Java Classes and Objects

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Introduction

In this lesson of Java course,we will learn about the Java classes and objects. In Java, a class acts as a blueprint that defines the attributes and methods of an object. It specifies the behavior and properties that objects of that class can have.

An object, on the other hand, is an instance of a class. It represents a specific concept or entity in the real world, such as a bank account or a car. Each object has its own unique state and behavior, which are determined by the properties and methods defined in its corresponding class.

1. Importance of Java Class and Objects

In Java, classes and objects are fundamental concepts in object-oriented programming (OOP). They play a crucial role in organizing code, improving code re-usability, and promoting modularity in programming. Classes allow for the creation of complex programs and data structures that can be easily managed and maintained.

Similarly, objects are used to represent real-world entities or concepts, with their own state and behavior that can be manipulated using methods. By utilizing classes and objects, Java programs can be written more efficiently and effectively, with the ability to create reusable code and implement complex designs.

2. Java Classes

A Java class is a blueprint for creating objects that define the attributes and behavior of the objects. Think of Java class as the architecture of your house or building which defines all the details, including.

  1. Number of rooms.
  2. Windows.
  3. Doors.
  4. Floor

Based on this design, we build a house and when this is created, we know about the rooms and other details of the house. In the same ways, a Java class contains variables, methods, and constructors that define the properties and behavior of the objects. Once you create an instance of the Java class using new operator, we know what is inside that object based on the blueprint we created while designing our class.

access_modifier class class_name {
  // instance variables
  variable_type variable_name;

  // constructor
  access_modifier class_name(parameter_list) {
    // constructor body
  }

  // methods
  access_modifier return_type method_name(parameter_list) {
    // method body
  }
}
  • Instance Variables/ Fields: They represent the state of the object. They are used to storing the data.
  • Method: Represents the behavior of the object. Method, as we know, perform operations.

3. How to create a Java Class

To create a Java class, you need to follow these steps:

  • Declare the access modifier of the class
  • Specify the class keyword
  • Give a name to the class
  • Define the instance variables, constructors, and methods of the class

Here is an example of a Java class:

public class Car {
  private String make;
  private String model;
  private int year;

  public Car(String make, String model, int year) {
    this.make = make;
    this.model = model;
    this.year = year;
  }

  public String getMake() {
    return make;
  }

  public String getModel() {
    return model;
  }

  public int getYear() {
    return year;
  }
}

4. Java Objects

In Java, an object is an instance of a class that has state and behavior. It is a runtime entity that is created from a class and can interact with other objects in the program. The syntax and structure of a Java object are:

class_name object_name = new class_name();

To create a Java object, you need to follow these steps:

  • Declare the class name
  • Give a name to the object
  • Use the new keyword to create the object
  • Call the class constructor to initialize the object

Here is an example of a Java object:

Car myCar = new Car("Toyota", "Camry", 2022);

5. Example: Java Class and Objects

Here is a complete example of a Java classes and objects:

public class BankAccount {
    private String accountNumber;
    private String accountHolder;
    private double balance;
    
    public BankAccount(String accountNumber, String accountHolder, double balance) {
        this.accountNumber = accountNumber;
        this.accountHolder = accountHolder;
        this.balance = balance;
    }
    
    public String getAccountNumber() {
        return accountNumber;
    }
    
    public String getAccountHolder() {
        return accountHolder;
    }
    
    public double getBalance() {
        return balance;
    }
    
    public void deposit(double amount) {
        balance += amount;
        System.out.println(amount + " deposited to account " + accountNumber);
    }
    
    public void withdraw(double amount) {
        if (balance >= amount) {
            balance -= amount;
            System.out.println(amount + " withdrawn from account " + accountNumber);
        } else {
            System.out.println("Insufficient balance");
        }
    }
}

Relation between a Java Classes and Objects

The class contains properties, also known as fields, which define the state of an object, as well as behaviors, also known as methods, which define the actions an object can perform. An object instantiates a class, meaning it creates a new instance of the class and has its own unique state and behavior. The object can access the properties and behaviors of its class through its instance variables and methods. The arrow between the class and the object represents the instantiation process.

Conclusion

In Java programming, classes and objects are fundamental concepts of object-oriented programming. A class acts a blueprint or a template that defines the behavior and properties of an object. An object is an instance of a class that has state and behavior. Classes and objects are important in Java programming because they help in organizing code, improving code re-usability, and making the code modular. They also enable the creation of complex programs and data structures that can be easily managed and maintained.

In Java, a class is defined using the class keyword and contains instance variables, constructors, and methods. An object is created using the new keyword and can access the properties and behaviors of its class through its instance variables and methods. As always, the source code for this course is available on our GitHub repository