Java Classes And Objects

In Java, classes and objects are fundamental concepts in object-oriented programming (OOP). A class is a blueprint for creating objects, which are instances of a class. Classes define properties (attributes) and behaviors (methods) that objects can have.

In this tutorial, we will go through the basics of creating and using classes and objects in Java.

Creating a class:

To create a class, use the class keyword followed by the class name. Inside the class, you can define fields and methods. Here's an example:

public class Dog {
    String name;
    String breed;
    int age;

    void bark() {
        System.out.println("Woof!");
    }

    void displayInfo() {
        System.out.println("Name: " + name + ", Breed: " + breed + ", Age: " + age);
    }
}

In this example, we create a Dog class with three fields (name, breed, and age) and two methods (bark and displayInfo).

Creating objects:

To create an object of a class, use the new keyword followed by the class name and parentheses. Here's an example:

Dog myDog = new Dog();

In this example, we create an object of the Dog class named myDog.

Accessing object fields:

To access an object's fields, use the object name followed by the dot (.) operator and the field name. Here's an example:

myDog.name = "Buddy";
myDog.breed = "Golden Retriever";
myDog.age = 3;

In this example, we set the values of the name, breed, and age fields of the myDog object.

Calling object methods:

To call an object's methods, use the object name followed by the dot (.) operator and the method name. Here's an example:

myDog.bark();          // Output: Woof!
myDog.displayInfo();   // Output: Name: Buddy, Breed: Golden Retriever, Age: 3

In this example, we call the bark() and displayInfo() methods of the myDog object.

Here's the complete example:

public class Main {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.name = "Buddy";
        myDog.breed = "Golden Retriever";
        myDog.age = 3;

        myDog.bark();          // Output: Woof!
        myDog.displayInfo();   // Output: Name: Buddy, Breed: Golden Retriever, Age: 3
    }
}

In summary, classes and objects are core concepts in Java and object-oriented programming. Classes define the structure and behavior of objects, and objects are instances of classes. By using classes and objects, you can create modular, reusable, and well-organized code.

Creating Objects in Java Class Example: Objects are instances of a class. Here's an example of creating objects in Java:

public class Car {
    String make;
    String model;

    public void start() {
        System.out.println("Car is starting.");
    }
}

public class CarExample {
    public static void main(String[] args) {
        // Creating objects
        Car myCar = new Car();
        Car friendCar = new Car();

        // Accessing object properties and methods
        myCar.make = "Toyota";
        myCar.model = "Camry";
        myCar.start();
    }
}

Instance Variables in Java Classes: Instance variables are fields that belong to each instance of a class. They represent the state of an object.
```
public class Person {
    String name; // Instance variable
    int age; // Instance variable
}
```

Methods in Java Classes and Their Role in Objects: Methods define the behavior of objects. They perform actions and can manipulate the object's state.

public class Circle {
    double radius; // Instance variable

    public double calculateArea() {
        return Math.PI * radius * radius;
    }
}

Java Class and Object Relationships: Classes define blueprints for objects. Objects are instances of classes, and they interact through methods and properties.

public class Dog {
    String name;
    int age;

    public void bark() {
        System.out.println("Woof! Woof!");
    }
}

public class DogOwner {
    public static void main(String[] args) {
        Dog myDog = new Dog();
        myDog.name = "Buddy";
        myDog.age = 3;
        myDog.bark();
    }
}

Access Modifiers for Class Members in Java: Access modifiers control the visibility of class members. Common modifiers include public, private, protected, and package-private.
```
public class MyClass {
    public int publicVar;
    private String privateVar;
    protected double protectedVar;
    int defaultVar; // Package-private by default
}
```

Inheritance and Polymorphism in Java Classes: Inheritance allows a class to inherit properties and methods from another class. Polymorphism enables objects to be treated as instances of their parent class.

public class Animal {
    public void makeSound() {
        System.out.println("Animal makes a sound");
    }
}

public class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Dog barks");
    }
}

Encapsulation in Java Classes and Objects: Encapsulation involves bundling data and methods within a class, and controlling access to the internal state.

public class BankAccount {
    private double balance;

    public void deposit(double amount) {
        if (amount > 0) {
            balance += amount;
        }
    }

    public double getBalance() {
        return balance;
    }
}