[Android] method onClick กับ performClick ต่างกันไงครับ

import java.util.Locale;

import android.app.Activity;
import android.os.Bundle;
import android.speech.tts.TextToSpeech;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends Activity {

   TextToSpeech ttobj;
   private Button btn;
   private EditText write;
   @Override
   protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.fragment_main);
      write = (EditText)findViewById(R.id.editText1);
      ttobj=new TextToSpeech(getApplicationContext(), 
      new TextToSpeech.OnInitListener() {
      @Override
      public void onInit(int status) {
         if(status != TextToSpeech.ERROR){
             ttobj.setLanguage(Locale.UK);
            }				
         }
      });
      
      btn = (Button)findViewById(R.id.button1);    
      write.setText("OK");
      btn.setOnClickListener(new OnClickListener() {
    	 
		@Override
		public void onClick(View v) {
			// TODO Auto-generated method stub
			speakText();
			
		}
	});
      btn.performClick();
   }
   @Override
   public void onPause(){
      if(ttobj !=null){
         ttobj.stop();
         ttobj.shutdown();
      }
      super.onPause();
   }
   @Override
   public boolean onCreateOptionsMenu(Menu menu) {
      // Inflate the menu; this adds items to the action bar if it is present.
      getMenuInflater().inflate(R.menu.main, menu);
      return true;
   }
   public void speakText(){
	  String toSpeak = write.getText().toString();
	  Toast.makeText(getApplicationContext(), toSpeak, Toast.LENGTH_SHORT).show();
      ttobj.speak(toSpeak, TextToSpeech.QUEUE_FLUSH, null);
      
   }
}

<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=".MainActivity" >

   <Button
      android:id="@+id/button1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_alignParentBottom="true"
      android:layout_alignParentRight="true"
      android:layout_marginBottom="188dp"
      android:layout_marginRight="67dp"
      android:text="@string/text1" />
   

   <EditText
      android:id="@+id/editText1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_above="@+id/button1"
      android:layout_centerHorizontal="true"
      android:layout_marginBottom="81dp"
      android:ems="10" >
      <requestFocus />
   </EditText>

   <TextView
      android:id="@+id/textView1"
      android:layout_width="wrap_content"
      android:layout_height="wrap_content"
      android:layout_alignParentTop="true"
      android:layout_centerHorizontal="true"
      android:layout_marginTop="20dp"
      android:text="@string/write"
      android:textAppearance="?android:attr/textAppearanceLarge" />
</RelativeLayout>