error dimane ye?? -_-

package com.androDev.ratingbar;

import android.os.Bundle;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.AlertDialog.Builder;
import android.content.DialogInterface;
import android.content.DialogInterface.OnClickListener;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.RatingBar;
import android.widget.RatingBar.OnRatingBarChangeListener;
import android.widget.TextView;

public class Rating extends Activity implements OnRatingBarChangeListener, OnClickListener{
    TextView tv;
    Button bt;
    RatingBar rb;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_rating);

        rb = (RatingBar) findViewById(R.id.rb);
        bt = (Button) findViewById(R.id.bt);
        bt.setOnClickListener((android.view.View.OnClickListener) this);//untu impementasi button
        rb.setOnRatingBarChangeListener(this);//untuk implementasi perubahan rating
    }

    public void onRatingChange(RatingBar ratingBar, float rate, boolean fromUser){
        tv = (TextView) findViewById(R.id.hasil);
        tv.setText("Bintang anda: "+rate);
    }

    public void onClick(View v){
        AlertDialog.Builder ab = new AlertDialog.Builder(this);
        setMessage("Apakah Anda Ingin Keluar??").setTitle("Konfirmasi Exit");
        setPositiveButton("IYA",new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub
                finish();
            }
        }).setNegativeButton("TIDAK",new DialogInterface.OnClickListener() {

            @Override
            public void onClick(DialogInterface dialog, int which) {
                // TODO Auto-generated method stub

            }
        });
        ab.show();
    }

    private Builder setPositiveButton(String string,
            OnClickListener onClickListener) {
        // TODO Auto-generated method stub
        return null;
    }

    private Activity setMessage(String string) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.rating, menu);
        return true;
    }

    @Override
    public void onRatingChanged(RatingBar arg0, float arg1, boolean arg2) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onClick(DialogInterface arg0, int arg1) {
        // TODO Auto-generated method stub

    }

}