Resolved Am having a compile error "java.lang.nullpointerexception"

[barillitos]

The name of the class is matrix and it compiles but when executed in blueJ.
A problem arises; the code snipped is stated here below where the getvalue method is found.

PHP Code:


import java.util.ArrayList;


public class TextLab03st
{
    public static void main(String args[]) 
    {
        System.out.println("\nTextLab03 80-POINT VERSION\n");
        
        Matrix m1 = new Matrix();
        m1.displayMatrix("Matrix m1 Default Display");
        System.out.println();
    
        Matrix m2 = new Matrix(3,5);
        m2.displayMatrix("Matrix m2 3 X 5 Display");
        System.out.println();
        int pos = 0;
        int count = 100;
        for (int r = 0; r < m2.getRows(); r++)
        {
            for (int c = 0; c < m2.getCols(); c++)
            {
                m2.setValue(r,c,count,pos);
                pos++;
                count++;
            }
        }       
        m2.displayMatrix("Matrix m2 3 X 5 Consecutive Integers Display");
        System.out.println();
        
        Matrix m3 = new Matrix(3,3,100);                
        m3.displayMatrix("Matrix m3 3 X 3 Initialized to 100 Display");
        System.out.println();
    }

class Matrix
{
    
    private ArrayList list; // one-dimensional array stores matrix values
    private int listSize;   // total number of elements in the matrix
    private int numRows;    // number of rows in the matrix
    private int numCols;    // number of cols in the matrix            
    
    public Matrix()
    {
       ArrayList list = new ArrayList();
       listSize = 0;
       numRows = 0;
       numCols = 0;
    }

    public Matrix(int r,int c)
    {
        numRows = r;
        numCols = c;
        
    }

    public Matrix(int r,int c ,int value)
    {
        numRows = r;
        numCols = c;
        
        for(int i=0; i<=r;i++)
        {
            for(int p=0; p<=c; c++)
            {
                value = p*i;
            }
        }
    }

    public int getRows()
    {
        return numRows;
    }

    public int getCols()
    {
        return numCols;
    }

    public int getSize()
    {
        int listSize = list.size();

        return listSize;    
    }

    public int getValue(int r,int c)
    {    
        int rW = getRows();
        int cL = getCols();
        
        int pstn = (r*cL) + c;
        return ((Integer)list.get(pstn)).intValue();
        
        //(list.get(pstn)).intvalue(); 
    }

    public void setValue(int r,int c,int value,int pst) 
    {
        int rW = getRows();
        
        list.add(pst,new Integer(value));
    }

    public void displayMatrix(String str)
    {
        System.out.println(str);
        
         for(int x=0; x<=getRows();x++)
        {
            for(int z=0; z<=getCols(); z++)
            {
                System.out.print(getValue(x,z) + " ");
            }
            System.out.println("");
        }
    }
} 


[servlet]

Can you paste line number from exception stacktrace

[Fou-Lu]

Originally Posted by servlet

Can you paste line number from exception stacktrace

I don't think that will be necessary:

PHP Code:

    public Matrix(int r,int c)
    {
        numRows = r;
        numCols = c;
        
    } 


You're array list does not exist in your second example. Attempting to call list.get will result in a nullpointerexception on your m2.displayMatrix call when you attempt to run getValue on a non-object list. Consider chaining your constructors to prevent this from happening.
That matrix should be cast to a generic type as well and applied to your data list. With a matrix, I wouldn't use an array list at all; the scalar 2 dimensional array makes more sense for a matrix (unless you're talking about hundreds of thousands of sparse entries, where some trickery will need to be performed which usually involves a list of some sorts; does not apply to dense entries for which a matrix usually is).

PHP Code:

class Matrix<T>
{
    private T[][] list;
    private int iRows;
    private int iCols;

    public Matrix()
    {
        this(0, 0);
    }
    public Matrix(int iRows, int iCols)
    {
        this(iRows, iCols, null);
    }
    public Matrix(int iRows, int iCols, T data)
    {
        this.iRows = iRows;
        this.iCols = iCols;
        this.list = (T[][])new Object[iRows][iCols];
        if (data != null)
        {
            for (int i = 0; i < iRows; ++i)
            {
                for (int j = 0; j < iCols; ++j)
                {
                    this.list[i][j] = data;
                }
            }
        }
    }
} 


Then change the method signatures to match T instead and rewrite for the (much easier) array instead of the collection.

This of course only works if you are either happy with only accepting objects and not primitives. If you wanted to do primitives you'd need to write an setValue overload for each type and wrap them into their corresponding datatype.

[barillitos]

thanks for the info