December 14, 2013

More on Classes and Access Modifiers : Inheritance

In the previous post, we saw how to create our own class and how to work with it. In this post, we shall explore more on classes and access modifiers like private, protected and public.

One of the most powerful feature of object oriented programming is inheriting information from another class. This feature is commonly named as inheritance. Like real world example, if your parents or your grandparents leave behind something (wealth, land/property) after their death, then as a heir you own that wealth. You are eligible to consume that wealth because you have an inherited relation with the people leaving behind that wealth.

Apply the same on classes. A class can inherit part or all of the information that belongs to another class. Suppose you have a class called A. You write another class B which inherits A. Then we call B a subclass, while A is the base class. Lets learn these things modifying the previous example.

using System;

namespace Spells
{
    class Program
    {
        static void Main(string[] args)
        {
            PublicExam pe1 = new PublicExam("Nabiha", 5, 12, 2002, 2013);

            Console.Read(); //keeps the console alive
        }
    }
    class StudentInfo
    {
        string name;
        int standard;
        int rollnum;
        int dob;    //newly added dob -> date of birth
        static int count = 0;
        public StudentInfo(string name, int stndrd, int roll,int dob)
        {
            this.name = name;
            standard = stndrd;
            rollnum = roll;
            this.dob = dob;
            count++;
        }
        public static int GetCount()
        {
            return count;
        }
    }
    class PublicExam : StudentInfo
    {
        const string PRIMARY = "Primary";
        const string JUNIOR = "Junior";
        const string SECONDARY = "Secondary";
        const string H_SECONDARY = "Higher Secondary";
        int currentyear;
        public PublicExam(string name, int stndrd, int roll,int dob,int year)
            : base(name, stndrd, roll, dob)
        {
            currentyear = year;
        }
    }
}

I have changed the code quite a lot. If you compare the code from the previous post, you will see that a new class called PublicExam now inherits StudentInfo class. Therefore StudentInfo is the base class while PublicExam is a subclass.

Notice line 34. This is the way you inherit a class. In this case, PublicExam class is inheriting StudentInfo class. Now have a good look inside the Main method. We are no longer creating objects of StudentInfo, rather we are interested in objects of PublicExam. In object oriented programming, you already know that when you create an object, you automatically call the constructor. In this case you are calling the PublicExam constructor which takes 5 arguments. This automatically makes a call to the base class constructor StudentInfo. Observe line 42, in this case you are actually specifying how to call the base class constructor. You can choose not to specify in this way; in that case you have to provide a base class constructor that takes no arguments. Go ahead and remove line 42. What do you see? An error that says "Error    1    'Spells.StudentInfo' does not contain a constructor that takes 0 arguments ". You can get rid of this error. For example -
using System;

namespace Spells
{
    class Program
    {
        static void Main(string[] args)
        {
            PublicExam pe1 = new PublicExam("Nabiha", 5, 12, 2002, 2013);

            Console.Read(); //keeps the console alive
        }
    }
    class StudentInfo
    {
        string name;
        int standard;
        int rollnum;
        int dob;    //newly added dob -> date of birth
        static int count = 0;
        public StudentInfo()
        { 
            //necessary codes here
        }
        public StudentInfo(string name, int stndrd, int roll,int dob)
        {
            this.name = name;
            standard = stndrd;
            rollnum = roll;
            this.dob = dob;
            count++;
        }
        public static int GetCount()
        {
            return count;
        }
    }
    class PublicExam : StudentInfo
    {
        const string PRIMARY = "Primary";
        const string JUNIOR = "Junior";
        const string SECONDARY = "Secondary";
        const string H_SECONDARY = "Higher Secondary";
        int currentyear;
        public PublicExam(string name, int stndrd, int roll,int dob,int year)
        {
            currentyear = year;
        }
    }
}

To sum up, if you do not provide a statement like base(...), the compiler will automatically call the base class constructor that has no argument. The constructor from line 21 to 24 takes no arguments. You would probably think that how two methods with the same name can co-exist? They can. This is called constructor overloading. Overloaded methods can have the same name, but they should differ on the number of arguments.
Does this code serve to our purpose? Though the base class constructor is called, but it is actually of no use to us, at least in this case. So we better specify how to call. That was what the first code was doing for us. We revert back to that again. Now we make the classes useful. You possibly have the students date of birth and which standard he/she reads in. Suppose when someone is in standard 5 and at least 10 years old, he/she is eligible to enter Primary exam. How do we do that?

using System;

namespace Spells
{
    class Program
    {
        static void Main(string[] args)
        {
            PublicExam pe1 = new PublicExam("Nabiha", 5, 12, 2000, 2013);

            Console.Read(); //keeps the console alive
        }
    }
    class StudentInfo
    {
        string name;
        int standard;
        int rollnum;
        int dob;    //newly added dob -> date of birth
        static int count = 0;
        public StudentInfo(string name, int stndrd, int roll,int dob)
        {
            this.name = name;
            standard = stndrd;
            rollnum = roll;
            this.dob = dob;
            count++;
        }
        public static int GetCount()
        {
            return count;
        }
    }
    class PublicExam : StudentInfo
    {
        const string PRIMARY = "Primary";
        const string JUNIOR = "Junior";
        const string SECONDARY = "Secondary";
        const string H_SECONDARY = "Higher Secondary";
        int currentyear;
        public PublicExam(string name, int stndrd, int roll,int dob,int year): base(name,stndrd,roll,dob)
        {
            currentyear = year;
        }

        public string WhichExam()
        {
            int age = currentyear - dob;   //the current age
            if (standard == 5 && age >= 10)
            {
                return PRIMARY;
            }
            else if (standard == 8 && age >= 13)
            {
                return JUNIOR;
            }
            else if (standard == 10 && age >= 16)
            {
                return SECONDARY;
            }
            else if (standard == 12 && age >= 18)
            {
                return H_SECONDARY;
            }
            else
            {
                return "No exams!";
            }
        }
    }
}
This code will let you down. You will see that dob and standard variables aren't recognized inside the function WhichExam. Your Visual Studio should say that these variables aren't accessible because of protection level. But hey! Didn't we just inherit everything from StudentInfo? True. Then what's wrong? In C#, everything by default is private. So all the variables inside StudentInfo are private and are only accessible inside StudentInfo class. So how do we access these variables? There are two possible ways -
  • Make the variables public.
  • Make the variables protected.
I will go with solution no. 2. Making the variables public isn't an intelligent solution. Because in that way, you expose your variables to the outer world that aren't quite necessary to them. You also make them vulnerable because you can accidentally modify their values anywhere else! See the working code using protected modifier on line 19 and 21.

using System;

namespace Spells
{
    class Program
    {
        static void Main(string[] args)
        {
            PublicExam pe1 = new PublicExam("Nabiha", 5, 12, 2000, 2013);
            PublicExam pe2 = new PublicExam("Abrar", 2, 22, 2003, 2013);
            Console.WriteLine("{0} will sit for {1}",pe1.GetStudentName(),pe1.WhichExam());
            Console.WriteLine("{0} will sit for {1}", pe2.GetStudentName(), pe2.WhichExam());
            Console.Read(); //keeps the console alive
        }
    }
    class StudentInfo
    {
        string name;
        protected int standard;
        int rollnum;
        protected int dob;    //newly added dob -> date of birth
        static int count = 0;
        public StudentInfo(string name, int stndrd, int roll,int dob)
        {
            this.name = name;
            standard = stndrd;
            rollnum = roll;
            this.dob = dob;
            count++;
        }
        public static int GetCount()
        {
            return count;
        }
        public string GetStudentName()
        {
            return name;
        }
    }
    class PublicExam : StudentInfo
    {
        const string PRIMARY = "Primary Exam";
        const string JUNIOR = "Junior Exam";
        const string SECONDARY = "Secondary Exam";
        const string H_SECONDARY = "Higher Secondary Exam";
        int currentyear;
        public PublicExam(string name, int stndrd, int roll,int dob,int year) : base(name,stndrd,roll,dob)
        {
            currentyear = year;
        }

        public string WhichExam()
        {
            int age = currentyear - dob;   //the current age
            if (standard == 5 && age >= 10)
            {
                return PRIMARY;
            }
            else if (standard == 8 && age >= 13)
            {
                return JUNIOR;
            }
            else if (standard == 10 && age >= 16)
            {
                return SECONDARY;
            }
            else if (standard == 12 && age >= 18)
            {
                return H_SECONDARY;
            }
            else
            {
                return "No exams!";
            }
        }
    }
}
Aha! Your code is ready to run. Hit F5 and see the output. Finally, you will notice that you can access methods from the base class by an object of the subclass. Subclass objects can have access to public methods inside the base class.

I hope the concepts of inheritance, access modifiers and how to control access to members and methods are quite clear. Just play with them, edit/modify these codes and see what you are allowed to do and what not.

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