CODEWITHSUNDEEP
java
Nick Div
Nick Div

Reputation: 5628

Hurdle with inheritance in Java

I dont understand the program below. I have mentioned the two errors I have encountered in the code. But I cannot understand the reason

import java.io.*;
class sdata
{
    float per;
    int m,tm=0,i,n;

    sdata(int n)throws Exception
    {
       DataInputStream dis2=new DataInputStream(System.in);
       for(i=1;i<=n;i++)
       {
        System.out.print("enter marks of subject"+i);
        int m=Integer.parseInt(dis2.readLine());
        tm=tm+m;
       }
       per=tm/n;
    }
}

class disdata extends sdata
{
    //below line gives an error "constructor sdata in class xyz.sdata cannot be applied to given types required:int found:no arguments"

    disdata() throws Exception{             
      System.out.println("TOTAL MARKS OF SUBJECTS:"+tm);
      System.out.println("PERCENTAGE OF STUDENT:"+per);
    }

}
class sresult
{
    public static void main(String args[])throws  Exception
    {
       DataInputStream dis=new DataInputStream(System.in);
       int n=Integer.parseInt(dis.readLine());

       disdata objj=new disdata();
       //the below line creates an error saying "cannot find symbol" 
       objj.sdata(n);
    }
}

Upvotes: 0

Views: 121

Answers (4)

Ian Roberts
Ian Roberts

Reputation: 122364

Java enforces proper chaining of constructors. The first statement in the body of a constructor must be either this(...) (a call to another constructor of the same class) or super(...) (a call to a superclass constructor), and if you don't include an explicit call then Java inserts an implicit call to super() before the rest of the constructor body. Since your sdata doesn't have a no-argument constructor this fails to compile.

You need to either

  1. Add a no-argument constructor to sdata or
  2. Put a super(0) call as the first thing in the disdata constructor to call the existing single-argument superclass constructor.

Upvotes: 1

PermGenError
PermGenError

Reputation: 46398

if your super class has an overloaded argument constructor your subclass has to make a call explicitly.

disdata() throws Exception{             
     super(some int vale youwanna pass);
            System.out.println("TOTAL MARKS OF SUBJECTS:"+tm);
        System.out.println("PERCENTAGE OF STUDENT:"+per);
    }

remember that super() should be the first line in the disdata() constructor.

disdata objj=new disdata();
    //the below line creates an error saying "cannot find symbol" 
        objj.sdata(n);

constructor are not methods. you are trying to call the constructor sdata(n) with objj which is wrong. use new operator to invoke it . like:

disdata objj=new disdata(n);

Upvotes: 2

Subhrajyoti Majumder
Subhrajyoti Majumder

Reputation: 41200

sdata is super class of disdata and when you are creating a object of disdata with no argument and in disdata constructor you did not call sdata int constructor so by default it will try to find no argument sdata constructor, which is not available, so it gives error.

you can do thing call sdata int constructor from disdata constructor or crate a no argument constructor in sdata.

class sdata {
    float per;
    int m, tm = 0, i, n;
    sdata(int n) throws Exception {...}
    //No argument constructor
    sdata(){}
}

Upvotes: 0

Ridcully
Ridcully

Reputation: 23655

You cannot invoke a constructor as a normal method, which you try to do with objj.sdata(n);. Constructors are no methods.

Upvotes: 0

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