Question

SO I have the following code

public void rand(int N){

  double[]x=new double[N];
  double[]y=new double[N];
  double[][]res=new double[N][N];
  for(int i=0;i<N;i++){x[i]=random.nextDouble();y[i]=random.nextDouble();}

}

but the it would return the nullpointerexceptionerror on the for loop….can anybody tell me what’s wrong with it?

Answer 1

Where did you initialize random? I don’t see it.

You’ll have another problem: the x and y arrays and the res matrix are declared, initialized, and immediately go out of scope when you leave the method. All that work is wasted.

I’d think about it more like this:

import java.util.Random;

/**
 * MatrixTest
 * @author Michael
 * @since 2/20/11
 */
public class MatrixTest
{
    private static final int DEFAULT_SIZE = 3;
    private double [] x;
    private double [] y;
    private int n;

    public static void main(String[] args)
    {
        MatrixTest m = new MatrixTest();
        System.out.println(m);
    }

    public MatrixTest()
    {
        this(DEFAULT_SIZE);
    }

    public MatrixTest(int n)
    {
        this.init(n);
    }

    public void init(int n)
    {
        Random random = new Random(System.currentTimeMillis());

        this.x = new double[n];
        this.y = new double[n];
        this.n = n;
        for (int i = 0; i < this.n; i++)
        {
            x[i] = random.nextDouble();
            y[i] = random.nextDouble();
        }
    }

    @Override
    public String toString()
    {
        final StringBuilder sb = new StringBuilder();
        sb.append("MatrixTest");
        sb.append("{x=").append(x == null ? "null" : "");
        for (int i = 0; x != null && i < x.length; ++i)
        {
            sb.append(i == 0 ? "" : ", ").append(x[i]);
        }
        sb.append(", y=").append(y == null ? "null" : "");
        for (int i = 0; y != null && i < y.length; ++i)
        {
            sb.append(i == 0 ? "" : ", ").append(y[i]);
        }
        sb.append(", n=").append(n);
        sb.append('}');
        return sb.toString();
    }
}