blocks|key|1890517|text|本教程可能已经过时了，它试图创建一个基于活动的UI，而不是向导生成的代码所首选的基于片段的UI。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1890518|视图在片段布局(fragment_main.xml)中，而不在活动布局(activity_main.xml)中。onCreate()在生命周期中太早，无法在activity视图层次结构中找到它，因此会返回一个null。在null上调用方法会导致NPE。|offset|length|style|CODE|1890519|首选的解决方案是将代码移动到片段onCreateView()，在膨胀的片段布局rootView上调用findViewById()|1890520|@Override
public+View+onCreateView(LayoutInflater+inflater,+ViewGroup+container,
++++Bundle+savedInstanceState)+{
++View+rootView+=+inflater.inflate(R.layout.fragment_main,+container,
++++++false);

++View+something+=+rootView.findViewById(R.id.something);+//+not+activity+findViewById()
++something.setOnClickListener(new+View.OnClickListener()+{+...+});

++return+rootView;
}|code-block|syntax|javascript|1890521|作为附注，片段布局最终将成为活动视图层次结构的一部分，并可通过activity+findViewById()发现，但仅在运行片段事务之后。挂起的片段事务在onCreate()之后在super.onStart()中执行。|1890522|entityMap^0|0|8|H|10|H|1K|A|2W|4|32|4|0|G|E|13|8|1E|E|0|0|14|E|25|A|2I|F|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@$D|U|E|V|F|G]|$D|W|E|X|F|G]|$D|Y|E|Z|F|G]|$D|10|E|11|F|G]|$D|12|E|13|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|14|8|@$D|15|E|16|F|G]|$D|17|E|18|F|G]|$D|19|E|1A|F|G]]|9|@]|A|$]]|$1|J|3|K|5|L|7|1B|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|1C|8|@$D|1D|E|1E|F|G]|$D|1F|E|1G|F|G]|$D|1H|E|1I|F|G]]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]]|R|$]]

The tutorial is probably outdated, attempting to create an activity-based UI instead of the fragment-based UI preferred by wizard-generated code.

The view is in the fragment layout (<code>fragment_main.xml</code>) and not in the activity layout (<code>activity_main.xml</code>). <code>onCreate()</code> is too early in the lifecycle to find it in the activity view hierarchy, and a <code>null</code> is returned. Invoking a method on <code>null</code> causes the NPE.

The preferred solution is to move the code to the fragment <code>onCreateView()</code>, calling <code>findViewById()</code> on the inflated fragment layout <code>rootView</code>:

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

 View something = rootView.findViewById(R.id.something); // not activity findViewById()
 something.setOnClickListener(new View.OnClickListener() { ... });

 return rootView;
}
</code></pre>

As a side note, the fragment layout will eventually be a part of the activity view hierarchy and discoverable with activity <code>findViewById()</code> but only after the fragment transaction has been run. Pending fragment transactions get executed in <code>super.onStart()</code> after <code>onCreate()</code>.

blocks|key|1890588|text|既然您已经在fragment_main.xml中声明了自己的视图，那么就将这段代码移到片段的onCreateView()方法中获取NPE的位置。这应该可以解决这个问题。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1890589|entityMap^0|6|H|1A|E|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

Since you have declared your View in the <code>fragment_main.xml</code>,move that piece of code where you get the NPE in the <code>onCreateView()</code> method of the fragment.
This should solve the issue.

blocks|key|1890597|text|同意，这是一个典型的错误，因为当人们开始开发Android时，他们通常并不真正了解碎片是如何工作的。为了减少混淆，我创建了一个简单的示例代码，我最初在Application+is+stopped+in+android+emulator上发布了它，但我也在这里发布了它。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1890598|下面是一个示例：|1890599|public+class+ContainerActivity+extends+FragmentActivity+implements+ExampleFragment.Callback
{
++++@Override
++++public+void+onCreate(Bundle+saveInstanceState)
++++{
++++++++super.onCreate(saveInstanceState);
++++++++this.setContentView(R.layout.activity_container);
++++++++if+(saveInstanceState+==+null)
++++++++{+++++++++++++++
+++++++++++++getSupportFragmentManager().beginTransaction()
++++++++++++++++.add(R.id.activity_container_container,+new+ExampleFragment())
++++++++++++++++.addToBackStack(null)
+++++++++++++.commit();
++++++++}
++++++++getSupportFragmentManager().addOnBackStackChangedListener(new+OnBackStackChangedListener()
++++++++{
++++++++++++public+void+onBackStackChanged()
++++++++++++{
++++++++++++++++int+backCount+=+getSupportFragmentManager().getBackStackEntryCount();
++++++++++++++++if+(backCount+==+0)
++++++++++++++++{
++++++++++++++++++++finish();
++++++++++++++++}
++++++++++++}
++++++++});
++++}

++++@Override
++++public+void+exampleFragmentCallback()
++++{
++++++++Toast.makeText(this,+"Hello!",+Toast.LENGTH_LONG).show();
++++}
}|code-block|syntax|javascript|1890600|activity_container.xml：|1890601|<?xml+version="1.0"+encoding="utf-8"?>
<RelativeLayout+xmlns:android="http://schemas.android.com/apk/res/android"
++++android:layout_width="match_parent"
++++android:layout_height="match_parent"
++++android:orientation="vertical"+>

++++<FrameLayout
++++++++android:id="@%2Bid/activity_container_container"
++++++++android:layout_width="match_parent"
++++++++android:layout_height="match_parent"+/>

</RelativeLayout>|1890602|ExampleFragment：|1890603|public+class+ExampleFragment+extends+Fragment+implements+View.OnClickListener
{
++++public+static+interface+Callback
++++{
++++++++void+exampleFragmentCallback();
++++}

++++private+Button+btnOne;
++++private+Button+btnTwo;
++++private+Button+btnThree;

++++private+Callback+callback;

++++@Override
++++public+void+onAttach(Activity+activity)
++++{
++++++++super.onAttach(activity);
++++++++try
++++++++{
++++++++++++this.callback+=+(Callback)+activity;
++++++++}
++++++++catch+(ClassCastException+e)
++++++++{
++++++++++++Log.e(this.getClass().getSimpleName(),+"Activity+must+implement+Callback+interface.",+e);
++++++++++++throw+e;
++++++++}
++++}

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

++++++++btnOne+=+(Button)+rootView.findViewById(R.id.example_button_one);
++++++++btnTwo+=+(Button)+rootView.findViewById(R.id.example_button_two);
++++++++btnThree+=+(Button)+rootView.findViewById(R.id.example_button_three);

++++++++btnOne.setOnClickListener(this);
++++++++btnTwo.setOnClickListener(this);
++++++++btnThree.setOnClickListener(this);
++++++++return+rootView;
++++}

++++@Override
++++public+void+onClick(View+v)
++++{
++++++++if+(btnOne+==+v)
++++++++{
++++++++++++Toast.makeText(getActivity(),+"One.",+Toast.LENGTH_LONG).show();
++++++++}
++++++++else+if+(btnTwo+==+v)
++++++++{
++++++++++++Toast.makeText(getActivity(),+"Two.",+Toast.LENGTH_LONG).show();
++++++++}
++++++++else+if+(btnThree+==+v)
++++++++{
++++++++++++callback.exampleFragmentCallback();
++++++++}
++++}
}|1890604|fragment_example.xml：|1890605|<?xml+version="1.0"+encoding="utf-8"?>
<RelativeLayout+xmlns:android="http://schemas.android.com/apk/res/android"
++++++++android:layout_width="match_parent"
++++++++android:layout_height="match_parent"+>

++++++++<Button
++++++++++++android:id="@%2Bid/example_button_one"
++++++++++++android:layout_width="match_parent"
++++++++++++android:layout_height="wrap_content"
++++++++++++android:layout_alignParentTop="true"
++++++++++++android:layout_centerHorizontal="true"
++++++++++++android:layout_marginTop="30dp"
++++++++++++android:text="@string/hello"+
++++++++++++android:layout_marginLeft="20dp"
++++++++++++android:layout_marginRight="20dp"/>

++++++++<Button
++++++++++++android:id="@%2Bid/example_button_two"
++++++++++++android:layout_width="wrap_content"
++++++++++++android:layout_height="wrap_content"
++++++++++++android:layout_alignLeft="@%2Bid/example_button_one"
++++++++++++android:layout_alignRight="@%2Bid/example_button_one"
++++++++++++android:layout_below="@%2Bid/example_button_one"
++++++++++++android:layout_marginTop="30dp"
++++++++++++android:text="@string/hello"+/>

++++++++<Button
++++++++++++android:id="@%2Bid/example_button_three"
++++++++++++android:layout_width="wrap_content"
++++++++++++android:layout_height="wrap_content"
++++++++++++android:layout_alignLeft="@%2Bid/example_button_two"
++++++++++++android:layout_alignRight="@%2Bid/example_button_two"
++++++++++++android:layout_below="@%2Bid/example_button_two"
++++++++++++android:layout_marginTop="30dp"
++++++++++++android:text="@string/hello"+/>

</RelativeLayout>|1890606|这应该是一个有效的示例，它展示了如何使用活动来显示片段，以及如何处理片段中的事件。以及如何与包含活动进行通信。|1890607|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/24526296/application-is-stopped-in-android-emulator/24527201#24527201^0|23|16|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@$A|17|B|18|1|19]]|C|$]]|$1|D|3|E|5|6|7|1A|8|@]|9|@]|C|$]]|$1|F|3|G|5|H|7|1B|8|@]|9|@]|C|$I|J]]|$1|K|3|L|5|6|7|1C|8|@]|9|@]|C|$]]|$1|M|3|N|5|H|7|1D|8|@]|9|@]|C|$I|J]]|$1|O|3|P|5|6|7|1E|8|@]|9|@]|C|$]]|$1|Q|3|R|5|H|7|1F|8|@]|9|@]|C|$I|J]]|$1|S|3|T|5|6|7|1G|8|@]|9|@]|C|$]]|$1|U|3|V|5|H|7|1H|8|@]|9|@]|C|$I|J]]|$1|W|3|X|5|6|7|1I|8|@]|9|@]|C|$]]|$1|Y|3|-4|5|6|7|1J|8|@]|9|@]|C|$]]]|Z|$10|$5|11|12|13|C|$14|15]]]]

Agreed, this is a typical error because people often don't really understand how Fragments work when they begin working on Android development. To alleviate confusion, I created a simple example code that I originally posted on <a href="https://stackoverflow.com/questions/24526296/application-is-stopped-in-android-emulator/24527201#24527201">Application is stopped in android emulator</a> , but I posted it here as well.

An example is the following:

<pre><code>public class ContainerActivity extends FragmentActivity implements ExampleFragment.Callback
{
 @Override
 public void onCreate(Bundle saveInstanceState)
 {
 super.onCreate(saveInstanceState);
 this.setContentView(R.layout.activity_container);
 if (saveInstanceState == null)
 { 
 getSupportFragmentManager().beginTransaction()
 .add(R.id.activity_container_container, new ExampleFragment())
 .addToBackStack(null)
 .commit();
 }
 getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener()
 {
 public void onBackStackChanged()
 {
 int backCount = getSupportFragmentManager().getBackStackEntryCount();
 if (backCount == 0)
 {
 finish();
 }
 }
 });
 }

 @Override
 public void exampleFragmentCallback()
 {
 Toast.makeText(this, "Hello!", Toast.LENGTH_LONG).show();
 }
}
</code></pre>

activity_container.xml:

<pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical" &gt;

 &lt;FrameLayout
 android:id="@+id/activity_container_container"
 android:layout_width="match_parent"
 android:layout_height="match_parent" /&gt;

&lt;/RelativeLayout&gt;
</code></pre>

ExampleFragment:

<pre><code>public class ExampleFragment extends Fragment implements View.OnClickListener
{
 public static interface Callback
 {
 void exampleFragmentCallback();
 }

 private Button btnOne;
 private Button btnTwo;
 private Button btnThree;

 private Callback callback;

 @Override
 public void onAttach(Activity activity)
 {
 super.onAttach(activity);
 try
 {
 this.callback = (Callback) activity;
 }
 catch (ClassCastException e)
 {
 Log.e(this.getClass().getSimpleName(), "Activity must implement Callback interface.", e);
 throw e;
 }
 }

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

 btnOne = (Button) rootView.findViewById(R.id.example_button_one);
 btnTwo = (Button) rootView.findViewById(R.id.example_button_two);
 btnThree = (Button) rootView.findViewById(R.id.example_button_three);

 btnOne.setOnClickListener(this);
 btnTwo.setOnClickListener(this);
 btnThree.setOnClickListener(this);
 return rootView;
 }

 @Override
 public void onClick(View v)
 {
 if (btnOne == v)
 {
 Toast.makeText(getActivity(), "One.", Toast.LENGTH_LONG).show();
 }
 else if (btnTwo == v)
 {
 Toast.makeText(getActivity(), "Two.", Toast.LENGTH_LONG).show();
 }
 else if (btnThree == v)
 {
 callback.exampleFragmentCallback();
 }
 }
}
</code></pre>

fragment_example.xml:

<pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
&lt;RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:layout_height="match_parent" &gt;

 &lt;Button
 android:id="@+id/example_button_one"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:layout_alignParentTop="true"
 android:layout_centerHorizontal="true"
 android:layout_marginTop="30dp"
 android:text="@string/hello" 
 android:layout_marginLeft="20dp"
 android:layout_marginRight="20dp"/&gt;

 &lt;Button
 android:id="@+id/example_button_two"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_alignLeft="@+id/example_button_one"
 android:layout_alignRight="@+id/example_button_one"
 android:layout_below="@+id/example_button_one"
 android:layout_marginTop="30dp"
 android:text="@string/hello" /&gt;

 &lt;Button
 android:id="@+id/example_button_three"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:layout_alignLeft="@+id/example_button_two"
 android:layout_alignRight="@+id/example_button_two"
 android:layout_below="@+id/example_button_two"
 android:layout_marginTop="30dp"
 android:text="@string/hello" /&gt;

&lt;/RelativeLayout&gt;
</code></pre>

And that should be a valid example, it shows how you can use an Activity to display a Fragment, and handle events in that Fragment. And also how to communicate with the containing Activity.

blocks|key|1890645|text|在上面发布的问题代码中有一个问题:您在oncreate方法中使用了R.layout.activity_main，但xml文件名为"+fragment_main.xml“，这意味着您正在尝试获取未显示的fragment_main.xml文件的视图，因此它会给出空指针异常。如下所示更改代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1890646|@Override
protected+void+onCreate(Bundle+savedInstanceState)+{
++++super.onCreate(savedInstanceState);
++++setContentView(R.layout.fragment_main);//+your+xml+layout+,where+the+views+are

++++View+something+=+findViewById(R.id.something);
++++something.setOnClickListener(new+View.OnClickListener()+{+...+});+//+NPE+HERE

++++if+(savedInstanceState+==+null)+{
++++++++getSupportFragmentManager().beginTransaction()
++++++++++++++++.add(R.id.container,+new+PlaceholderFragment()).commit();
++++}
}|code-block|syntax|javascript|1890647|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

in the posted code above in the question there is a problem :
you are using R.layout.activity_main in oncreate method, but the xml files name is "fragment_main.xml" , means you are trying to get the view of fragment_main.xml file which is not being shown so it gives null pointer exception. change the code like :

<pre><code>@Override
protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.fragment_main);// your xml layout ,where the views are

 View something = findViewById(R.id.something);
 something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE

 if (savedInstanceState == null) {
 getSupportFragmentManager().beginTransaction()
 .add(R.id.container, new PlaceholderFragment()).commit();
 }
}
</code></pre>

blocks|key|1676832|text|您正在尝试访问onCreate()中的UI元素，但是，现在访问它们还为时过早，因为在片段视图中可以使用onCreateView()方法创建。而onActivityCreated()方法可以可靠地处理它们上的任何操作，因为activity在这种状态下是完全加载的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1676833|entityMap^0|7|A|1F|E|1Z|J|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|G|$]]

You are trying to access UI elements in the <code>onCreate()</code> but , it is too early to access them , since in fragment views can be created in <code>onCreateView()</code> method. 
And <code>onActivityCreated()</code> method is reliable to handle any actions on them, since activity is fully loaded in this state.

blocks|key|1890793|text|在调用findViewById()、onCreate()和onCreateView()方法之后，我在初始化侦听器时得到了相同的NullPointerException。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1890794|但当我使用onActivityCreated(Bundle+savedInstanceState)+{...}时，它就能工作了。因此，我可以访问GroupView并设置我的侦听器。|1890795|我希望它能对你有所帮助。|1890796|entityMap^0|3|E|I|A|T|E|1R|K|0|5|1E|20|9|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|Z|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|K|$]]

I've got the same <code>NullPointerException</code> initializing a listener after calling <code>findViewById()</code> <code>onCreate()</code> and <code>onCreateView()</code> methods.

But when I've used the <code>onActivityCreated(Bundle savedInstanceState) {...}</code> it works. So, I could access the <code>GroupView</code> and set my listener.

I hope it be helpful.

blocks|key|1676982|text|尝试将访问视图转换为onViewCreated方法的fragment，因为有时当您尝试访问onCreate方法中的视图时，它们不会同时呈现，从而导致空指针异常。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1676983|+@Override
public+void+onViewCreated(View+view,+@Nullable+Bundle+savedInstanceState)+{
++++super.onViewCreated(view,+savedInstanceState);
+++++View+something+=+findViewById(R.id.something);
+++++something.setOnClickListener(new+View.OnClickListener()+{+...+});+//+NPE+HERE

+++++if+(savedInstanceState+==+null)+{
+++++++++++getSupportFragmentManager().beginTransaction()
++++++++++++.add(R.id.container,+new+PlaceholderFragment()).commit();
++++}
+}|code-block|syntax|javascript|1676984|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Try to shift your accessing views to the onViewCreated method of fragment because sometimes when you try to access the views in onCreate method they are not rendered at the time resulting null pointer exception.

<pre><code> @Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 View something = findViewById(R.id.something);
 something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE

 if (savedInstanceState == null) {
 getSupportFragmentManager().beginTransaction()
 .add(R.id.container, new PlaceholderFragment()).commit();
 }
 }
</code></pre>

blocks|key|1677050|text|你必须记住重要的一点是:当你声明了你的变量并试图在给它赋值之前检索它的值时，就会发生NullPointerException。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1677051|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You have to remember important thing is :
NullPointerException occurs when you have declared your variable and trying to retreive its value before assigning value to it.

This is a canonical question for a problem frequently posted on StackOverflow.

I'm following a tutorial. I've created a new activity using a wizard. I get <code>NullPointerException</code> when attempting to call a method on <code>View</code>s obtained with <code>findViewById()</code> in my activity <code>onCreate()</code>.

Activity <code>onCreate()</code>:

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

 View something = findViewById(R.id.something);
 something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE

 if (savedInstanceState == null) {
 getSupportFragmentManager().beginTransaction()
 .add(R.id.container, new PlaceholderFragment()).commit();
 }
}
</code></pre>

Layout XML (<code>fragment_main.xml</code>):

<pre><code>&lt;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="packagename.MainActivity$PlaceholderFragment" &gt;

 &lt;View
 android:layout_width="100dp"
 android:layout_height="100dp"
 android:id="@+id/something" /&gt;

&lt;/RelativeLayout&gt;
</code></pre>

NullPointerException accessing views in onCreate()

Android

这是一个经常在StackOverflow上发布的问题的典型问题。我正在学习一个教程。我已经使用向导创建了一个新的活动。当我试图在我的activity onCreate()中通过findViewById()获取的View上调用一个方法时，我得到了NullPointerException。活动onCreate()@Over...

问在onCreate()中访问视图的NullPointerException
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问在onCreate()中访问视图的NullPointerException
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