那些遇到过的问题

Android:尝试在空对象引用上调用虚方法(onTouch)

Cod*_*ody 6 java android nullpointerexception

我正在尝试在我的应用中的视图上的一个线性布局上设置手势检测.问题是我的应用程序崩溃,每当我用一个手势进行测试时.我收到的错误消息是臭名昭着的"尝试在空对象引用上调用虚方法".我已经完成了研究,我知道问题源于试图调用一个没有正确传递到一个代码中必要范围的Activity上的方法.话虽这么说,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);
        }
}
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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
}
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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;
    }

}
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我是否需要为GestureListener.java使用不同的构造函数来正确传入活动的实例？究竟是什么导致在null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)
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Squ*_*onk 5

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)
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问题出GestureListener在onTouch(...)方法中您班级的第59 行...

@Override
public boolean onTouch(View v, MotionEvent event) {
    return gDetector.onTouchEvent(event);
}
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基本上，您gDetector必须为空，并且尝试调用onTouchEvent(...)导致 NPE 的方法。

问题是您GestureListener在Activity方法中创建的实例configGestureRecognition()如下...

gestureListener = new GestureListener(PlayFrets.this);
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构造函数的版本GestureListener(Context context)没有初始化GestureDetector

使用将 aGestureDetector作为参数的构造函数...

public GestureListener(Context context, GestureDetector gDetector) {...}
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...并传入一个已初始化的GestureDetector或null允许它GestureDetector为您创建一个。

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