Android/Java：使用接口 [duplicate] 编辑来自另一个类/线程的文本视图

问：

我想编辑 MainActivity 类中的文本视图文本，并放置一个连接到接口的函数，然后该函数进入另一个名为 JobService 的类（用于发送带有线程的类）。但是我在构造函数部分遇到错误，它暗示了 myCallback 部分的空指针异常。即使我给字符串里面有代码（timestampJob），它也会进入空指针异常。

//MainActivity class

public class MainActivity extends AppCompatActivity implements MyCallback

    TextView timestampMails;

    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    timestampMails = (TextView) findViewById(R.id.timeStampMainMails);

    }

    //function for changeing the textview connected with interface to JobService class
    public void setTimestampMain(String time){
        timestampMails.setText(time);
    }

//Interface class

public interface MyCallback {
    public void setTimestampMain(String myString);
}

//JobService class

public class JobService extends android.app.job.JobService {

    //interface call
    MyCallback myCallback = null;
    //constructor
    public JobService(MyCallback callback){
        this.myCallback = callback;
    }
    public void timeStampJob(String time){
        //HERE IS THE EXCEPTION (I think)
        if(myCallback != null){
            myCallback.setTimestampMain(time);
        }
    }

    @Override
    public boolean onStartJob(JobParameters params) {
        Log.d(TAG, "Job started");
        doBackgroundWork(params);

        return true;
    }

    private void doBackgroundWork(JobParameters params) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i=0; i<1; i++){

                    //get device time function and populate function with that string
                    String deviceTime = new     SimpleDateFormat("HH:mm").format(Calendar.getInstance().getTime());
                    timeStampJob(deviceTime);

                 
                    if(jobCancelled){
                        return;
                    }

                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        throw new RuntimeException(e);
                    }
                }
                Log.d(TAG, "Job finished:");
                jobFinished(params, false);
            }
        }).start();

    }

    @Override
    public boolean onStopJob(JobParameters params) {
        Log.d(TAG, "Job cancelled before completion");
        jobCancelled = true;

        return true;
    }

Process: com.example.gmailemailsolution, PID: 3944
java.lang.RuntimeException: Unable to instantiate service
com.example.gmailemailsolution.JobService: java.lang.InstantiationException:
java.lang.Class<com.example.gmailemailsolution.JobService> has no zero
argument constructor (HERE)
at android.app.ActivityThread.handleCreateService(ActivityThread.java:3268)
at android.app.ActivityThread.-wrap5(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1608)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:173)
at android.app.ActivityThread.main(ActivityThread.java:6523)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:938)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:828)
Caused by: java.lang.InstantiationException:
java.lang.Class<com.example.gmailemailsolution.JobService> has no zero
argument constructor (HERE)
at java.lang.Class.newInstance(Native Method)
at android.app.ActivityThread.handleCreateService(ActivityThread.java:3265)
at android.app.ActivityThread.-wrap5(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1608)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:173)
at android.app.ActivityThread.main(ActivityThread.java:6523)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:938)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:828) 

尝试了接口类，用构造函数调用它并使 textview 静态，但没有让它工作。