不幸的是，应用程序停止了Android

Question

我正在创建一个android应用程序。而即时通讯面临'不幸的应用程序已停止'。我刚添加了Toast活动，但无法运行模拟器。在此之前，我只能在没有吐司活动的情况下查看用户界面。下面是我的java代码。真的很感谢任何帮助我进步！

package com.example.test; 

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





public class Test extends ActionBarActivity { 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_test); 
    if (savedInstanceState == null) { 
     getSupportFragmentManager().beginTransaction() 
       .add(R.id.container, new PlaceholderFragment()).commit(); 

    setupMessageButton(); 

    } 
} 


public void setupMessageButton(){ 

    Button messageButton = (Button) findViewById(R.id.homebtn); 
    messageButton.setOnClickListener(new View.OnClickListener() { 

     @Override 
     public void onClick(View v) { 
      Toast.makeText(Test.this, "Clicked!", Toast.LENGTH_LONG).show(); 


     } 
    }); 



} 


@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_test, container, 
       false); 
     return rootView; 
    } 
} 

}

下面是logcat的：

D/AndroidRuntime(855): Shutting down VM 
W/dalvikvm(855): threadid=1: thread exiting with uncaught exception (group=0x41465700) 
E/AndroidRuntime(855): FATAL EXCEPTION: main 
E/AndroidRuntime(855): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.Test}: java.lang.NullPointerException 
E/AndroidRuntime(855):at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211) 
E/AndroidRuntime(855):at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261) 
E/AndroidRuntime(855):at android.app.ActivityThread.access$600(ActivityThread.java:141) 
E/AndroidRuntime(855): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256) 
E/AndroidRuntime(855): at android.os.Handler.dispatchMessage(Handler.java:99) 
E/AndroidRuntime(855): at android.os.Looper.loop(Looper.java:137) 
E/AndroidRuntime(855): at android.app.ActivityThread.main(ActivityThread.java:5103) 
E/AndroidRuntime(855): at java.lang.reflect.Method.invokeNative(Native Method) 
E/AndroidRuntime(855): at java.lang.reflect.Method.invoke(Method.java:525) 
E/AndroidRuntime(855): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737) 
E/AndroidRuntime(855): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553) 
E/AndroidRuntime(855): at dalvik.system.NativeStart.main(Native Method) 
E/AndroidRuntime(855): Caused by: java.lang.NullPointerException 
E/AndroidRuntime(855): at com.example.test.Test.setupMessageButton(Test.java:36) 
E/AndroidRuntime(855): at com.example.test.Test.onCreate(Test.java:27) 
E/AndroidRuntime(855): at android.app.Activity.performCreate(Activity.java:5133) 
E/AndroidRuntime(855): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087) 
E/AndroidRuntime(855): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175) 
E/AndroidRuntime(855): ... 11 more 

Answer 1

试试这个..

我猜Buttonhomebtn是属于fragment_test.xml所以尽量像下方，也可去除setupMessageButton()方法。

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_test, container, 
       false); 
     Button messageButton = (Button) rootView.findViewById(R.id.homebtn); 
     messageButton.setOnClickListener(new View.OnClickListener() { 

       @Override 
       public void onClick(View v) { 
        Toast.makeText(getActivity(), "Clicked!", Toast.LENGTH_LONG).show(); 


       } 
     }); 
     return rootView; 
    } 
}