title: Android Studio can't find class Button
body: <p>This is AndroidManifest.XML</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="activitytest.example.com.activitytest"&gt;
  
  &lt;application
      android:allowBackup="true"
      android:icon="@mipmap/ic_launcher"
      android:label="@string/app_name"
      android:roundIcon="@mipmap/ic_launcher_round"
      android:supportsRtl="true"
      android:theme="@style/AppTheme"&gt;
      &lt;activity android:name=".FirstActivity"
          android:label="This is FirstActivity"&gt;
          &lt;intent-filter&gt;
              &lt;action android:name="android.intent.action.MAIN"/&gt;
              &lt;category android:name="android.intent.category.LAUNCHER"/&gt;
          &lt;/intent-filter&gt;
      &lt;/activity&gt;
  &lt;/application&gt;
  </code></pre>
  
  <p></p>
  
  <p>This is FirstActivity.java</p>
  
  <pre><code>package activitytest.example.com.activitytest;
  
  import android.support.v7.app.AppCompatActivity;
  import android.os.Bundle;
  import android.view.View;
  import android.widget.Toast;
  
  public class FirstActivity extends AppCompatActivity {
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.first_layout);
      Button button1 = (Button) findViewById(R.id.button_1);
      button1.setOnClickListener(new View.OnClickListener(){
          @Override
          public void onClick(View v){
              Toast.makeText(FirstActivity.this,"You clicked Button 1",
                      Toast.LENGTH_SHORT).show();
          }
      });
  }
  }
  </code></pre>
  
  <p>The error is on this:</p>
  
  <pre><code>Button button1 = (Button) findViewById(R.id.button_1);
  </code></pre>
  
  <p>can't find Button class ,how can I fix this?
  This is fisrt_layout.xml</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical" android:layout_width="match_parent"
  android:layout_height="match_parent"&gt;
  &lt;Button
      android:id="@+id/button_1"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:text="Button_1" /&gt;
  </code></pre>
  
  <p></p>
  
  <p>This is my first time study Android, help me please.Thank you.</p>