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Android：嘗試在空對象引用上調用虛擬方法（onTouch）

[英]Android: Attempt to invoke virtual method (onTouch) on a null object reference

 原文  2015-06-16 02:12:55  4   1    java/ android/ nullpointerexception

問題描述

我正在嘗試在我的應用程序視圖中的線性布局之一上設置手勢檢測。 問題是，每當我用揮舞的手勢對其進行測試時，我的應用就會崩潰。 我收到的錯誤消息是臭名昭著的“試圖在空對象引用上調用虛擬方法”。 我已經進行了研究，並且了解到問題源於試圖在未正確傳遞到代碼內必要范圍內的Activity上調用方法。 話雖這么說，我不知道如何在我的代碼中解決該問題。 以下是相關的代碼位：

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;
    }

}

我需要為GestureListener.java使用其他構造函數來正確傳入活動實例嗎？ 究竟是什么導致在空對象上調用onTouch（）方法？ 下面是詳細記錄該錯誤的日志。

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)

1 個解決方案

解決方案1
 4 已采納  2015-06-16 02:43:35

學習閱讀logcat，它可以告訴您確切的問題是什么... 

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)

問題出在onTouch(...)方法中GestureListener類的第59行... 

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

基本上，您的gDetector必須為null，並且嘗試調用導致NPE的onTouchEvent(...)方法。

問題在於，您可以在Activity方法configGestureRecognition()創建GestureListener的實例，如下所示... 

gestureListener = new GestureListener(PlayFrets.this);

構造函數GestureListener(Context context)的版本未初始化GestureDetector

使用將GestureDetector作為參數的構造函數... 

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

...並傳入初始化的GestureDetector或為null以允許它為您創建GestureDetector 。

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