This could be your first android project if you are learning. This demonstrates the following

satya - Friday, September 16, 2011 10:42:02 AM

Start with the layout 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"
    android:background="@drawable/background">
    
    <EditText android:layout_width="match_parent" 
        android:layout_height="wrap_content" 
        android:id="@+id/editText1" 
        android:text="0"
        android:inputType="numberDecimal"/>
        
    <EditText android:layout_width="match_parent" 
       android:layout_height="wrap_content" 
       android:text="0"
       android:id="@+id/editText2" 
       android:inputType="numberDecimal">
    </EditText>
       
<LinearLayout 
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    
    <Button android:text="+" android:id="@+id/plusButton" 
       android:layout_weight="1"
       android:layout_width="wrap_content" 
       android:layout_height="wrap_content">
    </Button>
    <Button android:text="-" android:id="@+id/minusButton" 
       android:layout_weight="1"
       android:layout_width="wrap_content" 
       android:layout_height="wrap_content">
    </Button>
    <Button android:text="*" android:id="@+id/multiplyButton" 
       android:layout_weight="1"
       android:layout_width="wrap_content" 
       android:layout_height="wrap_content">
    </Button>
    <Button android:text="/" android:id="@+id/divideButton" 
       android:layout_weight="1"
       android:layout_width="wrap_content" 
       android:layout_height="wrap_content">
    </Button>
</LinearLayout>    
</LinearLayout>

satya - Friday, September 16, 2011 10:42:29 AM

Ofourse the background impate you will need to procure

and place it in the drawable directory.

satya - Friday, September 16, 2011 10:43:24 AM

A single activity can do the rest


package com.ai.teach.calculator;

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;

public class CalculatorMainActivity extends Activity 
implements OnClickListener
{
    private EditText number1EditText;
    private EditText number2EditText;
    
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        gatherControls();
        setupButtons();
    }
    
    private void gatherControls()
    {
        number1EditText = (EditText)this.findViewById(R.id.editText1);
        number2EditText = (EditText)this.findViewById(R.id.editText2);
        number2EditText.requestFocus();
    }
    private void setupButtons()
    {
        Button b = (Button)this.findViewById(R.id.plusButton);
        b.setOnClickListener(this);

        b = (Button)this.findViewById(R.id.minusButton);
        b.setOnClickListener(this);
        
        b = (Button)this.findViewById(R.id.multiplyButton);
        b.setOnClickListener(this);
        
        b = (Button)this.findViewById(R.id.divideButton);
        b.setOnClickListener(this);
    }
    @Override
    public void onClick(View v) 
    {
        String sNum1 = number1EditText.getText().toString();
        String sNum2 = number2EditText.getText().toString();
        double num1 = getDouble(sNum1);
        double num2 = getDouble(sNum2);
        Button b = (Button)v;
        
        double value = 0; 
        if (b.getId() == R.id.plusButton)
        {
            value = plus(num1, num2);
        }
        else if (b.getId() == R.id.minusButton)
        {
            value = minus(num1, num2);
        }
        else if (b.getId() == R.id.multiplyButton)
        {
            value = multiply(num1, num2);
        }
        else if (b.getId() == R.id.divideButton)
        {
            value = divide(num1, num2);
        }
        number1EditText.setText(Double.toString(value));
    }
    
    private double plus(double n1, double n2)
    {
        return n1 + n2;
    }
    private double minus(double n1, double n2)
    {
        return n1 - n2;
    }
    private double multiply(double n1, double n2)
    {
        return n1 * n2;
    }
    private double divide(double n1, double n2)
    {
        if (n2 == 0)
        {
            return 0;
        }
        return n1 / n2;
    }
    private double getDouble(String s)
    {
        if (validString(s))
        {
            return Double.parseDouble(s);
        }
        return 0;
    }
    private boolean invalidString(String s)
    {
        return !validString(s);
    }
    private boolean validString(String s)
    {
        if (s == null)
        {
            return false;
        }
        if (s.trim().equalsIgnoreCase(""))
        {
            return false;
        }
        return true;
    }
}

satya - Friday, September 16, 2011 10:45:06 AM

Here is the android manifest.xml for the full measure


<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.ai.teach.calculator"
      android:versionCode="1"
      android:versionName="1.0">
    <uses-sdk android:minSdkVersion="8" />

    <application android:icon="@drawable/icon" 
              android:label="@string/app_name">
        <activity android:name=".CalculatorMainActivity"
                  android:label="@string/app_name">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

This is unchanged from automatically created android project.