พี่ๆที่เข้าใจหรือเขียน java ในแบบ android เป็นรบกวนช่วยดู code ผมทีครับติดปัญหาอยู่แก้ไม่ได้สักที

package com.Schoolmenu;

import java.util.ArrayList;

import org.apache.http.NameValuePair;
import org.apache.http.message.BasicNameValuePair;




import android.app.Activity;
import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.os.Bundle;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.content.DialogInterface;
import android.content.Intent;
import android.view.View.OnClickListener;
import android.view.View;


public class studentlogin extends Activity{
    private Button btnback,btnok;
    EditText un,pw;
    TextView error;
	

	@Override
    protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.slogin);
       
       
           
      	    
				
         un=(EditText)findViewById(R.id.euser);
         pw=(EditText)findViewById(R.id.epass);
         error=(TextView)findViewById(R.id.error);
         
         btnok=(Button)findViewById(R.id.btnok);
         btnok.setOnClickListener(new View.OnClickListener() {
	
        	 
        	 
        	 
        	 
        	 
			

			public void onClick(View v) {
				
				
				
				
				
				ArrayList<NameValuePair> postParameters = new ArrayList<NameValuePair>();
            	postParameters.add(new BasicNameValuePair("username", un.getText().toString()));
            	postParameters.add(new BasicNameValuePair("password", pw.getText().toString()));
            	//String valid = "1";
            	String response = null;
            	try {
            	    response = CustomHttpClient.executeHttpPost("http://192.168.1.2/android/check.php", postParameters);
            	    String res=response.toString();
            	   // res = res.trim();
            	    res= res.replaceAll("\\s+","");         	              	 
            	    //error.setText(res);
            	   
            	   if(res.equals("1"))
            		               		   
            	   {
            		 
   AlertDialog.Builder alert1 = new AlertDialog.Builder(this);   //----ตรง new alertdialog .buider(this) มัน error
   alert1.setMessage("รหัสถูกต้อง");
   alert1.setCancelable(false);
    alert1.setPositiveButton("yes", new DialogInterface.OnClickListener() {
						
	public void onClick(DialogInterface dialog, int which) {
							dialog.dismiss();
				           }
				       });
				alert1.show();
		
            		   
            		   
            		   
            	   //Intent welcome = new Intent (studentlogin.this, SchoolmenuActivity.class);
            	   //startActivity(welcome);
            	   //finish();
            	   //error.setText("ยินดีด้วยครับรหัสผ่านถูกต้อง");
       	    	
            	   }
            	   else
            	   {
            	   error.setText("เสียใจด้วยรหัสไม่ถูกต้อง");	   
            		   
            	   }
            	    	
            	    	
            	    	
            	} catch (Exception e) {
            		un.setText(e.toString());
            	}

            }
        });
				
		
              
         
         
         //---------------------การทำงานเมื่อทำการกดที่ปุ่ม  btnback---------//
         
         
         btnback= (Button)findViewById(R.id.btnback);
         btnback.setOnClickListener(new OnClickListener(){

  		public void onClick(View v) {
  	
  			gettwodialog();
  			
  	}});
         
  	}
  		private void gettwodialog() {
  	
  			AlertDialog.Builder builder= new AlertDialog.Builder(this);
  			builder.setMessage("กลับหน้าหลักหรือไม่");
  		    builder.setCancelable(false);
  		    builder.setPositiveButton("YES",new DialogInterface.OnClickListener() {
  			
  		     	
  				public void onClick(DialogInterface dialog, int id) {
  					Intent main=new Intent(studentlogin.this,SchoolmenuActivity.class);
  					startActivity(main);
  				}
  			});
  		
  			builder.setNegativeButton("NO",new DialogInterface.OnClickListener() {
  				
  				public void onClick(DialogInterface dialog, int id) {
  					dialog.cancel();
  				}	
  				});
  		    
  		    builder.show();			
	
			
			      
       
}
}