JavaClasses and Objects in Java

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Object-oriented programming (OOP) is a programming paradigm that allows you to use objects, their interactions, and their data. Object-oriented programming is a way of thinking about and building programs that allow programmers to reuse code and think more abstractly.

This programming tutorial talks about object-oriented programming and how we can work with it using Java.

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What is Object-Oriented Programming?

As a programming paradigm, object-oriented programming leverages abstraction to build models based on real-world data. Instead of “actions” and “logic,” object-oriented programming emphasizes “objects” and “data.”

Objects are made of three things:

  • State (what is the data it stores)
  • Behavior (what can it do)
  • Identity (unique name and address)

What is a Class

A class is a blueprint, or prototype, based on which objects are created. Classes give you the ability to create your own custom types, which makes your code easier to read and maintain.

When programming in Java (or any other object-oriented language), you must first create a class before you can use it in your program. When creating a class, you must define its fields (the data it contains), and methods (the actions it takes). You can then instantiate an object of this type using the new keyword followed by the name of the class, and any parameters for its constructor.

A class in Java can contain:

  • Fields – Variables that are defined for the class
  • Methods – Functions that are defined for the class
  • Constructor – A special type of method that creates an instance of a class
  • Block – Code surrounded by braces {}

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Support for OOP in Java

Java is an object-oriented programming language that has been around for more than two decades. Since Java is an object-oriented language, you can work with classes and objects in Java. When you create a class, you are creating your own custom type that you can fill with variables and functions. You can then use that class as a type for other variables or parameters for functions.

When you are creating objects in Java, you define a class that describes all attributes and behaviors of the objects in your program. Any object is an instance of a class that should exist to create an instance of the class.

What is an Object in Java?

An object is made up of data and functions that act on that data. The data is the attributes or characteristics, while the functions are called methods. Once an object is defined, its state can be modified through its methods.

An object contains state information (variables) and behavior information (methods). In other words, objects consist of state (data) and behavior (code). Objects created from the same classes share state and behavior information, but each object also has its own identity.

Member Data and Member Functions

The two main components of a class are variables and methods. Variables contain data, while methods are functions. Variables are containers that store data, while methods are functions that perform actions on data. For example, you could create a class called Rectangle, which would have variables representing its height and width, as well as a method to calculate its area.

Data members or variables can only store values of their declared type; for example, an integer variable can only hold integers. Methods can access, manipulate, and return variables or other values. Methods can also be reused; for instance, if you have a program with lots of Rectangle objects, the same method from your Rectangle class would be used to calculate the area of each object.

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What are Access Modifiers in Java

The Java programming language contains the following access modifiers to set access levels for classes, variables, methods, and constructors:

  • Public – accessible from everywhere
  • Protected – accessible from the same class and its subclasses
  • No modifier – accessible from the same package only (also known as default access)
  • Private – accessible from the same class only

Abstract Classes and Interfaces in Java

An interface is a particular type of class that does not have a state or implementation. On the other hand, an abstract class may contain both concrete and non-concrete methods. Abstract classes cannot be instantiated but can be inherited if necessary. An abstract class, unlike an interface, may potentially include method implementations. An abstract class can have both abstract and concrete members.

An interface can declare methods but without any implementation. An interface specifies a type and a set of operations on that type, but it does not specify any implementation details. It is up to the developer to define the implementations of the methods of an interface when creating a class that implements it. There are no data members in an interface; it can only contain declarations of actions.

Static Classes and Objects in Java

You can define a class or an instance of a class as static. To do this, you should precede the name of the class or the object with the static keyword. A static class is one that contains static members only. So, if you have a static class, all methods and variables or objects you defined in the class must be static.

If a class is static, you cannot create an instance of the class. To access the members of the class, you can use the class name instead. A static object resides in the memory throughout the application’s lifetime.

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Programming Classes and Objects in Java

Now that you know how to define a class, it is time to learn how to create objects using the new operator. To create an instance of a class in Java, you should take advantage of the new operator and a constructor of the class.

The new operator returns a reference to the object that was created, but programmers do not always need it. For example:

public class MyClass {
public static void main(String[] args) {
MyClass myObj = new MyClass ();

This allocates memory for the object to hold its state information and makes it available for use.

Final Thoughts on Java Classes and Objects

A good knowledge of OOP helps developers build applications that are robust, scalable, maintainable, and high performant. However, while there are several benefits of using OOP, there are certain downsides as well. Design patterns have evolved to address the shortcomings of OOP.

Read more Java programming and software development tutorials.

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