FindViewByID在setOnClickListener上返回null

问题描述：

当我尝试启动我的活动时，出现以下错误。FindViewByID在setOnClickListener上返回null

E/AndroidRuntime﹕ FATAL EXCEPTION: main 
Process: com.example.proofoftraining, PID: 19856 
java.lang.NullPointerException at com.example.app.MainActivity.setupAddActivityButton

这是一块在MainActivity守则：

... 
import android.app.Activity; 
import android.content.Context; 
import android.support.v4.app.FragmentTransaction; 
import android.support.v7.app.ActionBarActivity; 
import android.support.v7.app.ActionBar; 
import android.support.v4.app.Fragment; 
import android.support.v4.app.FragmentManager; 
import android.os.Bundle; 
import android.view.ContextMenu; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.ViewGroup; 
import android.support.v4.widget.DrawerLayout; 
import android.content.Intent; 
import android.widget.BaseAdapter; 
import android.widget.EditText; 
import android.widget.ListView; 

... 

import java.util.ArrayList; 
import java.util.List; 


public class MainActivity extends ActionBarActivity 
implements NavigationDrawerFragment.NavigationDrawerCallbacks { 

/** 
* Fragment managing the behaviors, interactions and presentation of the navigation drawer. 
*/ 
private NavigationDrawerFragment mNavigationDrawerFragment; 

/** 
* Used to store the last screen title. For use in {@link #restoreActionBar()}. 
*/ 
... 

private ActivitiesListAdapter adapter; 

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


@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
if (!mNavigationDrawerFragment.isDrawerOpen()) { 
// Only show items in the action bar relevant to this screen 
// if the drawer is not showing. Otherwise, let the drawer 
// decide what to show in the action bar. 
getMenuInflater().inflate(R.menu.main, menu); 
restoreActionBar(); 

//Set up the Activity List 
setupActivityListViewAdapter(); 
setupAddActivityButton(); 

return true; 
} 
return super.onCreateOptionsMenu(menu); 
} 




private void setupActivityListViewAdapter() { 
adapter = new ActivitiesListAdapter(MainActivity.this, R.layout.item_activities, new ArrayList<activity>()); 
ListView ActivitiesListView = (ListView)findViewById(R.id.listView_activities); 
ActivitiesListView.setAdapter(adapter); 
} 
private void setupAddActivityButton() { 
findViewById(R.id.action_activity_add).setOnClickListener(new View.OnClickListener() { 
@Override 
public void onClick(View v) { 
adapter.insert(new activity(0,0), 0); //Test activity(0,0) 
} 
}); 
} 
}

我的菜单main.xml中

<menu xmlns:android="http://schemas.android.com/apk/res/android" 
    xmlns:app="http://schemas.android.com/apk/res-auto" 
    xmlns:tools="http://schemas.android.com/tools" tools:context=".MainActivity"> 
    <item android:id="@+id/action_activity_add" 
     android:icon="@drawable/ic_action_new" 
     android:title="@string/action_activity_add" 
     app:showAsAction="ifRoom|withText" /> 
</menu>

我在哪里应该实现setupAddActivityButton（），从而使findViewByID将不会为空？

谢谢。

明显'action_activity_add'是不是你的'Activty'布局的一部分......请学习如何在Android的 – Selvin 2015-04-02 12:01:48

由于使用菜单！我明白。 – user4742258 2015-04-02 13:20:29

答

菜单项不能被findViewVyId()找到。

你应该看看如何在Android的活动，onPrepareOptionsMenu()使用menu，onOptionsItemSelected()

例如你可以看看这个教程，它很简单&直截了当 android menu tutorial

答

您可以处理菜单单击事件与下面的代码

@Override 
    public boolean onOptionsItemSelected(MenuItem p_item) 
    { 
     switch (p_item.getItemId()) 
     { 
      case R.id.action_activity_add: 
       // Write your code here. 
       break; 

      default: 
       break; 
     } 
     return super.onOptionsItemSelected(p_item); 
    }

FindViewByID在setOnClickListener上返回null

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