Why Toast appears more times than I need?

The problem is that a Toast appears as many times as you open Settings activity, while I need it only once after each click on CheckBox.

Thanks in advance to everyone, who tried to improve my code.

More detailed description of how the code works:

• I go to Settings activity and check a box
• Toast appears once
• I return to previous activity using Back button, do nothing, return to Settings activity once more and check a box.
• Toast appears twice
• I return to previous activity using Back button, do nothing, return to Settings activity once more and check a box.
• Toast appears three times

• And so on

  public class PrefActivity extends PreferenceActivity implements SharedPreferences.OnSharedPreferenceChangeListener {
  
  @Override
  public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      addPreferencesFromResource(R.xml.pref_screen);
      Context context = getApplicationContext();
      SharedPreferences settings = PreferenceManager.getDefaultSharedPreferences(context);
      settings.registerOnSharedPreferenceChangeListener(this);
  }
  
  public void onSharedPreferenceChanged(SharedPreferences settings, String key) {
      Toast mToast;
      mToast = Toast.makeText(this, "toast text", Toast.LENGTH_SHORT);
      if(key.equals("checkbox_key")){
          mToast.show();
      }
  }
  

  }

You should call settings.unregisterOnSharedPreferenceChangeListener() in your onPause() method when you leave the Activity. And I suggest to call settings.registerOnSharedPreferenceChangeListener() in your onResume() callback rather than onCreate().