title: Android checkbox check for null to avoid app crash
body: <p>I am very newbie on android studio and java. I have made a simple page which contain check boxes, and a button to print out in a toast message the checked box selected.
  The problem is if i clicked on submit button without selecting anything,the app stop working and crash.
  What i want to achieve is if i didn't select anything and i click on submit button it should send me a toast message "Please select a box" without making the app crash.</p>
  
  <p>XML code:</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;RelativeLayout 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"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical"
  android:padding="16dp"
  tools:context=".MainActivity"&gt;
  &lt;TextView
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:textStyle="bold"
      android:text="Favorite Courses: "
      android:textSize="20sp"
      android:layout_marginTop="30dp"
      android:layout_below="@+id/txDate"
      android:id="@+id/fvCourses"/&gt;
  &lt;CheckBox
      android:id="@+id/cbCs"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_toRightOf="@+id/fvCourses"
      android:layout_alignBottom="@+id/fvCourses"
      android:text="Computer Science" /&gt;
  &lt;CheckBox
      android:id="@+id/cbAcc"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_toRightOf="@+id/fvCourses"
      android:layout_below="@+id/cbCs"
      android:text="Accounting" /&gt;
  &lt;CheckBox
      android:id="@+id/cbGraphic"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_toRightOf="@+id/fvCourses"
      android:layout_below="@+id/cbAcc"
      android:text="Graphic Designer" /&gt;
  &lt;Button
      android:id="@+id/btnapply"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_below="@+id/radioGroup"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="330dp"
      android:text="Confirm" /&gt;
      &lt;/RelativeLayout&gt;
  </code></pre>
  
  <p>Java code:</p>
  
  <pre><code>package test.cb.app;
  
  import androidx.appcompat.app.AppCompatActivity;
  
  import android.widget.TextView;
  import android.widget.Toast;
  import android.widget.Button;
  import android.widget.CheckBox;
  public class MainActivity extends AppCompatActivity {
  
       TextView textView;
      CheckBox cbCs,cbAcc,cbGraphic;
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
          setContentView(R.layout.activity_main);
  
          cbCs = findViewById(R.id.cbCs);
          cbAcc = findViewById(R.id.cbAcc);
          cbGraphic = findViewById(R.id.cbGraphic);
  
          Button btnapply = findViewById(R.id.btnapply);
          btnapply.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View view) {
  
                  StringBuffer result = new StringBuffer();
                  result.append("Computer Science check : ").append(cbCs.isChecked());
                  result.append("\nAccounting check : ").append(cbAcc.isChecked());
                  result.append("\nGraphic check :").append(cbGraphic.isChecked());
                  Toast.makeText(MainActivity.this, result.toString(),
                          Toast.LENGTH_LONG).show();
              }
          });
          }
  
              }
  </code></pre>
  
  <p>I expect that i have to check for null values on the submit button.</p>
  
  <p>What would be the code to achieve that ?</p>
  
  <p>Any help would be greatly appriciated.</p>
  
  <p>Thank you!</p>