java - Android:尝试在空对象引用上调用虚方法 (onTouch)

标签 java android nullpointerexception

我正在尝试在我的应用程序 View 的线性布局之一上设置手势检测。问题是每当我用滑动手势测试它时，我的应用程序就会崩溃。我收到的错误消息是臭名昭著的“尝试在空对象引用上调用虚拟方法”。我已经完成研究，我知道问题源于尝试调用未正确传递到代码中必要范围的 Activity 上的方法。话虽如此，我不明白如何在我的代码中解决这个问题。以下是相关的代码:

Playfrets.java

public class PlayFrets extends Activity {
    LinearLayout gestureOverlay; 
    GestureListener gestureListener;        
    public void configGestureRecognition(){       //Toolbar gesture recognition
            gestureOverlay = (LinearLayout) findViewById(R.id.toolbarGestureOverlay);
            gestureListener = new GestureListener(PlayFrets.this);
            if (gestureOverlay == null){
                Toast.makeText(PlayFrets.this, "gestureOverlay object is null bro" ,Toast.LENGTH_SHORT).show();
            }
            gestureOverlay.setOnTouchListener(gestureListener);
        }
}

LoadNoteFiles.java

public class LoadNoteFiles extends AsyncTask<Void, Void, Void>{
private Context mContext;
Activity instance;  

public LoadNoteFiles(Context context){ //constructor
    mContext = context;
}

@Override
protected void onPreExecute(){
    PlayFrets.viewSwitch = new ViewSwitcher(mContext);
    PlayFrets.viewSwitch.addView(ViewSwitcher.inflate(mContext, R.layout.splash_screen, null));
    instance = (Activity)mContext;  //cast context from main activity into an activity to access setContentView method
    instance.setContentView(PlayFrets.viewSwitch);
}
...//do in background code omitted
@Override
protected void onPostExecute(Void v){
    PlayFrets.viewSwitch.addView(ViewSwitcher.inflate(mContext, R.layout.activity_play_frets, null));
    PlayFrets.viewSwitch.showNext();
    ((PlayFrets)mContext).configFretboard();
    ((PlayFrets)mContext).configGestureRecognition(); //test gesture recognition
}

GestureListener.java

class GestureListener extends SimpleOnGestureListener implements OnTouchListener {
    Context mContext;
    GestureDetector gDetector;
    static final double SWIPE_MIN_DISTANCE = PlayFrets.getActualHeight()*(0.80);  //must swipe across at least 80% of screen height to register a gesture
    static final int SWIPE_MAX_OFF_PATH = 150;
    static final int SWIPE_THRESHOLD_VELOCITY = 300;

    public GestureListener(Context context) {
        this.mContext = context;
    }

    public GestureListener(Context c, AttributeSet attrs) {
        super();
    }

    public GestureListener(Context context, GestureDetector gDetector) {
        if (gDetector == null){
            gDetector = new GestureDetector(context, this);
        }
        this.mContext = context;
        this.gDetector = gDetector;
    }

    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX,float velocityY) {
        //Log.e("gesture", String.valueOf(SWIPE_MIN_DISTANCE));
        if (Math.abs(e1.getY() - e2.getY()) > SWIPE_MAX_OFF_PATH) {
            if (Math.abs(e1.getX() - e2.getX()) > SWIPE_MAX_OFF_PATH|| Math.abs(velocityY) < SWIPE_THRESHOLD_VELOCITY) {
                return false;
            }
            else if (e1.getY() - e2.getY() > SWIPE_MIN_DISTANCE) {
                //float traveled = e1.getY() - e2.getY();
                //Toast.makeText(context, Float.toString(traveled) + ">" +String.valueOf(SWIPE_MIN_DISTANCE),Toast.LENGTH_SHORT).show();
                Toast.makeText(mContext, "Swiped Up",Toast.LENGTH_SHORT).show();
            } 
            else if (e2.getY() - e1.getY() > SWIPE_MIN_DISTANCE) {
                //float traveled = e2.getY() - e1.getY();
                //Toast.makeText(context, Float.toString(traveled) + ">" +String.valueOf(SWIPE_MIN_DISTANCE),Toast.LENGTH_SHORT).show();
                Toast.makeText(mContext, "Swiped Down",Toast.LENGTH_SHORT).show();
            }
        } 
        return super.onFling(e1, e2, velocityX, velocityY);
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        return gDetector.onTouchEvent(event);
    }

    public GestureDetector getDetector() {
        return gDetector;
    }

}

我是否需要为 GestureListener.java 使用不同的构造函数才能正确传递 Activity 实例？究竟是什么导致在空对象上调用 onTouch() 方法？下面是详细说明错误的日志。

日志文件

06-15 21:59:46.721: E/InputEventReceiver(6972): Exception dispatching input event.
06-15 21:59:46.721: E/MessageQueue-JNI(6972): Exception in MessageQueue callback: handleReceiveCallback
06-15 21:59:46.741: E/MessageQueue-JNI(6972): java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.view.GestureDetector.onTouchEvent(android.view.MotionEvent)' on a null object reference
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at com.example.guitarsim.GestureListener.onTouch(GestureListener.java:59)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.View.dispatchTouchEvent(View.java:8914)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2673)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2385)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2684)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2346)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2684)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2346)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2684)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2346)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2684)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2346)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2684)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2346)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:2552)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1767)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.app.Activity.dispatchTouchEvent(Activity.java:2840)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:2513)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.View.dispatchPointerEvent(View.java:9125)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4636)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4474)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4024)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4077)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4043)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4157)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4051)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4214)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4024)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4077)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4043)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4051)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4024)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6485)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6384)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6355)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6575)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:185)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.os.MessageQueue.nativePollOnce(Native Method)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.os.MessageQueue.next(MessageQueue.java:143)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.os.Looper.loop(Looper.java:130)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at android.app.ActivityThread.main(ActivityThread.java:5835)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at java.lang.reflect.Method.invoke(Native Method)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at java.lang.reflect.Method.invoke(Method.java:372)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

最佳答案

学习阅读 logcat，它会准确地告诉您问题出在哪里...

06-15 21:59:46.741: E/MessageQueue-JNI(6972): java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.view.GestureDetector.onTouchEvent(android.view.MotionEvent)' on a null object reference
06-15 21:59:46.741: E/MessageQueue-JNI(6972):   at com.example.guitarsim.GestureListener.onTouch(GestureListener.java:59)

问题出在 onTouch(...) 方法中 GestureListener 类的第 59 行...

@Override
public boolean onTouch(View v, MotionEvent event) {
    return gDetector.onTouchEvent(event);
}

基本上您的 gDetector 必须为 null，并且尝试调用它的 onTouchEvent(...) 方法会导致 NPE。

问题在于您在 Activity 方法 configGestureRecognition() 中创建 GestureListener 的实例，如下所示...

gestureListener = new GestureListener(PlayFrets.this);

构造函数 GestureListener(Context context) 的版本没有初始化 GestureDetector

使用以 GestureDetector 作为参数的构造函数...

public GestureListener(Context context, GestureDetector gDetector) {...}

...并传入初始化的 GestureDetector 或 null 以允许它为您创建一个 GestureDetector。

关于java - Android:尝试在空对象引用上调用虚方法 (onTouch)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/30857762/

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