What Are Java Classes and Objects and How Do You Implement Them?

The Concept of Java Classes and Objects

Classes and objects are the two most essential Java concepts that every programmer must learn. Classes and objects are closely related and work together. An object has behaviors and states, and is an instance of class. For instance, a cat is an object—it’s color and size are states, and its meowing and clawing furniture are behaviors. A class models the object, a blueprint or template that describes the state or behavior supported by objects of that type. 

Let’s explore Java classes and objects in a bit more detail, starting with the class.

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What Is a Class?

We can define a class as a container that stores the data members and methods together. These data members and methods are common to all the objects present in a particular package. 

Every class we use in Java consists of the following components, as described below:

Access Modifier

Object-oriented programming languages like Java provide the programmers with four types of access modifiers.

• Public
• Private
• Protected
• Default

These access modifiers specify the accessibility and users permissions of the methods and members of the class.

Class Name

This describes the name given to the class that the programmer decides on, according to the predefined naming conventions.

Body of the Class

The body of the class mainly includes executable statements.

Apart from these, a class may include keywords like "super" if you use a superclass,

"implements" if you are inheriting members, methods, and instances from the same class, and

"interface" if you are inheriting any members, methods, and instances from a different class.

Type of Classes

In Java, we classify classes into two types:

Built-in Classes

Built-in classes are just the predefined classes that come along with the Java Development Kit (JDK). We also call these built-in classes libraries. Some examples of built-in classes include:

• java.lang.System
• java.util.Date
• java.util.ArrayList
• java.lang.Thread

User-Defined Classes

User-defined classes are rather self-explanatory. The name says it all. They are classes that the user defines and manipulates in the real-time programming environment. User-defined classes are broken down into three types:

Concrete Class

Concrete class is just another standard class that the user defines and stores the methods and data members in.

Syntax:

class con{

 //class body;

}

Abstract Class 

Abstract classes are similar to concrete classes, except that you need to define them using the "abstract" keyword. If you wish to instantiate an abstract class, then it should include an abstract method in it. Otherwise, it can only be inherited.

Syntax:

abstract class AbstClas{

 //method();

 abstract void demo();

}

Interfaces

Interfaces are similar to classes. The difference is that while class describes an object’s attitudes and behaviors, interfaces contain the behaviors a class implements. These interfaces will only include the signatures of the method but not the actual method.

Syntax:

public interface demo{

 public void signature1();

 public void signature2();

}

public class demo2 implements demo{

 public void signature1(){

  //implementation;

}

public void signature2(){

  //implementation;

}

}

How to Create a Class

You can define a typical class in Java using the following syntax:

<Access Specifier> class <class name>{

//Body

} 

Example:

public class Student{

   String Name;

   int rollno;

   String section;

   void study() {

   }

   void write() {

   }

   void play() {

   }

}

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Rules for Creating Classes

The following rules are mandatory when you're working with Java classes:

• The keyword "class" must be used to declare a class
• Every class name should start with an upper case character, and if you intend to include multiple words in a class name, make sure you use the camel case
• A Java project can contain any number of default classes but should not hold more than one public class
• You should not use special characters when naming classes
• You can implement multiple interfaces by writing their names in front of the class, separated by commas
• You should expect a Java class to inherit only one parent class 

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What is an Object in Java?

An object in Java is the most fundamental unit of the object-oriented programming paradigm. It includes the real-world entities and manipulates them by invoking methods. An object in Java consists of the following:

1. Identity 
2. Behavior
3. State

Identity

This is the unique name given by the user that allows it to interact with other objects in the project.

Example:

Name of the student

Behavior

The behavior of an object is the method that you declare inside it. This method interacts with other objects present in the project.

Example:

Studying, Playing, Writing

State

The parameters present in an object represent its state based on the properties reflected by the parameters of other objects in the project.

Example:

Section, Roll number, Percentage

How Do You Create an Object in Java?

As discussed before, a class is a blueprint of any object. So, once you declare the class, all that you need to do to create a class is to instantiate it. There are three stages involved in creating an object. They are:

• Declaration
• Instantiation
• Initialization

Example:

public class Student {

   public Student (String name) {

      System.out.println("parameters sent are :" + name );

   }

   public static void main(String []args) {

      Student mystudent = new Student( "Steven" );

   }

}

Output:

Steven

Key Differences Between Java Classes and Objects

The key differences between a class and an object are:

Class:

• A class is a blueprint for creating objects
• A class is a logical entity
• The keyword used is "class"
• A class is designed or declared only once
• The computer does not allocate memory when you declare a class

Objects:

• An object is a copy of a class
• An object is a physical entity 
• The keyword used is "new"
• You can create any number of objects using one single class
• The computer allocates memory when you declare a class

And this brings us to the end of the article. We hope it has strengthened your understanding of the essential concepts relating to Java objects and classes, the rules used to declare them, and their key differences.

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