Android應用程序崩潰時使用按鈕OnClick監聽器

Question

我一直在測試一個簡單的android計算器應用程序，但它會一直崩潰，當我使用按鈕OnClick偵聽器。我嘗試使用android：onClick =「添加」，它工作正常，但只有當我把所有的txtA =（EditText）findViewById（R.id.txtA）;在「添加」子程序中。我真的想要使用OnClick監聽器，所以任何幫助將不勝感激。這裏是我的代碼：

package com.example.calculadora; 

import android.support.v7.app.ActionBarActivity; 
import android.support.v7.app.ActionBar; 
import android.support.v4.app.Fragment; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.view.ViewGroup; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 
import android.os.Build; 

public class Main extends ActionBarActivity { 
    private EditText txtA; 
    private EditText txtB; 
    private TextView lblSuma; 
    private Button btnsumar; 

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

     if (savedInstanceState == null) { 
      getSupportFragmentManager().beginTransaction() 
        .add(R.id.container, new PlaceholderFragment()) 
        .commit(); 
     } 
     txtA = (EditText) findViewById(R.id.txtA); 
     txtB = (EditText) findViewById(R.id.txtB); 
     lblSuma = (TextView) findViewById(R.id.lblSuma);   
     btnsumar = (Button) findViewById(R.id.btnSuma); 

     btnsumar.setOnClickListener(new OnClickListener() { 

      public void onClick(View arg0) { 

       //float A = Float.parseFloat(txtA.getText().toString()); 
       //float B = Float.parseFloat(txtB.getText().toString()); 
       //float Suma = A + B; 
       //lblSuma.setText(""+Suma); 
      } 

      }); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 

     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     if (id == R.id.action_settings) { 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 

    /** 
    * A placeholder fragment containing a simple view. 
    */ 
    public static class PlaceholderFragment extends Fragment { 

     public PlaceholderFragment() { 
     } 

     @Override 
     public View onCreateView(LayoutInflater inflater, ViewGroup container, 
       Bundle savedInstanceState) { 
      View rootView = inflater.inflate(R.layout.fragment_main, container, false); 
      return rootView; 
     } 
    } 

} 

這裏是logcat的：

04-13 10:48:17.966: W/ApplicationPackageManager(26281): getCSCPackageItemText() 
04-13 10:48:18.021: D/AndroidRuntime(26281): Shutting down VM 
04-13 10:48:18.021: W/dalvikvm(26281): threadid=1: thread exiting with uncaught exception (group=0x4180ac08) 
04-13 10:48:18.021: E/AndroidRuntime(26281): FATAL EXCEPTION: main 
04-13 10:48:18.021: E/AndroidRuntime(26281): Process: com.example.calculadora, PID: 26281 
04-13 10:48:18.021: E/AndroidRuntime(26281): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.calculadora/com.example.calculadora.Main}: java.lang.NullPointerException 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2305) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2363) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.ActivityThread.access$900(ActivityThread.java:161) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1265) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.os.Handler.dispatchMessage(Handler.java:102) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.os.Looper.loop(Looper.java:157) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.ActivityThread.main(ActivityThread.java:5356) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at java.lang.reflect.Method.invokeNative(Native Method) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at java.lang.reflect.Method.invoke(Method.java:515) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at dalvik.system.NativeStart.main(Native Method) 
04-13 10:48:18.021: E/AndroidRuntime(26281): Caused by: java.lang.NullPointerException 
04-13 10:48:18.021: E/AndroidRuntime(26281): at com.example.calculadora.Main.onCreate(Main.java:39) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.Activity.performCreate(Activity.java:5426) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105) 
04-13 10:48:18.021: E/AndroidRuntime(26281): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2269) 
04-13 10:48:18.021: E/AndroidRuntime(26281): ... 11 more 

Answer 1

試試這個..

我猜ButtonEditTextTextView是屬於fragment_main.xml所以試着像下面。

public static class PlaceholderFragment extends Fragment { 

    public PlaceholderFragment() { 
    } 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
      Bundle savedInstanceState) { 
     View rootView = inflater.inflate(R.layout.fragment_main, container, false); 

     txtA = (EditText) rootView.findViewById(R.id.txtA); 
     txtB = (EditText) rootView.findViewById(R.id.txtB); 
     lblSuma = (TextView) rootView.findViewById(R.id.lblSuma);   
     btnsumar = (Button) rootView.findViewById(R.id.btnSuma); 

     btnsumar.setOnClickListener(new OnClickListener() { 

      public void onClick(View arg0) { 

      } 

     }); 

     return rootView; 
    } 
} 

Answer 2

問題是與你的onclick代碼。它更改爲以下：

btnsumar .setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      // TODO Auto-generated method stub 

     } 
    });