Thread: Respawn moving objects on 2D array

I have a 2D array grid on which a couple of runnable vehicle objects (an 'x' and an 'o') are randomly placed. The grid repeatedly prints to console, and the x/y coordinates of the vehicle objects updates on their threads which gives the impression of the vehicles animating across the grid-lanes.

However, I'm stuck on how to make the vehicle objects respawn once they've reached the end of the grid. Any ideas how I can achieve this? Here's a screen shot and the code:

grid.png

public class gridMain {
 
    public static void main(String[] args) {
 
 
        Grid grid = new Grid(10, 20);
 
        Vehicle1 v1 = new Vehicle1(grid);
        Vehicle2 v2 = new Vehicle2(grid);
        grid.setVehicles(v1, v2);
 
 
        while(true) {
 
            grid.printGrid();
            try {
                Thread.sleep(200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
 
 
    }
}
 
 
public class Grid {
 
     //Grid width and height
    private int rows;
    private int columns;
    private String[][] gridArray;
    Vehicle1 v1;
    Vehicle2 v2;
 
 
public Grid(int rows, int columns) {
    this.rows = rows;
    this.columns = columns;
    gridArray = new String[rows][columns];
}
 
public int getRows() {
    return rows;
}
 
public int getColumns() {
    return columns;
}
 
public void setVehicles(Vehicle1 v1, Vehicle2 v2) {
    this.v1 = v1;
    this.v2 = v2;
    startVehicles();
}
 
public void startVehicles() {
    Thread t = new Thread(v1);
    t.start();
 
    Thread t2 = new Thread(v2);
    t2.start();
}
 
 
 
public void printGrid() {
 
 
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++) {
            if (i == v1.getX() && j == v1.getY()) {
                gridArray[v1.getX()][v1.getY()] = "o|";
 
            } else if (i == v2.getX() && j == v2.getY()) {
                gridArray[v2.getX()][v2.getY()] = "x|";
 
            } else {
                    gridArray[i][j] = " |";
                }
            }
        }
 
    System.out.println();
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++) {
            System.out.print(gridArray[i][j]);
        }
        System.out.println();
   }
}
 
 
}
 
public class Vehicle1 implements Runnable {
 
private int x; //Horizontal coordinate of vehicle
private int y; //Vertical coordinate of vehicle
private Grid g;
private int speed;
 
 
public Vehicle1 (Grid g) {
    int max = g.getRows();
    this.x = (int)(Math.random() * (max));
    this.y = 0;
    this.g = g;
    this.speed = (int) (200 + (600 - 200 ) * Math.random());
}
 
public int getX() {
return x;
}
 
public int getY() {
    return y;
}
 
@Override
public void run() {
 
 
    for(int i = y; i < g.getColumns(); i++) {
        y++;
        try {
            Thread.sleep(speed);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
 
}
}