Classes and Objects in Java

What is a Class?

A class is a blueprint or a template for creating objects (instances). It defines the properties (variables) and behaviors (methods) that objects created from the class will have.

Class Syntax Example:
        
        public class Car {
            // Properties (fields)
            String make;
            String model;
            int year;

            // Method (behavior)
            public void startEngine() {
                System.out.println("The engine is now running!");
            }
        }
In this example:
  • Properties: make, model and year are variables that describe the state of the object.
  • Method startEngine() is a behavior that represents an action that can be performed on objects of this class.

What is an Object?

An object is an instance of a class. When you create an object, you're allocating memory to store the values of the properties and enabling the behaviors defined in the class.

Creating an Object:

To create an object, you use the new keyword followed by the class constructor.

        
        public class Main {
            public static void main(String[] args) {
                // Creating an object of the Car class
                Car myCar = new Car();

                // Setting properties of the object
                myCar.make = "Toyota";
                myCar.model = "Corolla";
                myCar.year = 2020;

                // Calling the method of the object
                myCar.startEngine(); // Output: The engine is now running!
            }
        }

In the above example:

  • myCar is an object created from the Car class.
  • We assigned values to the properties: make, model, and year.
  • We called the startEngine() method, which produces the output: "The engine is now running!".

Constructor

A constructor is a special method that is used to initialize objects. It is automatically called when an object is created. If you don't define a constructor, Java provides a default no-argument constructor.

Constructor Example:
        
        public class Car {
            String make;
            String model;
            int year;

            // Constructor to initialize the object
            public Car(String make, String model, int year) {
                this.make = make;
                this.model = model;
                this.year = year;
            }

            // Method to start the engine
            public void startEngine() {
                System.out.println("The engine is now running!");
            }
        }

        public class Main {
            public static void main(String[] args) {
                // Creating an object using the constructor
                Car myCar = new Car("Honda", "Civic", 2022);

                // Calling method on the object
                myCar.startEngine(); // Output: The engine is now running!
            }
        }

In this example:

  • The constructor Car(String make, String model, int year) initializes the properties when the object is created.
  • We create an object with specific values for make, model, and year.

Accessing and Modifying Object Properties

To access or modify properties of an object, you use the dot operator (.).

        
        myCar.make = "Honda";  // Modifying the make of the car
        System.out.println(myCar.make);  // Accessing the make of the car

Encapsulation (Optional for Advanced Learning)

In object-oriented programming, encapsulation is a principle where you restrict direct access to certain object properties and methods. You can use getters and setters to access or modify private properties.

        
        public class Car {
            // Private properties
            private String make;
            private String model;

            // Getter for make
            public String getMake() {
                return make;
            }

            // Setter for make
            public void setMake(String make) {
                this.make = make;
            }
        }