NullPointerException while passing 2d array to the constructor

Question

When I try to pass the 2d array (myString) to constructor, Eclipse shows me NullPointerException in the following line:

DenseBoard<String> temp1 = new DenseBoard<String>(myString, "a");

Could smb please explain me what I did wrong? Thanks in advance!

public class Tester {

    public static void main(String[] args){

        String[][] myString = {{"A B C"}, {"D E F"}, {"G H I"}};
        DenseBoard<String> temp1 = new DenseBoard<String>(myString, "a");
        System.out.println(temp1);
    }
}

Class DenseBoard

import java.util.*;
public class DenseBoard <T> {

 private ArrayList<ArrayList<T>> myBoard;

 public DenseBoard(T[][] x, T fillElem){

      for(int i = 0; i < x.length; i++){
         ArrayList<T> values = new ArrayList<T>();
          for(int j = 0; j < x[0].length; j++){
              values.add(x[i][j]);
          }
          myBoard.add(values);
      }
  }

 public String toString(){
      String result = "";
      for(int i = 0; i < myBoard.size(); i++){
          for(int j = 0; j < myBoard.get(i).size(); j++){
              result += myBoard.get(i).get(j);
          }
          System.out.println();
      }
      return result;
  }

}

In addition to the answers, you should fix this: x.length, x is a 2d array you need something like x[0].length. — Zarwan
– Zarwan, Commented Sep 5, 2015 at 6:20

SaviNuclear · Accepted Answer · 2015-09-05 06:37:13Z

You havn't Initialized your variable myBoard and in for loop you have to use x[i].length instead of x[0].length

Try this:-

package p1;
import java.util.ArrayList;
public class A1{
    public static void main(String[] args) {
        String[][] myString = {{"A B C"}, {"D E F"}, {"G H I"}};
        DenseBoard<String> temp1 = new DenseBoard<String>(myString, "a");
        System.out.println(temp1);
    }
}

class DenseBoard <T> {
    private ArrayList<ArrayList<T>> myBoard;
    public DenseBoard(T[][] x, T fillElem){
        myBoard = new ArrayList<>();
        for(int i = 0; i < x.length; i++){
            ArrayList<T> values = new ArrayList<T>();
            for(int j = 0; j < x[i].length; j++){
                values.add(x[i][j]);
            }
            System.out.println(i);
            System.out.println(values);
            myBoard.add(values);
        }
    }
    public String toString(){
        String result = "";
        for(int i = 0; i < myBoard.size(); i++){
            for(int j = 0; j < myBoard.get(i).size(); j++){
                result += myBoard.get(i).get(j);
            }
            System.out.println();
        }
        return result;
    }
}

Shawn Mehan · Accepted Answer · 2015-09-15 05:11:42Z

1

Your myBoard is not initialized in java. Therefore, you should first intialize the object before you use it. Try this

private ArrayList<ArrayList<T>> myBoard = new ArrayList<>();

edited Sep 15, 2015 at 5:11

Shawn Mehan

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answered Sep 5, 2015 at 6:32

Ali Asad

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Comments

Codebender · Accepted Answer · 2015-09-05 06:13:18Z

0

Your myBoard is null.

You should initialize it with,

private ArrayList<ArrayList<T>> myBoard = new ArrayList<>();

answered Sep 5, 2015 at 6:13

Codebender

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Comments

HXSP1947 · Accepted Answer · 2015-09-05 06:13:39Z

0

It looks like myBoard is never instantiated.

answered Sep 5, 2015 at 6:13

HXSP1947

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