NullPointerException

[Adithya Bhat]

Hello guys i was working with multidimensional array and when i try to execute this following program i get this error.Can you guys please help me with this one.

java.lang.NullPointerException
at blur.copy(blur.java:15)
at blur.main(blur.java:30)

 class multi {
    public int[][] data;
    int row;
    int column;
   }

public class blur {

    public multi copy(multi i){
       multi res=new multi();
         res.row=i.row;
         res.column=i.column;
        for(int x=0;x<i.row;x++){
            for(int y=0;y<i.column;y++){
                res.data[x][y]=i.data[x][y];
            }
        }
            return res;
    }


    public static void main(String args[]){
    try{
        multi i=new multi();

     i.row=2;
     i.column=2;
     i.data=new int[][]{{1,2},{3,4}};
     blur obj=new blur();
     multi r=obj.copy(i);
      for(int x=0;x<r.row;x++){
            for(int y=0;y<r.column;y++){
               System.out.print(r.data[x][y]+",");
            }
         System.out.println();
      }
    }
  catch(Exception e){
      e.printStackTrace();
    }
   }
}

[Jelle Klap]

What do you suppose the data memer variable of the multi object created at line 10 refers to after the constructor completes?
Does that change anywhere in the body of the copy method?

[Vivek Singh]

Adithya Bhat wrote:

       multi res=new multi();
         res.row=i.row;
         res.column=i.column;
      

Hey you have not initialized the res.data so it throws NULL POINTER Exception.

[Campbell Ritchie]

Go through every object reference in that line (15) and insert a line before it rather like this

System.out.println("Obj1 is not null: %b. Obj2 is not null: %b%n", obj1 != null, obj2 != null);

That will tell you which object was null. You may be able to miss out the != null bits in that line.

[Adithya Bhat]

Thank you guys,code's working now.
But i don't get what Campbell is saying.

[Campbell Ritchie]

This is your code with one line added:

class multi {
    public int[][] data;
    int row;
    int column;
   }

public class blur {

    public multi copy(multi i){
       multi res=new multi();
         res.row=i.row;
         res.column=i.column;
/* The following line is an additonal line for testing */
        System.out.printf("res.data is not null: %b. i.data is not null: %b%n", res.data != null, i.data != null);
/* The preceding line is an additonal line for testing */
        for(int x=0;x<i.row;x++){
            for(int y=0;y<i.column;y++){
                res.data[x][y]=i.data[x][y];
            }
        }
            return res;
    }


    public static void main(String args[]){
    try{
        multi i=new multi();

     i.row=2;
     i.column=2;
     i.data=new int[][]{{1,2},{3,4}};
     blur obj=new blur();
     multi r=obj.copy(i);
      for(int x=0;x<r.row;x++){
            for(int y=0;y<r.column;y++){
               System.out.print(r.data[x][y]+",");
            }
         System.out.println();
      }
    }
  catch(Exception e){
      e.printStackTrace();
    }
   }
}

Correct the spelling and compiler errors (if any), run that, and see whether you get a helpful output just before the Exception.>

[Adithya Bhat]

Thanks Campbell,now i get what you are saying.

[Campbell Ritchie]

You're welcome

And welcome as a new member to JavaRanch