AtomicInteger misbehavior

I have an exercise: given a matrix of AtomicIntegers (initialized to 0), I have to run a thread for each rows, and a thread for each column, on the rows I subtract 1, on the columns I add 1, so at the end the matrix should remain as it was. The problem is that the matrix change! Here is the code: The object takes the matrix, a boolean which will tell him to operate on columns or rows, and the index of the column/row where it must operate.

import java.util.concurrent.atomic.AtomicInteger; public class FifthExerciseSafe implements Runnable{ private Thread thread; private boolean onRows;//tells if the object operates on the rows or on the columns private AtomicInteger[][] matrix; private int index; public FifthExerciseSafe(AtomicInteger[][] matrix, boolean onRows, int index){ this.matrix = matrix; this.onRows = onRows; this.index = index; } public void start(){ thread = new Thread(this); thread.start(); } public boolean isOnRows() { return onRows; } public void setOnRows(boolean onRows) { this.onRows = onRows; } public AtomicInteger[][] getMatrix() { return matrix; } public void setMatrix(AtomicInteger[][] matrix) { this.matrix = matrix; } @Override public void run() { if (this.onRows){ for(int i = 0; i < matrix[index].length; i++){ matrix[index][i].decrementAndGet(); } } else { for(int i = 0; i < matrix.length; i++){ matrix[i][index].incrementAndGet(); } } }//run public static void main(String args[]){ AtomicInteger[][] m = new AtomicInteger[3][3]; for (int i = 0; i < m.length; i++){ for(int j = 0; j < m.length; j++){ m[i][j]= new AtomicInteger(0); } } for(int i = 0; i < 3; i++){//I create 6 objects, 3 for columns and 3 for rows FifthExerciseSafe fes1 = new FifthExerciseSafe(m, true, i); FifthExerciseSafe fes2 = new FifthExerciseSafe(m, false, i); fes1.start(); fes2.start(); } for(int i = 0; i < m.length; i++){ for(int j = 0; j < m.length; j++){ System.out.print(m[i][j]+" "); } } }//main } 

The output should be: 000000 but sometimes it is: -100-100-1-10

It only happens on a netbook with an Intel Atom, on a desktop with a Core Duo I haven't seen it yet, but I'm wondering if it could happen there too.