How to create a Calculator App for Android

Layouts:

@ home.xml Layout: This layout is the welcome layout for Quick Calculator application. In this layout, a wallpaper is set as background of the layout. While we start this application, this layout runs for 3 seconds.

Listing 1: home.xml layout

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

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="35dp"
        android:layout_marginLeft="20dp"
        android:layout_marginTop="25dp"
        android:text="@string/app_name"
        android:textColor="#000"
        android:textSize="25sp" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="25dp"
        android:layout_marginLeft="70dp"
        android:layout_weight="0.04"
        android:text="@string/app_name_slogan"
        android:textColor="#000"
        android:textSize="15sp" />

</LinearLayout>

Java Class Files:

Every Android project contains a "res" (reources) folder. This folder is meant to comprise of number of subfolders.

Listing 2: calc.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".Home"
    android:background="#fff" >

    <EditText
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/disp"
        android:id = "@+id/display"
        android:hint="@string/dispHint" />
   <LinearLayout android:orientation="horizontal"
       android:layout_width="fill_parent"
       android:layout_height="wrap_content"
       android:layout_gravity="center"
       android:gravity="center"
       android:paddingTop="20dp">
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/seven"
     android:text="@string/seven"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/eight"
     android:text="@string/eight"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/nine"
     android:text="@string/nine"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/div"
     android:text="@string/div"
 /> 
   </LinearLayout>
    <LinearLayout android:orientation="horizontal"
       android:layout_width="fill_parent"
       android:layout_height="wrap_content"
       android:layout_gravity="center"
       android:gravity="center"
       android:paddingTop="20dp">
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/four"
     android:text="@string/four"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/five"
     android:text="@string/five"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/six"
     android:text="@string/six"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/mul"
     android:text="@string/mul"
 /> 
   </LinearLayout>
   <LinearLayout android:orientation="horizontal"
       android:layout_width="fill_parent"
       android:layout_height="wrap_content"
       android:layout_gravity="center"
       android:gravity="center"
       android:paddingTop="20dp">
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/one"
     android:text="@string/one"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/two"
     android:text="@string/two"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/three"
     android:text="@string/three"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/sub"
     android:text="@string/sub"
 /> 
   </LinearLayout>
   <LinearLayout android:orientation="horizontal"
       android:layout_width="fill_parent"
       android:layout_height="wrap_content"
       android:layout_gravity="center"
       android:gravity="center"
       android:paddingTop="20dp">
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/cancel"
     android:text="@string/cancel"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/zero"
     android:text="@string/zero"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/equal"
     android:text="@string/equal"
 />
    <Button 
     android:layout_width="55dp"
     android:layout_height="wrap_content"
     android:id = "@+id/add"
     android:text="@string/add"
 />
   </LinearLayout>
</LinearLayout>

Home class defined in the Home.java file.

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;

public class Home extends Activity{

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  // TODO Auto-generated method stub
  super.onCreate(savedInstanceState);
  setContentView(R.layout.home);
  Thread th = new Thread(){
   public void run(){
    try{
     sleep(1000);
    }
    catch(Exception e){
     e.printStackTrace();
    }
    finally{
     onPause();
     startActivity(new Intent("com.one.slate.CALC"));
    }
   }
  };
  th.start();
 }
 @Override
 public void onPause(){
  super.onPause();
  finish();
 }
 
}    

Calculator class defind into the Calc.java file.

import android.os.Bundle;
import android.app.Activity;
import android.text.Editable;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class Calc extends Activity implements View.OnClickListener{
 
 Button one, two, three, four, five, six, seven, eight, nine, zero, add, sub, mul, div, cancel, equal;
 EditText disp;
 int op1;
 int op2;
 String optr;
 
 @Override
    protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);
        
        one = (Button) findViewById(R.id.one);
        two = (Button) findViewById(R.id.two);
        three = (Button) findViewById(R.id.three);
        four = (Button) findViewById(R.id.four);
        five = (Button) findViewById(R.id.five);
        six = (Button) findViewById(R.id.six);
        seven = (Button) findViewById(R.id.seven);
        eight = (Button) findViewById(R.id.eight);
        nine = (Button) findViewById(R.id.nine);
        zero = (Button) findViewById(R.id.zero);
        add = (Button) findViewById(R.id.add);
        sub = (Button) findViewById(R.id.sub);
        mul = (Button) findViewById(R.id.mul);
        div = (Button) findViewById(R.id.div);
        cancel = (Button) findViewById(R.id.cancel);
        equal = (Button) findViewById(R.id.equal);
        
        disp = (EditText) findViewById(R.id.display);
        
        try{
         one.setOnClickListener(this);
    
         two.setOnClickListener(this);
         
         three.setOnClickListener(this);
         
         four.setOnClickListener(this);
         
         five.setOnClickListener(this);
         
         six.setOnClickListener(this);
         
         seven.setOnClickListener(this);
         
         eight.setOnClickListener(this);
         
         nine.setOnClickListener(this);
         
         zero.setOnClickListener(this);
         
         cancel.setOnClickListener(this);
         
         add.setOnClickListener(this);
  
         sub.setOnClickListener(this);
         
         mul.setOnClickListener(this);
         
         div.setOnClickListener(this);
         
         equal.setOnClickListener(this);
        }
        catch(Exception e){
         
        }
    }
 public void operation(){
  if(optr.equals("+")){
   op2 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
   op1 = op1 + op2;
   disp.setText("Result : " + Integer.toString(op1));
  }
  else if(optr.equals("-")){
   op2 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
   op1 = op1 - op2;
   disp.setText("Result : " + Integer.toString(op1));
  }
  else if(optr.equals("*")){
   op2 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
   op1 = op1 * op2;
   disp.setText("Result : " + Integer.toString(op1));
  }
  else if(optr.equals("/")){
   op2 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
   op1 = op1 / op2;
   disp.setText("Result : " + Integer.toString(op1));
  }
 }
 @Override
 public void onClick(View arg0) {
  Editable str =  disp.getText();
  switch(arg0.getId()){
   case R.id.one:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(two.getText());
    disp.setText(str);
   break;
   case R.id.two:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(two.getText());
    disp.setText(str);
   break;
   case R.id.three:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(three.getText());
    disp.setText(str);
   break;
   case R.id.four:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(four.getText());
    disp.setText(str);
   break;
   case R.id.five:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(five.getText());
    disp.setText(str);
   break;
   case R.id.six:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(six.getText());
    disp.setText(str);
   break;
   case R.id.seven:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(eight.getText());
    disp.setText(str);
   break;
   case R.id.eight:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(nine.getText());
    disp.setText(str);
  
   break;
   case R.id.nine:
    if(op2 != 0){
     op2 = 0;
     disp.setText("");
    }
    str = str.append(zero.getText());
    disp.setText(str);
  
   break;
   case R.id.cancel:
    op1 = 0;
    op2 = 0;
    disp.setText("");
    disp.setHint("Perform Operation :)");
  
   break;
   case R.id.add:
    optr = "+";
    if(op1 == 0){
   op1 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
    }
   else if(op2 != 0){
   op2 = 0;
   disp.setText("");
    }
    else{
   op2 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
   op1 = op1 + op2;
   disp.setText("Result : " + Integer.toString(op1));
    }
   break;
   case R.id.sub:
    optr = "-";
    if(op1 == 0){
   op1 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
    }
    else if(op2 != 0){
     op2 = 0;
    disp.setText("");
    }
    else{
   op2 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
   op1 = op1 - op2;
   disp.setText("Result : " + Integer.toString(op1));
    }
   break;
   case R.id.mul:
    optr = "*";
    if(op1 == 0){
     op1 = Integer.parseInt(disp.getText().toString());
     disp.setText("");
    }
    else if(op2 != 0){
     op2 = 0;
    disp.setText("");
    }
    else{
   op2 = Integer.parseInt(disp.getText().toString());
    disp.setText("");
   op1 = op1 * op2;
   disp.setText("Result : " + Integer.toString(op1));
    }
   break;
   case R.id.div:
   optr = "/";
  if(op1 == 0){
  op1 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
    }
  else if(op2 != 0){
   op2 = 0;
   disp.setText("");
    }
    else{
   op2 = Integer.parseInt(disp.getText().toString());
   disp.setText("");
   op1 = op1 / op2;
  disp.setText("Result : " + Integer.toString(op1));
    }
  break;
  case R.id.equal:
 if(!optr.equals(null)){
     if(op2 != 0){
    if(optr.equals("+")){
    disp.setText("");
    /*op1 = op1 + op2;*/
   disp.setText("Result : " + Integer.toString(op1));
     }
    else if(optr.equals("-")){
     disp.setText("");/*
    op1 = op1 - op2;*/
    disp.setText("Result : " + Integer.toString(op1));
     }
    else if(optr.equals("*")){
  dispsetText("");/*
   op1 = op1 * op2;*/
  disp.setText("Result : " + Integer.toString(op1));
      }
    else if(optr.equals("/")){
     disp.setText("");/*
    op1 = op1 / op2;*/
    disp.setText("Result : " + Integer.toString(op1));
      }
     }
     else{
      operation();
     }
    }
   break;
  }
 }
} 

XML : AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.androidvinod"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="17" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.one.slate.Home"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name="com.one.slate.Calc"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="com.one.slate.CALC" />

                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
    </application>
</manifest>