

• An abstract class is one that should not or can not be instantiated .

A concrete class can have instances• It may not make sense to attempt to fully

implement all methods in an abstract class– What should Animal.speak() do?


abstract keyword• declare a method with the abstract modifier to indicate that it

just a prototype. It is not implemented. It is implicitly marked virtual. It can't be marked private (since it must be overridden).

public abstract void speak();• A class that contains an abstract method must be declared

abstractpublic abstract class Animal{

public abstract void speak(); // more code }


Using abstract classes

• An abstract class can't be instantiated.• An abstract class can contain other non abstract

methods and ordinary variables• To use it, subclass it. Implement the abstract methods

in the subclass• If a subclass doesn't implement all of the base class

abstract methods, the subclass is also abstract and must be declared as such.

• Abstract classes provides a framework to be filled in by the implementer


Abstract class example public abstract class Accommodation{

protected boolean vacancy;protected int NumberOfRooms;

public abstract void reserveRoom(); public abstract void checkIn(); // etc...}

public class Motel: Accommodation{ //must implement all of the abstract //methods of Accommodation //(if we want the class to be instantiated) //code would follow}


Interfaces• An interface is a purely abstract class• An interface specifies a set of methods or properties that a class must implement (unless the class is

abstract)• Style: an interface name starts with an I.• Everything inside an interface is implicitly public and abstract. But it is an error to mark the interface

members as such.

public interface IDriveable{ bool startEngine(); void stopEngine(); bool turn(Direction dir);}


Using interfaces (1)• An interface defines some set of behavior for an object. Think of an interface as a badge that can be worn by a

class to say "I can do that".

public class Automobile: IDriveable {

// implements the methods of Driveable

public boolean startEngine()

{ if (notTooCold) engineRunning = true;

// more code

}

// other methods

}


Using interfaces (2)• Interface types act like class types.

– Variables can be of an interface type– method parameters can be of an interface type– A method return type can be an interface type– Any object that implements the interface can fill that spot.

• A class can implement as many interfaces as desired

public class C: Base, I1, I2, I3 { /* class code */}

• This is how C# deals with multiple inheritance ( C++)


Using interfaces (3)

• An interface can't contain any field ( Java)

• An interface can inherit from any number of interfaces.

public interface I: I1, I2, I3 {/*code*/}

• A concrete class that implements an interface must implement all of the methods in the interfaces of the hierarchy of that interface.


Method Hiding

class Super{

public Super() {. . .};public void DoSomething(){. . .}

}class Sub : Super{

public Sub() {. . .};public new void DoSomething(){. . .}

}

(not virtual)

This "hides" the method of theparent, making it inaccessiblethrough the derived class. Thisapplies to any class member.A compiler warning is givenunless the new keyword is used.

(not virtual)


Method Hiding

using System;using System.Collections;class Hello{

static void Main(){

Super super = new Super();

Sub sub = new Sub();

super.DoSomething();sub.DoSomething();

}}

(cont. ->)

class Super{

public Super() {}public void DoSomething(){

Console.WriteLine("Inside Super");}

}

class Sub : Super{

public Sub() {}public new void DoSomething(){

Console.WriteLine("Inside Sub");}

}


Method Hiding


static void Main(){


Sub sub = new Sub();ArrayList arr = new

ArrayList();arr.Add(super);arr.Add(sub);((Super)

arr[0]).DoSomething();((Super)

arr[1]).DoSomething();super.DoSomething();sub.DoSomething();

}}

(cont. ->)

class Super{

public Super() {}public void DoSomething(){


}

class Sub : Super{

public Sub() {}public new void DoSomething(){


}


Method Hiding


static void Main(){


Sub sub = new Sub();ArrayList arr = new

ArrayList();arr.Add(super);arr.Add(sub);((Super)

arr[0]).DoSomething();((Super)

arr[1]).DoSomething();super.DoSomething();sub.DoSomething();

}}

(cont. ->)

class Super{

public Super() {}public virtual void DoSomething(){


}

class Sub : Super{

public Sub() {}public override void DoSomething(){


}


Exceptions

public void SomeMethod(...){

File file = new File("Readme.txt");

.

.

.

file.Close();}

What happens if an error occursin here?

What happens if the filedoesn’t exist?


Part of the exception hierarchy


Exceptions: the general ideatry{

some code that might throw an exceptionmore code

}catch (most specific exception){

handle the exception}catch (less specific exception){

handle the exception}catch (any exception){

handle the exception}finally{

do this no matter what}still more code


try{

some code that might throw an exceptionmore code No exception thrown


handle the exception}catch (less specific exception){

handle the exception}catch (any exception){




try{



handle the exception}catch (less specific exception){ This exception

thrownhandle the exception

}catch (any exception){




try{



handle the exception}catch (less specific exception){ This exception thrown

handle the exceptionreturn;





try{



handle the exception}catch (less specific exception){ This exception thrown

handle the exceptionthrow;





using System.IO;public void SomeMethod(...){

File file = null;try{

file = new File("Readme.txt");more code

}catch (FileNotFoundException e){

Console.WriteLine("File " + e.FileName + " not found");}catch (Exception e){

Console.WriteLine(e);}finally{

if (file != null)file.Close();

}}


Exceptions

In a catch block, you can:

• Rethrow the same exception, notifying code higher in the call stack

• Throw a different exception, giving additional information to code higher in the call stack

• Handle the exception and fall out the bottom of the catch block


Exceptions

Remember that:

• Exceptions are not always "errors"

• Exceptions are not always infrequent

• Sometimes it's best not to catch an exception where it occurs

• There is a performance hit for exceptions


Exceptions

public void SomeMethod(...){

File file = null;try{

file = new File("Readme.txt");

more code}finally{

if (file != null)file.Close();

}}

Since there is no catch block,the exception is propagatedto the caller

Documents

2009 first semister Instructor: Abdalrahman Obidat 1 Revision 3 Instructor: Abdalrahman Obidat