雖然Eclipse的Eclipse應用程序，新秀

Question

我開始學習如何開發Android應用程序。這是我的第一個項目（測試），它不斷給我同樣的錯誤......「應用程序意外停止」，我不知道問題出在哪裏。

MainActivity.java

package com.example.layoutxml2; 

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

public class MainActivity extends ActionBarActivity { 

    @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(); 

      final TextView text1 = (TextView) findViewById(R.id.textView1); 
      Button carregar = (Button) findViewById(R.id.button1); 

      carregar.setOnClickListener(new View.OnClickListener() { 

       @Override 
       public void onClick(View v) { 
        // TODO Auto-generated method stub 
        text1.setText("Xau Pessoal!"); 

       } 
      }); 


     } 
    } 

    @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; 
     } 
    } 

} 

fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:tools="http://schemas.android.com/tools" 
    android:layout_width="match_parent" 
    android:layout_height="match_parent" 
    android:paddingBottom="@dimen/activity_vertical_margin" 
    android:paddingLeft="@dimen/activity_horizontal_margin" 
    android:paddingRight="@dimen/activity_horizontal_margin" 
    android:paddingTop="@dimen/activity_vertical_margin" 
    tools:context="com.example.layoutxml2.MainActivity$PlaceholderFragment" 
    android:background="#285178" > 

    <ImageView 
     android:id="@+id/imageView1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_alignParentTop="true" 
     android:layout_centerHorizontal="true" 
     android:contentDescription="@string/banner" 
     android:src="@drawable/androidbanner" /> 

    <TextView 
     android:id="@+id/textView1" 
     android:layout_width="wrap_content" 
     android:layout_height="wrap_content" 
     android:layout_below="@+id/imageView1" 
     android:layout_centerHorizontal="true" 
     android:layout_marginTop="20dp" 
     android:text="@string/saudacao" 
     android:textSize="26sp" 
     android:textColor="#ffffff" 
     /> 

    <Button 
     android:id="@+id/button1" 
     android:layout_width="fill_parent" 
     android:layout_height="wrap_content" 
     android:layout_alignLeft="@+id/imageView1" 
     android:layout_below="@+id/textView1" 
     android:layout_marginTop="32dp" 
     android:text="@string/carregar" 
     android:textColor="#ffffff" 
     android:onClick="selfdestruct" /> 

</RelativeLayout> 

logcat的

06-14 16:33:35.084: E/AndroidRuntime(1628): FATAL EXCEPTION: main 
06-14 16:33:35.084: E/AndroidRuntime(1628): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.layoutxml2/com.example.layoutxml2.MainActivity}: java.lang.NullPointerException 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.app.ActivityThread.access$1500(ActivityThread.java:117) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.os.Handler.dispatchMessage(Handler.java:99) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.os.Looper.loop(Looper.java:130) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.app.ActivityThread.main(ActivityThread.java:3683) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at java.lang.reflect.Method.invokeNative(Native Method) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at java.lang.reflect.Method.invoke(Method.java:507) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at dalvik.system.NativeStart.main(Native Method) 
06-14 16:33:35.084: E/AndroidRuntime(1628): Caused by: java.lang.NullPointerException 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at com.example.layoutxml2.MainActivity.onCreate(MainActivity.java:28) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611) 
06-14 16:33:35.084: E/AndroidRuntime(1628):  ... 11 more 

很抱歉，如果錯誤是明顯的，但我在這個東西真的新，我想改善。 謝謝你的幫助:) Regards， Ricardo Moreira

Answer 1

你在Fragment佈局中聲明Button，但在活動中使用它們。將代碼移入片段類到onCreateView方法中並使用rootview.findViewById（）...

package com.example.layoutxml2; 
import android.os.Bundle; 
import android.support.v4.app.Fragment; 
import android.support.v7.app.ActionBarActivity; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.Button; 
import android.widget.TextView; 

public class MainActivity extends ActionBarActivity { 

    @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(); 
     } 
    } 

    @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); 

      final TextView text1 = (TextView) rootView.findViewById(R.id.textView1); 
      Button carregar = (Button) rootView.findViewById(R.id.button1); 

      carregar.setOnClickListener(new View.OnClickListener() { 

       @Override 
       public void onClick(View v) { 
        // TODO Auto-generated method stub 
        text1.setText("Xau Pessoal!"); 

       } 
      }); 

      return rootView; 
     } 
    } 
}