調用canvas.drawText（）時發生NullPointerException。爲什麼？

Question

這是我之前詢問的問題的後續內容。基本上我有一個遊戲，主要動作發生在一個自View類擴展的自定義類的畫布上。

我將分數保存在一個字符串中，我試圖將它繪製到屏幕的左上角。

在我的onDraw（）方法的底部，我有下面的代碼行：

canvas.drawText(score, 5, 5, null); 

當我試圖運行遊戲，登錄貓給了我以下錯誤：

03-02 22:30:29.955: ERROR/AndroidRuntime(8140): FATAL EXCEPTION: main 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140): java.lang.NullPointerException 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.graphics.Canvas.drawText(Native Method) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at com.mattdrewery.supercatch.GameView.onDraw(GameView.java:143) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.View.draw(View.java:6880) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewGroup.drawChild(ViewGroup.java:1646) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.View.draw(View.java:6883) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.widget.FrameLayout.draw(FrameLayout.java:357) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewGroup.drawChild(ViewGroup.java:1646) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewGroup.drawChild(ViewGroup.java:1644) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1373) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.View.draw(View.java:6883) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.widget.FrameLayout.draw(FrameLayout.java:357) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1862) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewRoot.draw(ViewRoot.java:1522) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewRoot.performTraversals(ViewRoot.java:1258) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.view.ViewRoot.handleMessage(ViewRoot.java:1859) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.os.Handler.dispatchMessage(Handler.java:99) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.os.Looper.loop(Looper.java:123) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at android.app.ActivityThread.main(ActivityThread.java:3683) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at java.lang.reflect.Method.invokeNative(Native Method) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at java.lang.reflect.Method.invoke(Method.java:507) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) 
03-02 22:30:29.955: ERROR/AndroidRuntime(8140):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

您的Paint參數不能爲空。

初始化它的某處（不在onDraw）;

Paint mPaint = new Paint(); 

canvas.drawText(score, 5, 5, mPaint);