每日一题，Java中的线程安全是指？

private static Vector<Integer> vector = new Vector<Integer>();
public static void main(String[] args) {
    while (true) {
        for (int i = 0; i < 10; i++) {
            vector.add(i);
        }
        Thread removeThread = new Thread(() -> {
            for (int i = 0; i < vector.size(); i++) {
                vector.remove(i);
            }
        });
        Thread printThread = new Thread(() -> {
            for (int i = 0; i < vector.size(); i++) {
                System.out.print((vector.get(i)));
            }
        });
        removeThread.start();
        printThread.start();
        //不要同时产生过多的线程，否则会导致操作系统假死
        while (Thread.activeCount() > 20);
    }
}