[Android Software Development] nemoc 23:
Try this code.

myDC.java

package com.mydc;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.AdapterView.OnItemSelectedListener;
import android.widget.TextView;
import android.widget.Toast;

public class myDC extends Activity {

double initial_price;
double discount;
double final_price;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

Spinner spinner = (Spinner) findViewById(R.id.spinner1);
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(
this, R.array.discount, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
spinner.setAdapter(adapter);
spinner.setOnItemSelectedListener(new OnItemSelectedListener(){

@Override
public void onItemSelected(AdapterView<?> parent, View view, int pos, long id) {

if (pos==0){
discount=10;
}
else if (pos==1){
discount=15;
}
else if (pos==2){
discount=18;
}
}

@Override
public void onNothingSelected(AdapterView<?> arg0) {
// TODO Auto-generated method stub
}
});

Button calculate = (Button) findViewById(R.id.calculate);
calculate.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
EditText amount1 = (EditText) findViewById(R.id.amount1);
if (amount1.length()>0 ) {
initial_price=Double.parseDouble(amount1.getText().toString());
TextView final_price_text = (TextView) findViewById(R.id.final_price);
final_price=initial_price-(initial_price*discount/100);
final_price_text.setText("Final Price : "+final_price);
}
else {
Toast.makeText(getBaseContext(), "Please enter price", Toast.LENGTH_SHORT).show();
}
}
});
}
}

main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent">
<Spinner
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/spinner1"></Spinner>
<TextView
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="Enter the price" />
<EditText
android:layout_height="wrap_content"
android:id="@+id/amount1"
android:text=""
android:layout_width="match_parent"
android:numeric="decimal"></EditText>
<Button
android:text="Calculate Result"
android:id="@+id/calculate"
android:layout_width="wrap_content"
android:layout_height="wrap_content"></Button>
<TextView
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:layout_width="fill_parent"
android:id="@+id/final_price"></TextView>

</LinearLayout>

strings.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
<string name="hello">Hello World, main!</string>
<string name="app_name">myDC</string>
<string-array name="discount">
<item>10% discount A</item>
<item>15% discount B</item>
<item>18% discount C</item>
</string-array>
</resources>