java - 显示当前时间

Question

这个问题已经有答案了:

How to update a TextView in an activity constantly in an infinite loop? (2 个回答)

已关闭 4 个月前。

我目前正在尝试向自定义 ListView 显示当前时间，但我不断收到 NullPointerException，并且想知道我做错了什么，因为我已经添加到数组并获取其值。

自定义适配器和 setter / getter 位于:1 2

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

    listView = (ListView) findViewById(R.id.list_view);
    boss_title = getResources().getStringArray(R.array.boss_array);
    bossTime = (TextView) findViewById(R.id.boss_time);

    adapter = new BossAdapter(getApplicationContext(), R.layout.boss_layout);
    listView.setAdapter(adapter);
    int i = 0;


    Thread t = new Thread() {

        @Override
        public void run() {
            try {
                while (!isInterrupted()) {
                    Thread.sleep(1000);
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {
                            Calendar c = Calendar.getInstance();
                            SimpleDateFormat df = new SimpleDateFormat("HH-mm-ss");
                            String formattedTime = df.format(c.getTime());

                            String [] boss_time_array = new String[6];
                            for(int i =0; i < 6; i++){
                                boss_time_array[i] = formattedTime;
                            }

                            boss_time = boss_time_array;

                        }
                    });
                }
            } catch (InterruptedException e) {
            }
        }
    };

    t.start();

    for (String boss : boss_title) {
        Boss bossObject = new Boss(boss_icon[i], boss, boss_time[i]);

        adapter.add(bossObject);
        i++;
    }

}

日志猫

 8541-8541/baegmon.com.bosstimer E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: baegmon.com.bosstimer, PID: 8541
java.lang.RuntimeException: Unable to start activity ComponentInfo{baegmon.com.bosstimer/baegmon.com.bosstimer.MainActivity}:     java.lang.NullPointerException: Attempt to read from null array
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
        at android.app.ActivityThread.access$800(ActivityThread.java:151)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5257)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
 Caused by: java.lang.NullPointerException: Attempt to read from null array
        at baegmon.com.bosstimer.MainActivity.onCreate(MainActivity.java:67)
        at android.app.Activity.performCreate(Activity.java:5990)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
        at android.app.ActivityThread.access$800(ActivityThread.java:151)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5257)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

如何修改它，使其不返回空指针错误并向我的 ListView 返回正确的时间？

Answer 1

您会收到 NullPointerException，因为创建适配器时线程尚未完成。线程启动后，您将立即读取 boss_title 的结果。这是不可能的，因为您必须等待它完成。

您应该为此使用 AsyncTask 并在 onPostExecute ( http://developer.android.com/reference/android/os/AsyncTask.html ) 内创建适配器。

此外，SimpleDateFormat 类不是线程安全的:

public static Date dateToString(Date date, final String format, final Locale locale)
  ThreadLocal<SimpleDateFormat> formater = new ThreadLocal<SimpleDateFormat>() {
    @Override
    protected SimpleDateFormat initialValue() {
      return new SimpleDateFormat(format, locale);
    }
  };
  return formater.get().format(string);
}