Java Concurrency Race Condition

Consider the ArrayRace program provided below. Which of the following best describes the primary concurrency issue that may affect its output?

public class ArrayRace {
    private int[] data = {0};
    
    public void increment() {
        data[0]++;
    }
    
    public static void main(String[] args) throws InterruptedException {
        ArrayRace ar = new ArrayRace();
        Thread t1 = new Thread(() -> { for(int i = 0; i < 1000; i++) ar.increment(); });
        Thread t2 = new Thread(() -> { for(int i = 0; i < 1000; i++) ar.increment(); });
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println("Data: " + ar.data[0]);
    }
}