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Java Classes and Objects

Understanding Java classes and objects is essential for writing effective and efficient Java code. This blog attempts to understand, in detail, the basics of Java classes and objects, including the creation of objects, defining classes, and using those in your code. Moreover, we’ll try and explore some best practices when working with classes and objects in Java. So, without further ado, let’s get cracking!

Introduction to Java Classes and Objects
Constructors and Methods in Java
Variables in Java
Types of Variables in Java
Understanding Superclass in Java
Understanding Subclass in Java
Creating Classes and Objects in Java
Defining Classes and Objects in Java with Examples
Conclusion

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Introduction to Java Classes and Objects

Objects are the basic building blocks of object-oriented programming languages like Java. An object is a class instance that contains real-world values. It’s a physical thing with a state (fields) and behavior (methods). We can make numerous objects from a single class. Each object’s fields and methods will be unique.

Classes define the structure and behavior of objects. A class is a blueprint for creating objects. When you define a class, you’re guiding Java on how to create an object of that type. A class can have fields (data), methods (actions), and constructors (specialized functions). If you don’t specify any constructors, Java will automatically create one for you when it needs to instantiate an object of your class.

Constructors and Methods in Java

A constructor is a special Java function that creates and configures class entities. This procedure has the same name as its class, and, unlike other methods, it does not have a return type. Following the object’s creation with the “new” keyword, the constructor assigns predefined values to its variables.

For example, you have a “Person” class with variables like “name” and “age.” You can create a constructor for the class that receives name and age parameters and uses them to initialize the variables. This allows you to create new objects with varied names and ages, simply.

On the other hand, a method in Java is a block of code that performs a specified task. It is often specified within a class and can be called a class object. Methods can have parameters, values supplied to the method for use, and return types, which define the data type that the method will return after completing its work.

For example, using the “Person” class, from above, you could create a method named “sayHello” to output a customized greeting to the console. The method may accept an argument, specifying the person’s name to greet and return a message; thus, indicating that the person was greeted.

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Variables in Java

Variables, in Java, are containers that retain values of specified data types. Variables can retain data that may be changed or utilized in a program.

Variables in Java can be used for a variety of purposes, including performing mathematical computations, storing user input, and transporting data across methods or classes. Understanding how to use variables, effectively, is crucial when learning Java programming. In order to use a variable in Java, it must first be declared with a specified data type.

Types of Variables in Java

Variables in Java are classified into three types, namely instance variables, class variables, which are also known as static variables, and local variables. Here is a quick brief of each type.

Instance Variables – Variables that are declared within a class but outside any method or constructor are known as instance variables. Since instance variables are specific to each class instance, each object created from the class has its own set of instance variables. To prohibit direct access from outside the class, instance variables are often declared with the ‘private’ access modifier.
Example

public class Person {
    private String name;     // instance variable
    private int age;         // instance variable
}

Class Variables (Static Variables) – Class or static variables are those that are declared with the ‘static’ keyword within a class but outside any methods or constructors. Class variables are shared by all class instances, implying that they belong to the class rather than to any single object. Class variables are typically declared with the ‘public’ access modifier.
Example

public class Person {
     public static int totalPersons = 0;     // class variable
}

Local Variables – These are variables that are declared within a method or constructor. Local variables can only be accessed within the method or constructor they are declared in. Local variables do not have a default value and must be initialized before being utilized.
Example:

public class Person {
public void printName(String name) {    // local variable
     System.out.println("Name: " + name);
    }
}

Understanding Superclass in Java

A subclass in Java that inherits properties and methods from another class is called a superclass. In comparison to the superclass, a subclass is more specialized, which means that it can contain additional properties and methods that are particular to its purpose.

Assume that you have an “Animal” superclass with properties and methods like “name,” “age,” and “makeSound().” You can create a “Dog” subclass that deviates from “Animal” and includes dog-specific properties and methods such as “breed” and “bark().” The subclass can access and use the properties and methods of the superclass, as well as add its own.

Employing inheritance, code can be reused; thus, reducing duplication, to result in more efficient and easier-to-maintain code. Besides being a fundamental notion in object-oriented programming, inheritance also works as a strong tool for developing complicated applications.

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Understanding Subclass in Java

A subclass, in Java, inherits properties and methods from another class, called superclass. In comparison to superclass, subclass is more specialized, which means that it can contain additional properties and methods that are particular to its purpose.

Assume that you have a “Vehicle” superclass with attributes and methods like “make,” “model,” and “start().” You can construct a “Car” subclass that derives from “Vehicle” and includes car-specific attributes and methods like “numDoors” and “drive().” The subclass can access and use the properties and methods of the superclass, as well as add its own.

Subclassing allows you to create specialized versions of a class without having to rewrite all the code from scratch. It is a fundamental notion in object-oriented programming, as well as a strong tool for developing complicated applications.

Creating Classes and Objects in Java

Defining a Class
The class keyword is used to define a class in Java. For example,

class Account {
  int accountId; //class variable
  String accountHolderName; //class variable
  public int getAccountId() {
    return accountId; //method
  }
  public String getAccountHolderName() {
    return accountHolderName; //method
  }  
}

Here, “Account” is a class, defining attributes “accountId” and “accountHolderName” and methods to access them.

Creating Objects
We create objects by using the new keyword. For example,

Account acc1 = new Account(); //creating object
acc1.accountId = 101; //setting value
acc1.accountHolderName = "John"; //setting value

Here, “acc1” is an object of class “Account” with values “101” and “John” for “accountId” and “accountHolderName”, respectively.

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Defining Classes and Objects in Java with Examples

Now that we have understood more about classes and objects in Java, let’s combine everything and define a complete “Employee” class with instance variables, class variables, constructors, and methods.

  public class Employee {
  // Instance variables
  String name;
  int age;
  double salary;
  // Class variable
  static int numberOfEmployees;
  // Constructor
  Employee(String name, int age, double salary) {
    this.name = name;
    this.age = age;
    this.salary = salary;
    // Increment the numberOfEmployees class variable
    numberOfEmployees++;
  }
  // Method to display employee information
  void displayInfo() {
    System.out.println("Name: " + name);
    System.out.println("Age: " + age);
    System.out.println("Salary: " + salary);
  }
}

Here’s an example of how to use the “Employee class to create and manipulate Employee objects

public class Main {
  public static void main(String[] args) {
    // Creating Employee objects
    Employee employee1 = new Employee("John Doe", 30, 50000);
    Employee employee2 = new Employee("Jane Smith", 28, 55000);
    // Displaying employee information
    employee1.displayInfo();
    employee2.displayInfo();
    // Displaying the total number of Employee objects
    System.out.println("Total Employees: " + Employee.numberOfEmployees);
  }
}

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Conclusion

Having an understanding of Java classes and objects is crucial for any aspiring Java developer. Classes form the blueprint for objects, the basic building blocks of Java programs. By defining classes, you can create objects with specific properties and behaviors, making your code more efficient and easier to maintain.

The object-oriented programming model of Java enables code reusability and modularity, both of which are critical when developing complex applications. You can reuse code across many portions of your program by creating classes and objects, making it easier to maintain and modify.

Working with Java objects requires an understanding of constructors and methods. Constructors enable the creation and initialization of objects, whereas methods let you perform specified activities on those objects.

Java classes and objects are key notions in modern programming, and anyone wishing to develop fast and scalable Java programs must understand these concepts thoroughly.

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