Question

first of all here is the code, you can just copy an paste

import java.util.ArrayList;

public class RepetionCounter implements Runnable{
private int x;
private int y;
private int[][] matrix;
private int xCounter;
private int yCounter;
private ArrayList<Thread> threadArray;
private int rowIndex;
private boolean[] countCompleted; 

public RepetionCounter(int x, int y, int [][]matrix)
{
    this.x = x;
    this.y = y;
    this.matrix = matrix;
    this.threadArray = new ArrayList<Thread>(matrix.length);
    this.rowIndex = 0;
    for(int i = 0; i < matrix.length; i++){
        threadArray.add(new Thread(this));
    }
    countCompleted = new boolean[matrix.length];
}

public void start(){
    for (int i = 0; i < threadArray.size(); i++){
        threadArray.get(i).start();
        this.rowIndex++;
    }
}

public void count(int rowIndex)
{
    for(int i = 0; i < matrix[rowIndex].length; i++){
        if (matrix[rowIndex][i] == x){
            this.xCounter++;
        } else if (matrix[rowIndex][i] == y){
            this.yCounter++;
        }
    }
}

@Override
public void run() {
    count(this.rowIndex);
    countCompleted[this.rowIndex] = true;
}

public int getxCounter() {
    return xCounter;
}

public void setxCounter(int xCounter) {
    this.xCounter = xCounter;
}

public int getyCounter() {
    return yCounter;
}

public void setyCounter(int yCounter) {
    this.yCounter = yCounter;
}

public boolean[] getCountCompleted() {
    return countCompleted;
}

public void setCountCompleted(boolean[] countCompleted) {
    this.countCompleted = countCompleted;
}

public static void main(String args[]){
    int[][] matrix = {{0,2,1}, {2,3,4}, {3,2,0}};
    RepetionCounter rc = new RepetionCounter(0, 2, matrix);
    rc.start();
    boolean ready = false;
    while(!ready){
        for(int i = 0; i < matrix.length; i++){
            if (rc.getCountCompleted()[i]){
                ready = true;
            } else {
                ready = false;
            }
        }
    }
    if (rc.getxCounter() > rc.getyCounter()){
        System.out.println("Thre are more x than y");
    } else {System.out.println("There are:"+rc.getxCounter()+" x and:"+rc.getyCounter()+" y");

    }
}

}

What I want this code to do: I give to the object a matrix and tow numbers, and I want to know how much times these two numbers occurs in the matrix. I create as many thread as the number of rows of the matrix (that’ why there is that ArrayList), so in this object I have k threads (supposing k is the number of rows), each of them count the occurrences of the two numbers.
The problem is: if I run it for the first time everything work, but if I try to execute it another time I get and IndexOutOfBoundException, or a bad count of the occurrences, the odd thing is that if I get the error, and modify the code, after that it will works again just for once.
Can you explain to me why is this happening?

Answer 1

You are using the same instance of RepetitionCounter for each thread:

for(int i = 0; i < matrix.length; i++){
    threadArray.add(new Thread(this));
}

so they will all share the same rowIndex. The code is pretty confusing as it is, so I suggest you encapsulate the logic for the threads in a separate Runnable class with individual row ids:

class ThreadTask implements Runnable {
    private int rowId;
    private int[][] matrix;

    public ThreadTask(int[][] matrix, int rowId) {
        this.matrix = matrix; // only a reference is passed here so no worries
        this.rowId = rowId;
    }

    public void run() {
       // search on my row
    }
}

then:

for(int i = 0; i < matrix.length; i++) {
     threadArray.add(new Thread(new ThreadTask(matrix, i)));
}