Question

我目前正在尝试向我的自定义ListView显示当前时间但是我一直在收到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它的值。

自定义适配器和设置器/ 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++;
    }

}

logcat的

 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)

如何修改它以使其不返回空指针错误并将正确的时间返回到列表视图？

Answer 1

尝试从以下网站实施此代码。

Calendar c = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("dd:MMMM:yyyy HH:mm:ss a");
String strDate = sdf.format(c.getTime());

希望有任何疑问或疑虑，请告诉我。

Answer 2

您得到NullPointerException，因为在创建适配器时线程尚未完成。线程启动后，您将立即从boss_title中读取结果。这是不可能的，因为你必须等待它完成。

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

SimpleDateFormat类也不是线程安全的：

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);
}

Answer 3

 Handler handler=new Handler();
 handler.post(new Runnable(){

        @Override
        public void run() {

             // upadte textView here

            handler.postDelayed(this,500); // set time here to refresh textView

        }

    });

只需在hanlder方法中传递textview。这很好。

显示当前时间

3 个答案: