Related Topics: Java IoT

Java IoT: Article

Intro to Object-Oriented Programming with Java

Lesson 2

Java is an object-oriented language and Java programs consist of classes that represent objects in the real world. Classes in Java may have methods and attributes.

Let's create and discuss a class named Car. This class may have one or more methods, which can tell what the objects of this class can do: start the car, stop it, accelerate, lock the doors, and so on.

This class also may have some attributes or properties: color of the car, number of doors, size of engine, and so on.

Our class Car may represent some common features for many different cars: all cars have such properties as color and the number of doors, and all of them perform similar actions. We can be more specific and create another Java class called ToyotaCorolla. It's still a car, but with some properties specific to the model Toyota Corolla.

We will be often using such term as an object, which is an instance of a class. The phrase "to create an instance of a class" means to create a copy of the object in the computer's memory, based on the definition of this class.

Factory specifications of a Toyota Corolla plays a similar role as definition of Java classes. The process of building real cars based on these specs is an equivalent to creating instances of this class in Java.

In many cases, a program can't use the Java class until its instance has been created. Obviously, you can create thousands of cars based on the same Toyota Corolla specifications. Even though they all represent the same class, they may have different values in their properties - some of them are red, some of them have two doors, while others have four, etc. In other words, we may create multiple instances of the class Toyota Corolla:

class ToyotaCorolla{
    String color;
    int numberOfDoors;
    void startEngine {
    void stopEngine {

Data Types

Java variables have to be declared before usage - they must have an assigned data type. There are 8 primitive data types in Java: 4 data types are for integer values, 2 are for values with a decimal point, 1 char, and 1 boolean (allows only the values true or false).

All of these primitives have corresponding wrapper classes that contain useful methods dealing with respective data types.

The data type char allows you to store only one character, while classes String or StringBuffer are used for holding a longer text, i.e.

String lastName="Smith"; char grade = 'A';

Please note that the char data type uses 2 bytes of memory to store the data.

Below are some examples of variable declarations and initializations.

int chairs = 12;                            
boolean cancelJob = false;         
double nationalIncome = 23863494965745.78; 
float hourlyRate = 12.50f;   // add an f at the end of         
                             //float  literals
long totalCars  = 4637283648392l;  // add an l at the end              
                                   // of long literals

The Tax Calculation Program

Let's design and write a program that will calculate the state tax.

First, we need to decide what Java class(es) we need to write. Second, we'll think about properties and methods that our class(es) should have.

Start your text editor and enter the following:

class Tax{

The open and close curly brackets are used in various contexts to enclose the body of a class, a method, and some other Java elements. Every open curly brace must have the closing one. Save your class in the file called Tax.java. You can't go wrong with that. Let's think about the data that this class would need to perform the state tax calculation.

Obviously, you will need to know the gross income of a person for the last year. This is a good candidate for a property of this class. Properties in Java are represented by variables. Before declaring a variable for the gross income, we need to decide what types of data we'll be storing in it. These are some of the Java data types:

int, double, float, char, String...

Let's add the variable grossIncome of the data type double to our class, because this type allows numbers with a decimal point:

 class Tax{   
       double grossIncome;

We also need to know what state the person lives in - different states may have different taxation rules. Possible values of this variable are: "NY", "NJ", "CT", etc. The String data type represents text data:

class Tax{   
       double grossIncome;
       String state;

Let's add one more property for dependents. This will be an integer variable, since a person can not have two and a half dependents.

class Tax{   
       double grossIncome;
       String state;
       int  dependents;

It's time to add some methods to our class. We definitely need to be able to calculate the state tax based on the values of gross income, number of dependents, and state. Let's create a method called calcTax():

class Tax{   
  double grossIncome;
  String state;
  int  dependents;
  public double calcTax() {
    return 234.55;  //  returning a hard-coded value

This method's signature tells us the following:

  • Any external class could access this method (public).
  • This method will return a value of type double.
  • The name of the method is calcTax.
  • This method does not need any values from outside - the empty parentheses mean that the method does not have any arguments.
How do we decide if a method should return a value? If your method performs some calculations and has to give the resulting value back to a calling program, it has to return a value. If, for example, a method just prints some information, it may not need to return any value, but you still need to mention this in a method signature by using a special keyword void:

public void printCustomers() {...}

Java has a return statement and this is how a method returns data contained in a variable myResult to a calling program:

return myResult;

Our class Tax has to be instantiated before we start using it. Let's create one more class called TestTax. This class will just instantiate and use our class Tax. Here's what the class TestTax should do: 1. Create an instance of the class Tax. 2. Assign some values (gross income, state...) to the variables of the class Tax. 3. Call the method calcTax(). 4. Print the result on the screen.

The class TestTax will be stored in a separate file named TestTax.java.

class TestTax{
     public static void main(String[] args){
            Tax   t = new Tax(); // creating an instance
            t.grossIncome= 50000;  // assigning the values
            t.dependents= 2;
            t.state= "NJ";
            double yourTax = t.calcTax(); //calculating tax 
           // Printing the result 
           System.out.println("Your tax is " + yourTax);

In the code above, we've declared a variable t of type Tax.

The method main() is an entry point to our program. This method creates an instance of the class Tax using the Java operator new. The variable t points to a place in the memory where the Tax object was created. From now on, if we want to work with the class, we will be using the variable t.

The following three lines assign values to the properties of the object Tax.

After that, we'll calculate tax by calling the method calcTax() and the result will be assigned to our variable yourTax. The last line of method main() just displays the result on the system console.

At this point we already have two classes communicating with each other (the TestTax and the Tax).

To make our example more realistic, let's add some code to perform tax calculations in the method calcTax().

Let's say if the gross income was less that $30,000, we will take 5% for state tax. If it's greater than $30,000, we will take 6%.

public double calcTax() {         
  double  stateTax=0;
  if (grossIncome < 30000) {
    stateTax= grossIncome*0.06;
  return stateTax;

Variable Scopes

If you declare a variable inside any method, the variable has a local scope (the variable stateTax is local). This means that it's visible only for the code within this method. When the method is finished, the variable automatically gets destroyed. If a variable has to be visible through more than one method in a class, you should declare it on a class level. In the class Tax, grossIncome, dependents, and state are class variables. These variables are "alive" when the class exists in memory. They could be shared and reused by all methods within the class and they can even be visible from external classes, for example TestTax class is accessing them.

In the previous examples we were using hard-coded values for calculations (values that didn't change). Later on, we'll make sure that our program can perform tax calculations for any income, state, and number of dependents.

Let's introduce some new object-oriented terms. One of them is called inheritance, which is an ability to create a new object, based on an existing object.

We were planning to use the class Tax for all states, but what if the state of New Jersey introduces educational tax deductions? If you have a kid in college, this makes you eligible for an additional $500 deduction from your taxes. In this case, we have to either change the method calcTax(), or create another class that will be based on our class Tax plus have this new functionality to calculate educational deductions.

In real life, every person inherits some features from his or her parents. This similar process exists in Java. The special keyword extends is used to indicate that one class has been inherited from another:

class NJTax extends Tax{

The class NJTax will have all features the class Tax has, plus you can add some new properties and methods to it. The class Tax is called a superclass, and the NJTax is called a subclass. You could also use such terms as ancestor and descendent respectively. This new class will have access to variables and methods of its superclass (unless they have a private access level, but let's worry about that later). It's time to create a method called adjustForStudents() in the class NJTax.

class NJTax extends Tax{
    double adjustForStudents (double stateTax){
       double adjustedTax = stateTax - 500;
       return adjustedTax;

How will the class TestTax create an instance of the class NJTax? Here you go:

NJTax t= new NJTax();

Now you can call methods defined in the class Tax as well as in the NJTax using the variable t, for example:

NJTax t= new NJTax(); double yourTax = t.calcTax();

double totalTax = t. adjustForStudents (yourTax);

Please note that we've added a new functionality to the tax calculation program without changing the code of the class Tax. Another important note is that even though the variable t refers to an instance of the class NJTax, we are calling the method defined in its superclass:


The above code fragment also shows how you can pass a value calculated from one method to another one. We are passing the value of the variable yourTax to the method adjustForStudents() as an argument.

Method Overriding

The next important term of object-oriented programming is method overriding. Imagine that a superclass has 20 methods. Most of them are generic for all states, but there is one method that is not valid for the state of New Jersey. Instead of modifying this method in the superclass, we could create another method in the subclass with the same name and argument list. It's time to modify our previous example. The goal is to change the calcTax() method without modifying the superclass. We'll declare the method calcTax() in the subclass - NJTax. By doing that, we're suppressing the method of the superclass with the same name and signature. Let's try a simple test - create a method calcTax() in NJTax and only write one line:
public double calcTax() {         
  return 1000; 

Compile the code and run the TestTax program - it will print the number 1000 as a result, which proves that only the calcTax() from the subclass has been called.

The advantages of using method overriding as opposed to direct code modification are:

  1. The source code of the superclass may not be available, but you still need to change its functionality.
  2. Somebody else may still need the original version, that's why you can't modify it.

More Stories By Yakov Fain

Yakov Fain is a Java Champion and a co-founder of the IT consultancy Farata Systems and the product company SuranceBay. He wrote a thousand blogs (http://yakovfain.com) and several books about software development. Yakov authored and co-authored such books as "Angular 2 Development with TypeScript", "Java 24-Hour Trainer", and "Enterprise Web Development". His Twitter tag is @yfain

Comments (1)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.

Latest Stories
The platform combines the strengths of Singtel's extensive, intelligent network capabilities with Microsoft's cloud expertise to create a unique solution that sets new standards for IoT applications," said Mr Diomedes Kastanis, Head of IoT at Singtel. "Our solution provides speed, transparency and flexibility, paving the way for a more pervasive use of IoT to accelerate enterprises' digitalisation efforts. AI-powered intelligent connectivity over Microsoft Azure will be the fastest connected pat...
There are many examples of disruption in consumer space – Uber disrupting the cab industry, Airbnb disrupting the hospitality industry and so on; but have you wondered who is disrupting support and operations? AISERA helps make businesses and customers successful by offering consumer-like user experience for support and operations. We have built the world’s first AI-driven IT / HR / Cloud / Customer Support and Operations solution.
ScaleMP is presenting at CloudEXPO 2019, held June 24-26 in Santa Clara, and we’d love to see you there. At the conference, we’ll demonstrate how ScaleMP is solving one of the most vexing challenges for cloud — memory cost and limit of scale — and how our innovative vSMP MemoryONE solution provides affordable larger server memory for the private and public cloud. Please visit us at Booth No. 519 to connect with our experts and learn more about vSMP MemoryONE and how it is already serving some of...
Darktrace is the world's leading AI company for cyber security. Created by mathematicians from the University of Cambridge, Darktrace's Enterprise Immune System is the first non-consumer application of machine learning to work at scale, across all network types, from physical, virtualized, and cloud, through to IoT and industrial control systems. Installed as a self-configuring cyber defense platform, Darktrace continuously learns what is ‘normal' for all devices and users, updating its understa...
Codete accelerates their clients growth through technological expertise and experience. Codite team works with organizations to meet the challenges that digitalization presents. Their clients include digital start-ups as well as established enterprises in the IT industry. To stay competitive in a highly innovative IT industry, strong R&D departments and bold spin-off initiatives is a must. Codete Data Science and Software Architects teams help corporate clients to stay up to date with the mod...
As you know, enterprise IT conversation over the past year have often centered upon the open-source Kubernetes container orchestration system. In fact, Kubernetes has emerged as the key technology -- and even primary platform -- of cloud migrations for a wide variety of organizations. Kubernetes is critical to forward-looking enterprises that continue to push their IT infrastructures toward maximum functionality, scalability, and flexibility. As they do so, IT professionals are also embr...
Platform9, the leader in SaaS-managed hybrid cloud, has announced it will present five sessions at four upcoming industry conferences in June: BCS in London, DevOpsCon in Berlin, HPE Discover and Cloud Computing Expo 2019.
At CloudEXPO Silicon Valley, June 24-26, 2019, Digital Transformation (DX) is a major focus with expanded DevOpsSUMMIT and FinTechEXPO programs within the DXWorldEXPO agenda. Successful transformation requires a laser focus on being data-driven and on using all the tools available that enable transformation if they plan to survive over the long term. A total of 88% of Fortune 500 companies from a generation ago are now out of business. Only 12% still survive. Similar percentages are found throug...
When you're operating multiple services in production, building out forensics tools such as monitoring and observability becomes essential. Unfortunately, it is a real challenge balancing priorities between building new features and tools to help pinpoint root causes. Linkerd provides many of the tools you need to tame the chaos of operating microservices in a cloud native world. Because Linkerd is a transparent proxy that runs alongside your application, there are no code changes required. I...
In his general session at 21st Cloud Expo, Greg Dumas, Calligo’s Vice President and G.M. of US operations, discussed the new Global Data Protection Regulation and how Calligo can help business stay compliant in digitally globalized world. Greg Dumas is Calligo's Vice President and G.M. of US operations. Calligo is an established service provider that provides an innovative platform for trusted cloud solutions. Calligo’s customers are typically most concerned about GDPR compliance, application p...
Modern software design has fundamentally changed how we manage applications, causing many to turn to containers as the new virtual machine for resource management. As container adoption grows beyond stateless applications to stateful workloads, the need for persistent storage is foundational - something customers routinely cite as a top pain point. In his session at @DevOpsSummit at 21st Cloud Expo, Bill Borsari, Head of Systems Engineering at Datera, explored how organizations can reap the bene...
"NetApp's vision is how we help organizations manage data - delivering the right data in the right place, in the right time, to the people who need it, and doing it agnostic to what the platform is," explained Josh Atwell, Developer Advocate for NetApp, in this SYS-CON.tv interview at 20th Cloud Expo, held June 6-8, 2017, at the Javits Center in New York City, NY.
Druva is the global leader in Cloud Data Protection and Management, delivering the industry's first data management-as-a-service solution that aggregates data from endpoints, servers and cloud applications and leverages the public cloud to offer a single pane of glass to enable data protection, governance and intelligence-dramatically increasing the availability and visibility of business critical information, while reducing the risk, cost and complexity of managing and protecting it. Druva's...
Kubernetes as a Container Platform is becoming a de facto for every enterprise. In my interactions with enterprises adopting container platform, I come across common questions: - How does application security work on this platform? What all do I need to secure? - How do I implement security in pipelines? - What about vulnerabilities discovered at a later point in time? - What are newer technologies like Istio Service Mesh bring to table?In this session, I will be addressing these commonly asked ...
BMC has unmatched experience in IT management, supporting 92 of the Forbes Global 100, and earning recognition as an ITSM Gartner Magic Quadrant Leader for five years running. Our solutions offer speed, agility, and efficiency to tackle business challenges in the areas of service management, automation, operations, and the mainframe.