Java Inheritance with the use of super

Question

i'm trying to create a simple java inheritance program along with the use of super and this keyword. showing here the marks of a student in three subjects for 2 semesters sem1 and sem2. i want to show total marks ie. S1T(Sem1 total), S2T(Sem 2 total) and also grand total..

//Student Record Keeping System
class Sem1
{
    int a,b,c,S1T,S1GT;
    Sem1(int a,int b,int c)
    {
        this.a=a;
        this.b=b;
        this.c=c;
    }

    void total()
    {
        S1T=a+b+c;
        S1GT=S1T;
    }


    void display()
    {
        System.out.println("S11: "+a);
        System.out.println("S12: "+b);
        System.out.println("S13: "+c);
        System.out.println("S1Total: "+S1T);
        System.out.println("S1Gtotal: "+S1GT);
        System.out.println("");
    }

    }

class Sem2 extends Sem1
{
    int p,q,r,S2T,S2GT;
    Sem2(int p,int q,int r)
    {
        this.p=p;
        this.q=q;
        this.r=r;
    }

    void total()
    {   
        S2T=p+q+r;
        S2GT=S2T+S1T;
        }


    void display()
    {
        super.display();
        System.out.println("S21: "+p);
        System.out.println("S22: "+q);
        System.out.println("S23: "+r);
        System.out.println("S2Total: "+S2T);
        System.out.println("S2Gtotal: "+S2GT);
        System.out.println("");
    }

}

here is main class

class StudentRcd
{
    public static void main(String abc[])
    {
        Sem1 obj = new Sem1(10,20,30);
        obj.total();
        obj.display();

        Sem2 obj1 = new Sem2(20,30,40);
        obj1.total();
        obj1.display();
    }
}

error: constructor Sem2 in class Sem2 cannot be applied to given types; { ^ required: int,int,int found: no arguments reason: actual and formal argument lists differ in length

kindly help me out here..

in your Sem2 constructor, your first line should be super(p,q,r); — Trevor Freeman
– Trevor Freeman, Commented Jul 15, 2013 at 17:46
i don't test it but i think your code even compile cause your parent class doesn't have constructor without parameters, you have to call super(int , int ,int ) in child class — nachokk
– nachokk, Commented Jul 15, 2013 at 17:46

Andy Thomas · Accepted Answer · 2013-07-15 18:01:21Z

1

I don't think you really want two separate classes. The two classes are mostly the same, except for the computation of the semester 2 total. It may work better to have two separate instances of one class, and compute the full-year total separately.

If you did want two classes, related by inheritance, then you'd either need to call super() in Sem2's constructor, because Sem1 lacks a default constructor. This would likely require you to provide additional values in Sem2's constructor, since the marks for semester 1 differ from those of semester 2.

class Sem2 extends Sem1
{
   int p,q,r,S2T,S2GT;
   Sem2( int a, int b, int c, int p,int q,int r)
   {
       super( a, b, c );
       this.p=p;
       this.q=q;
       this.r=r;
   }

edited Jul 15, 2013 at 18:01

answered Jul 15, 2013 at 17:47

Andy Thomas

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1 Comment

nachokk Over a year ago

why not protected? now if they are in the same package has access

user1697575 · Accepted Answer · 2013-07-15 17:46:16Z

1

You forgot to call constructor of the super class in Sem2. e.g. you should have:

Sem2(int p,int q,int r)
{
    super(p,q,r);
    this.p=p;
    this.q=q;
    this.r=r;
}

answered Jul 15, 2013 at 17:46

user1697575

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Comments

Trevor Freeman · Accepted Answer · 2013-07-15 17:53:01Z

1

Firstly, as commented, your compilation problem is due to not calling the Sem1 constructor in your Sem2 constructor since Sem1 does not have a 0 argument / default constructor. You need to add super(p,q,r); as the first line of your Sem2 constructor.

Secondly, and more importantly, you appear to be using inheritance incorrectly. There appears to be no reason to have a Sem2 object that inherits from Sem1. I imagine you could simply have two instance of a Semester object instead of having essentially identical Sem1 and Sem2 classes.

In fact, having Sem2 as a subclass of Sem1 breaks the general guidelines for inheritance since Sem2 is NOT a Sem1 (there is no "is a" relationship). Sem1 and Sem2 are both semesters, however, so you could have a Semester class and then Sem1 and Sem2 subclasses if there was unique logic required for each of them.

answered Jul 15, 2013 at 17:53

Trevor Freeman

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1 Comment

Sahil Sood Over a year ago

sir, i got it right.. i'm learning the language and thanks for a great suggestion.. thankyou.

Nick Palmer · Accepted Answer · 2013-07-15 17:47:34Z

0

Sem2 needs to call the super constructor super(int, int, int). Sem2 either needs to take three more grades, or a SEM1 which it can use to call a super constructor.

answered Jul 15, 2013 at 17:47

Nick Palmer

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