ontouchevent in android

Question

我正在做一個應用程序，它將顯示ontouch的位置。 我的代碼如下，我編輯了我的代碼。 公共類AndroidTabLayoutAcActivity延伸活動{

DemoView demoview1 ; 
DemoView2 demoview2; 
DemoView3 demoview3; 
DemoView4 demoview4; 
LowerView lowerview; 

float d,b; 

//ViewFlipper c; 
//ViewFlipper a; 
String text="Oxygen"; 
String text1="Carbon"; 
String text2="Phosphate"; 
String text3="Hydrogen"; 

//Called when the activity is first created. 
@Override 
public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
      demoview1 = new DemoView(this); 
      setContentView(demoview1); 
       demoview1.requestFocus(); 

} 

public class DemoView extends View implements OnTouchListener{ 
    String n1=""; 
      String n2=""; 
      Canvas canvas; 
    public DemoView(Context context){ 
     super(context); 
     setOnTouchListener(this); 

    } 
    public boolean onTouch(View view, MotionEvent event) { 
      // if(event.getAction() != MotionEvent.ACTION_DOWN) 
      // return super.onTouchEvent(event); 

      d = event.getX(); 
      b = event.getY(); 

      n1=Float.toString(d); 
      n2=Float.toString(b); 
       onDraw(canvas); 
      return true; 
     } 

    @Override protected void onDraw(Canvas canvas) { 
     super.onDraw(canvas); 

     int x = 150; 
     int y = 100; 
     Paint paint = new Paint(); 
     paint.setStyle(Paint.Style.FILL); 

     // make the entire canvas white 
     paint.setColor(Color.CYAN); 
     canvas.drawPaint(paint); 
     paint.setAntiAlias(true); 
     paint.setColor(Color.RED); 
     canvas.drawCircle(x-24, y+30, 11, paint); 
     paint.setColor(Color.WHITE); 
     canvas.drawCircle(x+11, y-11, 7, paint); 
     canvas.drawCircle(x+6, y+48, 7, paint); 
     canvas.drawCircle(x+21, y+43, 7, paint); 
     //carbon molecule 
     paint.setColor(Color.BLACK); 
     canvas.drawCircle(x, y, 11, paint); 
     canvas.drawCircle(x-11, y+19, 11, paint); 
     canvas.drawCircle(x, y+38, 11, paint); 
     canvas.drawCircle(x+20, y+30, 11, paint); 
     canvas.drawCircle(x-11, y-19, 11, paint); 

     //hydrogen molecule 
     paint.setColor(Color.WHITE); 
     canvas.drawCircle(x-25, y-12, 7, paint); 
     canvas.drawCircle(x+2, y-27, 7, paint); 
     canvas.drawCircle(x-20, y+9, 7, paint); 
     canvas.drawCircle(x-10, y+45, 7, paint); 
     canvas.drawCircle(x-34, y+40, 7, paint); 
     canvas.drawCircle(x+51, y+18, 7, paint); 
     canvas.drawCircle(x-33, y-52, 7, paint); 

     //oxygen molecule 
     paint.setColor(Color.RED); 
     canvas.drawCircle(x+41, y+30, 11, paint); 
     canvas.drawCircle(x+20, y+9, 11, paint); 
     canvas.drawCircle(x-23, y-37, 11, paint); 
     canvas.drawText(n1, x+100, y+100, paint); 
     canvas.drawText(n2, x+100, y+100, paint); 

    } 

} 

}

我的日誌貓文件顯示下面的錯誤，但我沒有得到那爲什麼它顯示以下錯誤..

05-07 12:26:31.354: E/AndroidRuntime(385): FATAL EXCEPTION: main 
05-07 12:26:31.354: E/AndroidRuntime(385): java.lang.NullPointerException 
05-07 12:26:31.354: E/AndroidRuntime(385): at com.example.an.AndroidTabLayoutAcActivity$DemoView.onDraw(AndroidTabLayoutAcActivity.java:150) 
05-07 12:26:31.354: E/AndroidRuntime(385): at com.example.an.AndroidTabLayoutAcActivity$DemoView.onTouch(AndroidTabLayoutAcActivity.java:136) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.view.View.dispatchTouchEvent(View.java:3762) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:863) 
05-07 12:26:31.354: E/AndroidRuntime(385): at com.android.internal.policy.impl.PhoneWindow$DecorView.superDispatchTouchEvent(PhoneWindow.java:1671) 
05-07 12:26:31.354: E/AndroidRuntime(385): at com.android.internal.policy.impl.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1107) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.app.Activity.dispatchTouchEvent(Activity.java:2086) 
05-07 12:26:31.354: E/AndroidRuntime(385): at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchTouchEvent(PhoneWindow.java:1655) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.view.ViewRoot.handleMessage(ViewRoot.java:1785) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.os.Handler.dispatchMessage(Handler.java:99) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.os.Looper.loop(Looper.java:123) 
05-07 12:26:31.354: E/AndroidRuntime(385): at android.app.ActivityThread.main(ActivityThread.java:4627) 
05-07 12:26:31.354: E/AndroidRuntime(385): at java.lang.reflect.Method.invokeNative(Native Method) 
05-07 12:26:31.354: E/AndroidRuntime(385): at java.lang.reflect.Method.invoke(Method.java:521) 
05-07 12:26:31.354: E/AndroidRuntime(385): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 
05-07 12:26:31.354: E/AndroidRuntime(385): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 
05-07 12:26:31.354: E/AndroidRuntime(385): at dalvik.system.NativeStart.main(Native Method) 

Answer 1

我假設，

你得到這些行錯誤，

canvas.drawText(n1, x+100, y+100, paint); 
canvas.drawText(n2, x+100, y+100, paint); 

當你setContentView(demoview1)
那麼你demoVIew的onDraw()稱爲第一，仍然onTouch()不是， 那麼你沒有的String n1, n2一個值尚未..都是空，因此它給你NullPointerException異常

因此，嘗試，

String n1 = "", n2=""; 
     public DemoView(Context context){ 
      super(context); 
      setOnTouchListener(this); 
     } 
     public boolean onTouch(View view, MotionEvent event) { 
     // if(event.getAction() != MotionEvent.ACTION_DOWN) 
     // return super.onTouchEvent(event); 

     d = event.getX(); 
     b = event.getY(); 

     n1 = d +""; 
     n2 = b +""; 
      return true; 
    } 

其實我認爲你必須初始化String n1 = "", n2 = "";