我的Android應用程序中的NullPointerException

Question

我對以下內容有點問題。我在我的筆記本電腦上有一個運行在Java中的TCP服務器，並且正在開發一個Android應用程序，它將在X按鈕被按下時向服務器發送一個字符串。我一直在四處尋找，並沒有找到任何解脫。

我測試過我的手機能夠連接到筆記本電腦（同樣的Wifi），它工作得很好，但我所做的應用程序不會。

主要活動代碼：

public class MainActivity extends Activity { 


private String serverIpAddress = "10.30.13.110"; 
private int serverPort = 50505; 
private boolean connected = false; 
final Button button1 = (Button) findViewById(R.id.Button1); 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 

    button1.setOnClickListener(onClickListener); 
} 


private OnClickListener onClickListener = new OnClickListener() { 
    @Override 
    public void onClick(final View v) { 
     switch(v.getId()){ 
      case R.id.Button1: 
       if (!connected) { 
        Thread cThread = new Thread(new ClientThread()); 
        cThread.start(); 
       } 
      break; 

     } 
    } 
}; 

public class ClientThread implements Runnable { 

    public void run() { 
     try { 
      Log.d("ClientActivity", "C: Connecting..."); 
      Socket socket = new Socket(serverIpAddress, serverPort); 
      connected = true; 
      while (connected) { 
       try { 
        Log.d("ClientActivity", "C: Sending command."); 
        PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())), true); 
         out.println("Hey Server!"); 
         Log.d("ClientActivity", "C: Sent."); 
       } catch (Exception e) { 
        Log.e("ClientActivity", "Error", e); 
       } 
      } 
      socket.close(); 
      Log.d("ClientActivity", "C: Closed."); 
     } catch (Exception e) { 
      Log.e("ClientActivity", "Error", e); 
      connected = false; 
     } 
    } 
} 
} 

logcat的錯誤：

04-24 18:07:44.158: E/AndroidRuntime(20554): FATAL EXCEPTION: main 
04-24 18:07:44.158: E/AndroidRuntime(20554): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.project1test/com.example.project1test.MainActivity}: java.lang.NullPointerException 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2173) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2297) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.ActivityThread.access$700(ActivityThread.java:152) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1282) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.os.Handler.dispatchMessage(Handler.java:99) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.os.Looper.loop(Looper.java:137) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.ActivityThread.main(ActivityThread.java:5328) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at java.lang.reflect.Method.invokeNative(Native Method) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at java.lang.reflect.Method.invoke(Method.java:511) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at dalvik.system.NativeStart.main(Native Method) 
04-24 18:07:44.158: E/AndroidRuntime(20554): Caused by: java.lang.NullPointerException 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.Activity.findViewById(Activity.java:1886) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at com.example.project1test.MainActivity.<init>(MainActivity.java:23) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at java.lang.Class.newInstanceImpl(Native Method) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at java.lang.Class.newInstance(Class.java:1319) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.Instrumentation.newActivity(Instrumentation.java:1071) 
04-24 18:07:44.158: E/AndroidRuntime(20554): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2164) 

我很新到Android編程，因此任何指導，將不勝感激。謝謝

Answer 1

當你撥Activity.findViewById爲button1你投擲NullPointerException。撥打Activity.setContentView後，您應該初始化您的Button，通常在Activity.onCreate完成。例如：

Button button1; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.your_layout_that_contains_button1); 

    button1 = (Button) findViewById(R.id.Button1); 
    button1.setOnClickListener(onClickListener); 
}