Classes and objects in Java: differences, uses, and complete programming examples

The world of programming, especially when we talk about Java , is fundamentally based on the difference between classes and objects . It might sound like the same old thing: theory on one hand, practice on the other. However, knowing how to distinguish between the two and applying it naturally is what makes the difference between chaotic code and robust, elegant, and maintainable code. Understanding this pairing goes far beyond exams or interview questions: it's part of the daily life of any Java developer and the foundation of the famous Object-Oriented Programming (OOP).

In this article, you'll find a thorough, clear explanation, complete with examples, of what classes are , what objects are exactly , how they relate to each other, the advantages and potential drawbacks of this paradigm, and above all, how to put all this knowledge into practice in the Java universe. If you're looking for quick answers or a simple list of differences, you'll find much more here: a comprehensive, applied overview with real-world nuances that will be useful whether you're a beginner programmer or a veteran looking to refine your approach.

What is Object-Oriented Programming and why does it matter in Java?

The central pillar of languages ​​like Java is Object-Oriented Programming (OOP) . This paradigm goes beyond simply writing code: it involves building programs from modular components called objects , which group both data (properties, attributes) and behaviors (methods, functions). Classes define the nature of these objects, and objects are concrete instances that live and work in the program's memory.

And why is this so important? Because organizing software this way allows you to build larger, more understandable, and much easier-to-maintain applications. Everything has its place, changes don't require redoing everything from scratch, and reuse becomes the norm, not the exception. Java , along with C++, Python , and C#, is based on these principles, and its entire design and evolution are rooted in object-oriented programming ( OOP ).

Classes in Java: the recipe for creating objects

In the Java world, a class is nothing more than a kind of blueprint or architectural plan. It defines what a set of objects will be like: it describes their attributes (what they have) and their methods (what they can do). It is not an "actual object" ; it is only a description of how they should be.

Imagine a cookie factory: the recipe and the mold are the class ; each cookie that comes out represents the concrete objects . Each one may have its own flavor, color, or size, but they all came from the same initial model.

When defining a class , you write the attributes (fields/variables) that characterize that concept, and the methods that allow you to work with that data. This is where you shape what will later be used by thousands or millions of different objects.

Initial class example in Java:

public class Person { private String name; private int age; // Constructor public Person(String name, int age) { this. name = name; this. age = age; } // Method public void greet() { System. out. println("Hello, I am " + name + " and I am " + age + " years old."); } }

In this case, the Person class acts as a template. It has two attributes (name and age), a constructor that initializes the object, and a method that prints a greeting. But this "template" doesn't yet occupy memory as such, nor does it perform any function in the program; it is simply the definition.

Objects in Java: Actual Instances and Occupied Memory

If the class is the recipe, the object is each cookie coming out of the oven. When creating an object of a class in Java, memory is allocated and its specific attributes are initialized: each object has its own values, unique and independent of those of other objects, although they all share the same "mold".

Creating and using objects in Java is usually done with the keyword new:

Person person1 = new Person("Ana", 28); Person person2 = new Person("Juan", 34);

Every time it runs new Persona(...) Memory is reserved, data is initialized, and a unique object is obtained. If you now modify persona1.edad, that will not affect at all persona2.

The object not only has values: it can execute methods, interact with other objects, and its physical and logical existence is key to the program's actual functioning. All private, public, or protected attributes, as well as methods, are already ready to be used by that specific object.

Key differences between classes and objects

This is one of the eternal doubts, and to avoid confusion the essential differences can be highlighted:

• Class: It's the plan, the theory, the description. It only exists once. It doesn't dynamically occupy memory.
• Object: It's the concrete instance, the physical thing, what lives in memory. There can be infinite objects (if memory allows) from a single class.
• Class: Defines attributes and methods, but does not assign concrete values ​​to them.
• Object: It has its own values ​​and can execute methods defined in the class.
• Class: It is not directly manipulated (its “state” cannot be changed), it only serves as a base.
• Object: It can be modified, updated, used and manipulated throughout the program.

Another key difference is the timing of memory allocation: a class only occupies memory once (by definition), but each object occupies a specific memory area. This allows each object to have an independent lifespan and its own data.

Main components of a class in Java

Classes in Java can be as simple or complex as needed, but they always have these basic components :

• Attributes (fields/properties): Variables that describe the characteristics of the object (example: name, age, color, size).
• Methods: Functions that allow you to manipulate attributes or execute actions (example: wave, start, accelerate).
• Constructors: Special methods that are initiated when an object (instance) is created, which allow attributes to be initialized and the object to be prepared before use.
• Access modifiers: Words like public, private, protected that control the visibility and access of attributes/methods.
• Inheritance and polymorphism: A class can inherit from another class (parent/child) and extend or modify its behavior. This gives rise to abstract classes, interfaces, etc.

For example, if we design a class Car For a video game, its attributes could be the model, the maximum speed, the color… and its methods could include acelerar(), frenar() o encender().

How to create and use objects in Java: practical examples

Practice is everything. Virtually every resource you see about OOP in Java uses examples like people, cars, animals, or companies because they're easily translatable into logical structures in code.

Simple example: create a person and display their information

public class Person { private String name; private int age; public Person(String name, int age) { this. name = name; this. age = age; } public void showInformation() { System. out. println("Name: " + name); System. out. println("Age: " + age); } } public class Main { public static void main(String[] args) { Person person1 = new Person("Luis", 22); person1. showInformation(); } }

The constructor initializes the object's values, the method allows the information to be displayed, and the object persona1 lives in memory with those specific data.

Example of using static attributes

public class Counter { private static int account = 0; public Counter() { account++; } public static int getAccount() { return account; } public static void main(String[] args) { Counter c1 = new Counter(); Counter c2 = new Counter(); System.out.println("Total objects created: " + Counter.getAccount()); } }

The variable cuenta It is shared by all objects of the class and allows us to know how many objects have been created in total. This illustrates the difference between instance attributes (specific to each object) and static attributes (shared among all objects).

Attributes in Java: Types and Uses

Attributes are the data or characteristics of each object. They are placed within the class but outside of the methods, and can have different modifiers:

• Instance attributes: They are specific to each object, each one has its own particular copy and unique state.
• Static attributes: Shared by all instances. If you change it from one object, it affects all.
• Final attributes: Once initialized, they cannot change value.
• Class attributes: In Java they are usually equivalent to static attributes.

The choice between public, private, or protected depends on whether you want to control access from outside the class (best practice: use private and getter/setter methods for greater control and encapsulation). If you want to learn more about customizing reports in Power BI , you can also expand your knowledge of data management and visualization.

Constructors: initializing the state of objects

In Java, every time you create an object, a constructor is executed . This is a special method whose name matches the class name and never has a return type. It is used to set the initial values ​​of attributes or to execute startup logic.

There are different types of constructors, which you can overload depending on your needs:

• Default constructor: It takes no parameters. If you don't define any, Java automatically creates an empty one.
• Constructor with parameters: Receives initial values ​​for the attributes.
• Copy constructor: Allows you to create a new object from another, copying its values.
• Private builder: It is often used in patterns such as Singleton so that objects cannot be created directly from outside.

Inheritance, Superclasses, and Subclasses in Java

One of the most powerful concepts in OOP and Java is inheritance : the ability to create new classes based on others, inheriting their attributes and methods. This allows for building logical hierarchies and avoiding code duplication.

The superclass is the base (parent) class from which subclasses (children) inherit. A subclass can extend or modify inherited behaviors and even add new methods or attributes.

Example:

public class Animal { protected String name; public void makeSound() { System.out.println("Generic Sound"); } } public class Dog extends Animal { public void makeSound() { System.out.println("Woof woof"); } }

Advantages and disadvantages of Object-Oriented Programming

Object -oriented programming (and Java in particular) has numerous advantages, which explain why this paradigm dominates in professional development:

• Allows code reuse: Thanks to inheritance and modular organization, you can leverage existing work and adapt it to new contexts.
• Divide the programs into modules: Each class is a self-contained “piece,” making large programs easier to manage.
• Facilitates maintenance: Modifying one class does not affect the others, so changes are localized and safe.
• Promotes clarity and documentation: Classes and objects reflect real-world concepts, helping to understand the problem domain.

But there are also some drawbacks or challenges:

• Higher memory consumption: When creating many objects, each one occupies its own memory.
• Initial learning curve: At first, understanding the relationship between classes, objects, inheritance, polymorphism, etc. can be confusing.
• Possible loss of efficiency: Programs may run somewhat slower than in more "flat" models (such as structured programming).

Complete examples of applied classes and objects in Java

Now, let's look at an example that covers everything we've learned so far and shows the relationship between classes, objects, attributes, constructors, and methods.

// We define the class public class Animal { private String name; private int age; public Animal(String name) { this. name = name; this. age = 0; } public int getAge() { return age; } public void setAge(int newAge) { this. age = newAge; } public String getName() { return name; } } // Using the class public class Example { public static void main(String[] args) { Animal myAnimal = new Animal("Falco"); myAnimal.setAge(3); System.out.println("The name is: " + getName() + " and is " + getAge() + " years old"); } }

How memory and references affect Java

When you create an object in Java, what you actually have in the program variables are references to the memory space occupied by that object. Therefore, if you assign an object to another variable, both variables point to the same instance.

Example:

Person personA = new Person("Mario", 45); Person personB = personA; personB.setAge(46); System.out.println(personA.getAge()); // Print 46

This happens because both personaA , the personaB point to the same object in memory. To create an independent copy, it is recommended to use the .

Best practices: encapsulation and accessor methods

One of the most important recommendations in Java is to protect attributes by making them private , and allowing access or modification only through public methods called getters and setters . This protects the object's internal state and allows for additional checks or validations if needed.

For example:

public class Person { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } }