GFG Day 8: Classes, Objects, and Methods

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• In Java, classes and objects are basic concepts of object-oriented programming (OOP).

Java Classes

• A class is like a blueprint or template for creating objects. It defines properties (variables) and behaviors (methods) that an object will have. A class does not allocate memory until an object is created from it.

Syntax

class ClassName {
    // Fields or Variables
    dataType variableName;

    // Methods
    returnType methodName() {
        // code
    }
}

• A class in Java is a template with the help of which we create real-world entities known as objects that share common characteristics and properties.

• Here, the below Java code demonstrates the basic use of class in Java. It defines a Student class with two instance variables (id and n), creates an object s1 and prints values of s1.

// File: Student.java
class Student {
    int id;
    String n;

    // Added constructor to initialize both fields
    public Student(int id, String n) {
        this.id = id;
        this.n = n;
    }
}

// File: Main.java
public class Main {
    public static void main(String[] args) {

        // Creating Student object using the new constructor
        Student s1 = new Student(10, "Advance Java");

        System.out.println(s1.id);  
        System.out.println(s1.n);  
    }
}

Output

10
Advance Java

Java Object

• An object is an actual instance of a class it's like a real house built from the plan. When you create an object, you're allocating memory and accessing the class's fields and methods.

  • State: It is represented by attributes of an object. It also reflects the properties of an object.

  • Behavior: It is represented by the methods of an object. It also reflects the response of an object to other objects.

  • Identity: It gives a unique name to an object and enables one object to interact with other objects.

Note: Objects (non-primitive types) are always allocated on the heap, while their reference variables are stored on the stack.

Syntax to create object

ClassName objectName = new ClassName();

Initializing a Java Object

• The new operator instantiates a class by allocating memory for a new object and returning a reference to that memory. The new operator also invokes the class constructor.

// Java Program to Demonstrate the 
// use of a class with instance variable 

// Class Declaration
public class Dog {

    // Instance Variables
    String name;
    String breed;
    int age;
    String color;

    // Constructor Declaration of Class
    public Dog(String name, String breed, int age,
               String color)
    {
        this.name = name;
        this.breed = breed;
        this.age = age;
        this.color = color;
    }

    // method 1
    public String getName() { 
      return name; 
    }

    // method 2
    public String getBreed() { 
      return breed; 
    }

    // method 3
    public int getAge() { 
      return age; 
    }

    // method 4
    public String getColor() { 
      return color; 
    }

    @Override public String toString()
    {
        return ("Name is: " + this.getName()
                + "\nBreed, age, and color are: "
                + this.getBreed() + "," + this.getAge()
                + "," + this.getColor());
    }

    public static void main(String[] args)
    {
        Dog tuffy
            = new Dog("tuffy", "papillon", 5, "white");
        System.out.println(tuffy.toString());
    }
}

Output

Name is: tuffy
Breed, age, and color are: papillon,5,white

Explanation:

• The Dog class has instance variables for name, breed, age, and color.

• A constructor initializes these variables using passed arguments.

• Constructors have no return type and share the class name.

• Object tuffy is created with: new Dog("tuffy", "papillon", 5, "white").

• toString() returns a string representation of the object.

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Java Methods

• A method is a block of code that performs a specific task. It helps in:

  • Code reusability (write once, use many times)

  • Organizing logic

  • Reducing code duplication

• All methods in Java must belong to a class. Methods are similar to functions and expose the behavior of objects.

Java program to demonstrate how to create and use a method

// Creating a method
public class Geeks
{
    public void printMessage() {
        System.out.println("Hello, Advance Java!");
    }
    public static void main(String[] args) {
        // Create an instance of the Method class
        Geeks obj = new Geeks();

        // Calling the method
        obj.printMessage(); 
    }
}

Output

Hello, Advance Java!

Explanation:

• printMessage() is a simple method that prints a message.

• It has no parameters and does not return anything.

• Here, first we create a method that prints Hello, Advance Java!

Types of Methods in Java

1. Predefined Method

• Predefined methods are the method that is already defined in the Java class libraries. It is also known as the standard library method or built-in method.

Math.random()    // returns random value
Math.PI()     // return pi value

2. User-defined Method

• The method written by the user or programmer is known as a user-defined method. These methods are modified according to the requirement.

sayHello         // user define method created above in the article
Greet() 
setName()

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Happy Learning

Thanks For Reading! :)

SriParthu 💝💥