Real World Example of Class, Methods & Objects

First lets understand what is Class, Methods & Objects in real world, below are some classes

• A class is a template or blueprint used to categorize objects. For example, we can have classes like Animal, Birds, Vehicles and Furniture. Each class can have multiple objects, which represent real-world entities.
  • The Animal class can have objects such as elephant, tiger, dog etc.
  • The Birds class can have objects like sparrow, peacock etc.
  • Similarly, other classes will have their respective objects.
• Every class has its own methods, which define the actions that objects can perform:
  • The Animal class may have methods like eat(), run(), sleep() etc.
  • The Birds class may have methods like fly(), eat() etc.
• Since objects represent real-world entities, they are used to call methods and perform actions.

What is Class in Java ?

• Class is a blueprint or prototype or template for creating objects in Java.
• It is not a real-world entity, means it does not exist physically; it only defines how an object should behave.
• Class does not occupy memory (but objects occupies memory).
• NOTE : Class metadata (such as the number of variables, variable names, method names, constructors, etc.) is stored in the Method Area.

• Syntax :

  access-modifiers class ClassName
  {
  	Fields (Instance Variables) – Store object data.
      Constructors – Initialize objects.
      Methods – Define object behavior.
      Static Members – Shared among all objects.
      Nested Classes – Class inside another class.
      Blocks – Static and instance blocks for initialization.
  }

• Example :

  public class Animal
  {
      //fields
  	int eyes;
      String color;
      
      //method
      void run()
      {
          //body
      }
  }

What is Method in Java ?

• A method in Java is a block of code that performs a specific task and can be reused multiple times.

• Syntax :

  access-modifiers return-type methodName(List of Parameters)
  {
  	// body
  }

• Example :

  public void run(String name)
  {
      System.out.println(name+" is running");
  }

• We can write computations, data processing, input/output operations, object manipulation, conditional logic etc inside methods to perform specific tasks efficiently.

What is Object in Java ?

• Object is an instance of a class - Objects are created from a class blueprint and represent real data.
• Object is a real-world entity - It represents tangible things like a car, person, or book in programming.
• Object occupies memory - When an object is created (new ClassName()), it gets memory allocated in the heap.

• Objects has its own :
  • Identity - Unique memory reference assigned by JVM
  • State / Attribute - Data stored in instance variables
  • Behaviour - Actions (methods)

• Syntax :
  • 1. Creating Objects:

    • ClassName objectName = new ClassName();

      • ClassName objectName; — Declares a reference variable objectName of type ClassName.
      • new ClassName(); — Creates a new object of ClassName by allocating memory and calling its constructor.
      • = — Assigns the reference (address) of the newly created object to the variable objectName.
      • So overall: objectName now holds a reference to a new ClassName object in memory.
  • 2. Access Methods using Objects:

    • objectName.methodName();

  • 3. Access Fields/Variables using Objects

    • objectName.field_name;

• Example :

  public class Animal
  {
      String color;   //instance variable
      
      void run()  //method
      {
          System.out.println("Im running");
      }
      public static void main(String[] args)
      {
          Animal buzo = new Animal();     //creating object (buzo)
          buzo.run();                     //accessing run() method from buzo object
          buzo.color = "Black";           //setting value in instance variable using . operator
  
          System.out.println("Buzo color is "+buzo.color);    //accessing instance variable
      }
  }

Program 1:

• Below is the simple program in which we have created one Animal1 class, one run() method and called the run() method using jumbo object.

• // Define a class named Animal1
  public class Animal1 
  {
      // Method to display a running message
      public void run() 
      {
          System.out.println("I'm running"); // Print a message to the console
      }
  
      // Main method - entry point of the program
      public static void main(String[] args) 
      {
          // Create an instance (object) of the Animal1 class
          Animal1 jumbo = new Animal1();
  
          // Call the run method using the object 'jumbo'
          jumbo.run();
      }
  }

  Output:

  I'm running

Program 2:

• Below is the simple program having one Animal2 class, two methods i.e. run() and eat() method and jambo object is accessing both the methods.

• // Define a class named Animal2
  public class Animal2 
  {
      // Method to display a running message
      public void run() 
      {
          System.out.println("I'm running"); // Print a message to the console
      }
  
      // Main method - entry point of the program
      public static void main(String[] args) 
      {
          // Create an instance (object) of the Animal2 class
          Animal2 jumbo = new Animal2();
  
          // Call the run method using the object 'jumbo'
          jumbo.run();
  
          // Call the eat method using the object 'jumbo'
          jumbo.eat();
      }
  
      // Method to display an eating message
      public void eat() 
      {
          System.out.println("I'm eating...!!"); // Print a message to the console
      }
  }

  Output:

  I'm running
  I'm eating...!!

Program 3:

• Below is the simple program having one Animal3 class, two methods i.e. run() and eat() methods. Then two objects i.e. jambo and buzo objects are accessing both the methods.

• // Define a class named Animal3
  public class Animal3 
  {
      // Method to display a running message
      public void run() 
      {
          System.out.println("I'm running"); // Print a message indicating the animal is running
      }
  
      // Main method - entry point of the program
      public static void main(String[] args) 
      {
          // Create an instance (object) of Animal3 named 'jumbo'
          Animal3 jumbo = new Animal3();
  
          // Call the run method using the 'jumbo' object
          jumbo.run();
  
          // Call the eat method using the 'jumbo' object
          jumbo.eat();
  
          // Create another instance (object) of Animal3 named 'buzo'
          Animal3 buzo = new Animal3();
  
          // Call the eat method using the 'buzo' object
          buzo.eat();
  
          // Call the run method using the 'buzo' object
          buzo.run();
      }
  
      // Method to display an eating message
      public void eat() 
      {
          System.out.println("I'm eating...!!"); // Print a message indicating the animal is eating
      }
  }

  Output:

  I'm running
  I'm eating...!!
  I'm eating...!!
  I'm running

Program 4:

• Below is the another program with parameters in methods.

• // Define a class named Animal4
  public class Animal4 
  {
      // Method to display that an animal is running
      public void run(String name) 
      {
          System.out.println(name + " is running");
      }
  
      // Main method - program entry point
      public static void main(String[] args) 
      {
          // Create an object 'jumbo' of Animal4
          Animal4 jumbo = new Animal4();
          jumbo.run("Jumbo"); // Call run method with "Jumbo"
          jumbo.eat("Jumbo"); // Call eat method with "Jumbo"
  
          // Create another object 'buzo' of Animal4
          Animal4 buzo = new Animal4();
          buzo.eat("Buzo"); // Call eat method with "Buzo"
          buzo.run("Buzo"); // Call run method with "Buzo"
      }
  
      // Method to display that an animal is eating
      public void eat(String name) 
      {
          System.out.println(name + " is eating...!!");
      }
  }

  Output:

  Jumbo is running
  Jumbo is eating...!!
  Buzo is eating...!!
  Buzo is running

Program 5:

• Below is the another program with multiple parameters in methods.

• // Define a class named Animal5
  public class Animal5 
  {
      // Method to display that an animal has run a certain distance
      public void run(String name, int distance_km) 
      {
          System.out.println(name + " has run " + distance_km + " km");
      }
  
      // Main method - program entry point
      public static void main(String[] args) 
      {
          // Create an object 'jumbo' of Animal5
          Animal5 jumbo = new Animal5();
          jumbo.run("Jumbo", 5); // Call run method with name "Jumbo" and distance 5 km
          jumbo.eat("Jumbo", "grass"); // Call eat method with name "Jumbo" and food "grass"
  
          // Create another object 'buzo' of Animal5
          Animal5 buzo = new Animal5();
          buzo.eat("Buzo", "meat"); // Call eat method with name "Buzo" and food "meat"
          buzo.run("Buzo", 12); // Call run method with name "Buzo" and distance 12 km
      }
  
      // Method to display that an animal is eating a specific dish
      public void eat(String name, String dish) 
      {
          System.out.println(name + " is eating " + dish);
      }
  }

  Output:

  Jumbo has run 5 km  
  Jumbo is eating grass  
  Buzo is eating meat  
  Buzo has run 12 km

Program 6:

• Below is the another program with instance variables and method parameter.

• // Define a class named Animal6
  public class Animal6 
  {
      // Declare instance variables
      int no_of_eyes; // Variable to store the number of eyes
      String color; // Variable to store the color of the animal
  
      // Method to display the details of an animal
      public void details(String name) 
      {
          System.out.println("-------Details of " + name + "-------");
          System.out.println("Eyes : " + no_of_eyes);
          System.out.println("Color : " + color);
      }
  
      // Main method - program entry point
      public static void main(String[] args) 
      {
          // Create an object 'jumbo' of Animal6 and assign values
          Animal6 jumbo = new Animal6();
          jumbo.no_of_eyes = 2;
          jumbo.color = "Brown";
          jumbo.details("Jumbo"); // Call details method for 'jumbo'
  
          // Create another object 'buzo' of Animal6 and assign values
          Animal6 buzo = new Animal6();
          buzo.no_of_eyes = 2;
          buzo.color = "Black";
          buzo.details("Buzo"); // Call details method for 'buzo'
      }
  }

  Output:

  -------Details of Jumbo-------
  Eyes : 2
  Color : Brown
  -------Details of Buzo-------
  Eyes : 2
  Color : Black

Program 7: (Best Practice Program)

• In above programs we have created main method in Animal class which is not good practice, we should always create main method in main class only, below is the program.

• // Define a class named Animal7
  class Animal7 
  {
      // Method to display a running message
      void run() 
      {
          System.out.println("I'm running");
      }
  }
  
  // Define the main class MainApp7
  public class MainApp7 
  {
      // Main method - program entry point
      public static void main(String[] args) 
      {
          // Create an object 'buzo' of Animal7
          Animal7 buzo = new Animal7();
          buzo.run(); // Call the run method
      }
  }

  Output:

  I'm running

Program 8: (Best Practice Program)

• Here we have created Animal8, Birds8 class with different methods and also created Main class as separate class which is good practice.

• // Define a class named Animal8
  class Animal8 
  {
      // Method to display a running message
      void run() 
      {
          System.out.println("I'm running");
      }
  }
  
  // Define a class named Birds8
  class Birds8 
  {
      // Method to display a flying message
      void fly() 
      {
          System.out.println("I'm flying");
      }
  }
  
  // Define the main class MainApp8
  public class MainApp8 
  {
      // Main method - program entry point
      public static void main(String[] args) 
      {
          // Create an object 'buzo' of Animal8 and call the run method
          Animal8 buzo = new Animal8();
          buzo.run();
  
          // Create an object 'sparrow' of Birds8 and call the fly method
          Birds8 sparrow = new Birds8();
          sparrow.fly();
      }
  }

  Output:

  I'm running
  I'm flying