Beginig Android Application Developement by developing simple calculator

File-> New-> Android Application Project

Application Name: Calculator

Project Name: Calculator

Package name: com.calculator.scientific

Minimum Required SDK:  API 8
Target SDK: API 15

click next and then click finish;

open MainActivity.java from src->comcom.calculator.scientific from your project and replace with following code.
//--------------------------------------------------------------
package com.calculator.scientific

import android.app.Activity;
import android.os.Bundle;
import android.text.TextUtils;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends Activity implements OnClickListener {

final int MENU_RESET_ID = 1;
final int MENU_QUIT_ID = 2;



  EditText etNum1;
  EditText etNum2;

  Button btnAdd;
  Button btnSub;
  Button btnMult;
  Button btnDiv;

  TextView tvResult;

  String oper = "";

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

    // find the elements
    etNum1 = (EditText) findViewById(R.id.etNum1);
    etNum2 = (EditText) findViewById(R.id.etNum2);

    btnAdd = (Button) findViewById(R.id.btnAdd);
    btnSub = (Button) findViewById(R.id.btnSub);
    btnMult = (Button) findViewById(R.id.btnMult);
    btnDiv = (Button) findViewById(R.id.btnDiv);

    tvResult = (TextView) findViewById(R.id.tvResult);

    // set a listener
    btnAdd.setOnClickListener(this);
    btnSub.setOnClickListener(this);
    btnMult.setOnClickListener(this);
    btnDiv.setOnClickListener(this);

  }

  @Override
  public void onClick(View v) {
    // TODO Auto-generated method stub
    float num1 = 0;
    float num2 = 0;
    float result = 0;

    // check if the fields are empty
    if (TextUtils.isEmpty(etNum1.getText().toString())
        || TextUtils.isEmpty(etNum2.getText().toString())) {
      return;
    }

    // read EditText and fill variables with numbers
    num1 = Float.parseFloat(etNum1.getText().toString());
    num2 = Float.parseFloat(etNum2.getText().toString());

    // defines the button that has been clicked and performs the corresponding operation
    // write operation into oper, we will use it later for output
    switch (v.getId()) {
    case R.id.btnAdd:
      oper = "+";
      result = num1 + num2;
      break;
    case R.id.btnSub:
      oper = "-";
      result = num1 - num2;
      break;
    case R.id.btnMult:
      oper = "*";
      result = num1 * num2;
      break;
    case R.id.btnDiv:
      oper = "/";
      result = num1 / num2;
      break;
    default:
      break;
    }

    // form the output line
    tvResult.setText(num1 + " " + oper + " " + num2 + " = " + result);
  }

  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
    // TODO Auto-generated method stub
    menu.add(0, MENU_RESET_ID, 0, "Reset");
    menu.add(0, MENU_QUIT_ID, 0, "Quit");
    return super.onCreateOptionsMenu(menu);
  }

  // process menu item clicks
  @Override
  public boolean onOptionsItemSelected(MenuItem item) {
    // TODO Auto-generated method stub
    switch (item.getItemId()) {
    case MENU_RESET_ID:
      // clear the fields
      etNum1.setText("");
      etNum2.setText("");
      tvResult.setText("");
      break;
    case MENU_QUIT_ID:
      // exit the application
      finish();
      break;
    }
    return super.onOptionsItemSelected(item);
  }

}

//--------------------------------------------------------------------------

Open activity_main.xml  from res->layout from your project and paste the following code

//----------------------------------------------------------


<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <LinearLayout
        android:id="@+id/linearLayout1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="10pt"
        android:layout_marginRight="10pt"
        android:layout_marginTop="3pt" >

        <EditText
            android:id="@+id/etNum1"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginRight="5pt"
            android:layout_weight="1"
            android:inputType="numberDecimal" >
        </EditText>

        <EditText
            android:id="@+id/etNum2"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginLeft="5pt"
            android:layout_weight="1"
            android:inputType="numberDecimal" >
        </EditText>
    </LinearLayout>

    <LinearLayout
        android:id="@+id/linearLayout2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="5pt"
        android:layout_marginRight="5pt"
        android:layout_marginTop="3pt" >

        <Button
            android:id="@+id/btnAdd"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="+"
            android:textSize="8pt" >
        </Button>

        <Button
            android:id="@+id/btnSub"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="-"
            android:textSize="8pt" >
        </Button>

        <Button
            android:id="@+id/btnMult"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="*"
            android:textSize="8pt" >
        </Button>

        <Button
            android:id="@+id/btnDiv"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="/"
            android:textSize="8pt" >
        </Button>
    </LinearLayout>

    <TextView
        android:id="@+id/tvResult"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginLeft="5pt"
        android:layout_marginRight="5pt"
        android:layout_marginTop="3pt"
        android:gravity="center_horizontal"
        android:textSize="12pt" >
    </TextView>

</LinearLayout>





--------------------------------------------------------

Run The Application by Right Click on the project and RunAs-> Android Application