What potential concurrency issue exists in the code above?

public class Checker {
    private int data = 0;
    
    public void updateData() {
        if(data == 0) {
            data = 100;
        }
    }
    
    public void printData() {
        System.out.println(data);
    }
    
    public static void main(String[] args) {
        Checker chk = new Checker();
        Thread t1 = new Thread(() -> chk.updateData());
        Thread t2 = new Thread(() -> chk.printData());
        t1.start();
        t2.start();
    }
}