Application crashes

This is my code below it complies properly and when i click on the calculate button the app crashes i dont understand y?????

package thumbplay.tip;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class tip extends Activity {

double tbill;
double tippercent;
double tip_cal;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_tip);
    final EditText billamt = (EditText) findViewById(R.id.tbill);

// final EditText tip=(EditText)findViewById(R.id.tpercent);
final EditText tip_percent = (EditText) findViewById(R.id.tpercent);
final TextView totalbill = (TextView) findViewById(R.id.gt);
//final TextView tipamount=(TextView)findViewById(R.id.tipamount);
//final TextView hpays=(TextView)findViewById(R.id.hpays);
final Button calculate = (Button) findViewById(R.id.caculate);

    calculate.setOnClickListener(new OnClickListener() {
                                     public void onClick(View v) {
                                          tbill = Double.parseDouble(billamt.toString());
                                         tippercent = Double.parseDouble(tip_percent.toString());
                                         tip_cal = (tbill * tippercent) / 100;
                                         totalbill.setText(Double.toString(tip_cal));
                                     }
                                 }
    );
}

}