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Question

最近我创建了一个包含类printName(String s)和类Timer的函数TimerTask。但是，当我连续使用此函数时，它显示此错误：

a
Exception in thread "Timer-1" Exception in thread "Timer-0" java.lang.NullPointerException
a
    at ProxyPattern.TimerTest$1.run(TimerTest.java:21)
a
c
    at java.util.TimerThread.mainLoop(Timer.java:555)
c
c
    at java.util.TimerThread.run(Timer.java:505)
c
java.lang.NullPointerException
c
    at ProxyPattern.TimerTest$1.run(TimerTest.java:21)
b
    at java.util.TimerThread.mainLoop(Timer.java:555)
b
b
    at java.util.TimerThread.run(Timer.java:505)
b
b
a
a

这是我的代码：

public class TimerTest {
    Timer timer;
    TimerTask timerTask;

    public void printName(String s) {
        timer = new Timer();
        timerTask = new TimerTask() {
            @Override
            public void run() {
                for (int i = 0; i < 5; i++) {
                    System.out.println(s);
                }
                timerTask.cancel();
                timerTask = null;
            }
        };
        timer.schedule(timerTask, 0, 1);
    }

    public static void main(String[] args) {
        TimerTest timerTest = new TimerTest();
        timerTest.printName("a");
        timerTest.printName("b");
        timerTest.printName("c");
    }
}

我希望它打印“ a”五次，然后打印“ b”五次，最后打印“ b”五次。就是这样：

a
a
a
a
a
a
b
b
b
b
b
c
c
c
c
c

如何解决？