嘗試將消息從一個活動發送到另一個活動時，出現空指針異常

Question

當我嘗試實現將字符串消息從一個活動發送到擴展Service的BluetoothService活動時，為什么會有NullPointerException錯誤。

這是發送字符串所涉及的代碼的主要部分：

private BluetoothService mChatService = null;

public void onCaptureButtonClicked(View view) throws IOException{                                   
            try{
                    sendMessage("1");
                }catch(NullPointerException e){
                    e.printStackTrace();
                }

            takePhoto(1);
        }
        }

private void sendMessage(String message) {

             if (mChatService.getState() != BluetoothService.STATE_CONNECTED) {
             return;
         }

         if (message.length() > 0) {
             byte[] send = message.getBytes();
             mChatService.write(send);
         }
     }

這是錯誤logcat：

02-15 18:21:19.960: W/System.err(1252): java.lang.NullPointerException
02-15 18:21:19.960: W/System.err(1252):     at com.example.application.parss.CameraActivity.sendMessage(CameraActivity.java:751)
02-15 18:21:19.960: W/System.err(1252):     at com.example.application.parss.CameraActivity.onCaptureButtonClicked(CameraActivity.java:245)
02-15 18:21:19.960: W/System.err(1252):     at java.lang.reflect.Method.invokeNative(Native Method)
02-15 18:21:19.960: W/System.err(1252):     at java.lang.reflect.Method.invoke(Method.java:507)
02-15 18:21:19.960: W/System.err(1252):     at android.view.View$1.onClick(View.java:2180)
02-15 18:21:19.960: W/System.err(1252):     at android.view.View.performClick(View.java:2585)
02-15 18:21:19.960: W/System.err(1252):     at android.view.View$PerformClick.run(View.java:9299)
02-15 18:21:19.960: W/System.err(1252):     at android.os.Handler.handleCallback(Handler.java:587)
02-15 18:21:19.960: W/System.err(1252):     at android.os.Handler.dispatchMessage(Handler.java:92)
02-15 18:21:19.960: W/System.err(1252):     at android.os.Looper.loop(Looper.java:130)
02-15 18:21:19.960: W/System.err(1252):     at android.app.ActivityThread.main(ActivityThread.java:3691)
02-15 18:21:19.960: W/System.err(1252):     at java.lang.reflect.Method.invokeNative(Native Method)
02-15 18:21:19.970: W/System.err(1252):     at java.lang.reflect.Method.invoke(Method.java:507)
02-15 18:21:19.970: W/System.err(1252):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:912)
02-15 18:21:19.970: W/System.err(1252):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:670)
02-15 18:21:19.970: W/System.err(1252):     at dalvik.system.NativeStart.main(Native Method)

編輯：

我以這種方式初始化了mChatSession：

 mChatService = new BluetoothService(this, mHandler);

並稱為：mChatService.write（send）;

但是類似的NullPointerException錯誤仍然存​​在。

Answer 1

我想您因為沒有初始化BluetoothService而收到了異常。

private BluetoothService mChatService = null;

而且，您正在sendMessage()方法中使用mChatService.getState()而不進行初始化。

Answer 2

嘗試

     if(mChatService == null)
         return;
     if (message.length() > 0 && mChatService.getState() == BluetoothService.STATE_CONNECTED) {
         byte[] send = message.getBytes();
         mChatService.write(send);
     }

Answer 3

BluetoothService mChatService = new BluetoothService(args);

您需要在某個時候為該變量分配一個實例。