錯誤：空指針異常

Question

我無法從Android向Java桌面服務器獲取價值。我只得到一個輸出。我已經使循環運行了5次。它運行一次並在android上輸出一次，然後崩潰。它應該給android 5次輸出。在每個輸出之後，它應該將價值傳遞給java桌面服務器。

我在做什麼和我想要的是什麼：我正在獲得信號強度。我想要5個樣本。所以我運行代碼5次。只要我一次獲得值，我就可以將它傳遞給Java Desktop Server代碼。我已經寫在java中作爲server.java，它運行得很好。

客戶端代碼：

public class MainActivity extends Activity { 
     protected static final long TIME_DELAY = 1000; 
     TextView mTextView; 
     private WifiManager wifiManager; 
     Handler handler=new Handler(); 
     int count =0; String data =""; 

     private String messsage; 

     private static final IntentFilter FILTER = new IntentFilter(
       WifiManager.SCAN_RESULTS_AVAILABLE_ACTION); 

     @Override 
     protected void onCreate(Bundle savedInstanceState) { 
      super.onCreate(savedInstanceState); 
      setContentView(R.layout.activity_main); 
      mTextView = (TextView) findViewById(R.id.text_id);   
      wifiManager = (WifiManager) getSystemService(WIFI_SERVICE);   
      wifiManager.startScan(); 

     } 


     @Override 
     public void onStart() { 
      super.onStart(); 
      // Register the scan receiver 
      registerReceiver(scanReceiver, FILTER); 
     } 

     @Override 
     public void onStop() { 
      super.onStop(); 
      // Unregister the receiver 
      unregisterReceiver(scanReceiver); 
     } 

     public void onClickRefresh(View v) { 
      count=0; 
      wifiManager.startScan(); 

      //Log.d(TAG, "onClickRefresh"); 
     } 


     BroadcastReceiver scanReceiver = new BroadcastReceiver() { 
      @Override 
      public void onReceive(Context context, Intent intent) { 
      // Log.d(TAG, "scanReceiver received!"); 
       // Refreshes the chart once we have scan results 

       // Request a scan again 

       if(count<5){ 
       final StringBuilder sb = new StringBuilder(); 

       List<ScanResult> wifiScanList = wifiManager.getScanResults(); 
       for (ScanResult result : wifiScanList) { 
         if (result.SSID.equals("Khosla ka Ghosla")) { 
          sb.append(""+result.level); 
         } 
         if (result.SSID.equals("panny")) { 
          sb.append(""+result.level); 
         } 
         if (result.SSID.equals("ferbora")) { 
          sb.append(""+result.level); 
         } 

        //  mTextView.setText("getting called " +count + sb);     
         messsage=sb.toString(); 
         mTextView.setText(messsage); 

        } count++; 
        new Asynctask1().execute(messsage); 
       wifiManager.startScan(); 
       } 
      } 
     }; 

      public class Asynctask1 extends AsyncTask<String, Void, Void> { 
       private PrintWriter printwriter; 
      private static final String IP_ADDRESS = "192.168.0.8"; 
      private static final int DEST_PORT = 4444; 
      private EditText mTextField; 
      protected Void doInBackground(String... messages) { 
       String message = messages[0]; 
       Socket client = null; 
       try { 
        client = new Socket(IP_ADDRESS, DEST_PORT); // connect to server 
       } catch (UnknownHostException e) { 
        e.printStackTrace(); 
       } catch (IOException e) { 
        e.printStackTrace(); 
       } 
       // Write to server. 
       try { 
        printwriter = new PrintWriter(client.getOutputStream(), true); 
        printwriter.write(messsage); // write the message to output stream 
        printwriter.flush(); 
        printwriter.close(); 
       } 
       catch (IOException e) { 
        e.printStackTrace(); 
       } finally { 
        try { 
         client.close(); 
        } catch (IOException e) { 
         e.printStackTrace(); 
        } 
       } 
       return null; 
      } 
     } 
}  

我得到的錯誤是空指針異常。我不知道爲什麼它是空的。它給出了錯誤：finally {try {client.close（）;.

除此之外，日誌貓不會顯示太多東西。你能指導我嗎？

Answer 1

您在doInBackground方法中返回null。