Question

我正在尝试使用listview作为菜单，并在用户点击时触发活动（这是在谷歌眼镜上）我已经通过了上下文，但由于某种原因，空指针异常不断弹出。想知道为什么会这样。

代码：

public class HeadListView extends ListView implements SensorEventListener {

private static final float INVALID_X = 10;
private Sensor mSensor;
private int mLastAccuracy;
private SensorManager mSensorManager;
private float mStartX = INVALID_X;
private static final int SENSOR_RATE_uS = 200000;
private static final float VELOCITY = (float) (Math.PI / 180 * 2); // scroll one item per 2¬∞
public int location;
Context myContext;

public HeadListView(Context context, AttributeSet attrs, int defStyle) 
{
  super(context, attrs, defStyle);
  myContext=context;
  init();
}

public int getLocation() 
{
  return location;
}

public void setLocation(int location) 
{
  this.location = location;
}

public HeadListView(Context context, AttributeSet attrs) {
  super(context, attrs);
  init();
}

public HeadListView(Context context) {
  super(context);
  init();
}

public void init() {
  mSensorManager = (SensorManager) getContext().getSystemService(Context.SENSOR_SERVICE);
  mSensor = mSensorManager.getDefaultSensor(Sensor.TYPE_ROTATION_VECTOR);
}

public void activate() {
  if (mSensor == null)
    return;

  mStartX = INVALID_X;
  mSensorManager.registerListener(this, mSensor, SENSOR_RATE_uS);
}

public void deactivate() {
  mSensorManager.unregisterListener(this);
  mStartX = INVALID_X;
}

  @Override
public void onAccuracyChanged(Sensor sensor, int accuracy) {
  mLastAccuracy = accuracy;
}
public boolean onKeyDown(int keycode, KeyEvent event) 
  {
      if (keycode == KeyEvent.KEYCODE_DPAD_CENTER) 
      {
          // user tapped touchpad, should get and store the idd ItemsDto, and the   byteArray

        try 
        {
            Log.e("Position: ", String.valueOf(location));
            //declare intent
            if(location==0)
            {

                Intent intn_test = new Intent("de.tud.ess.ANACTIVITY");                         

                myContext.startActivity(intn_test);

            }


        } 
        catch (Exception e) 
        {
            Log.e("MenuScreen:onCreate", e.toString());
        }


            //overridePendingTransition(R.layout.fade_in, R.layout.fade_out);  //fade in/out transition
        return true;
    }
    if (keycode == KeyEvent.KEYCODE_BACK) 
    {
        //goes back to the previous activity, in this case the scanner

        return true;
    }
    return false;
}

抛出空指针异常的部分是onKeyDown。这让我感到困惑，因为我（我以为我是正确的）在清单中声明了活动。

    <activity
        android:name="de.tud.ess.AnActivity"
        android:label="@string/title_activity_an" >
        <intent-filter>
            <action android:name="de.tud.ess.ANACTIVITY" />

            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity>

我唯一的想法是listview在一个单独的线程上运行，我可能不得不使用ui函数运行来获得正确触发活动的意图。任何帮助，将不胜感激。

Answer 1

您的两个构造函数没有为myContext赋值。如果这些构造函数中的任何一个是Android用于创建View的构造函数，myContext将为null并在onKeyDown()中导致NullPointerException。

您可能希望在所有三个构造函数中为myContext赋值，因此您应该更新这两个：

public HeadListView(Context context, AttributeSet attrs) {
  super(context, attrs);
  myContext = context;
  init();
}

public HeadListView(Context context) {
  super(context);
  myContext = context;
  init();
}

在尝试使用intent时，Intent在listview中显示空指针

1 个答案: