Day 2: Understanding Class and Object in Java

1. Class in Java

What is a Class?

• A class is a blueprint or template used to create objects.
• A class is a logical entity (it does not occupy memory by itself).
• A class is called a logical entity because it only defines structure and behavior, and memory is allocated only when an object is created.
• class is a Java keyword.

=> Syntax of a Class

class ClassName {

}

Rules for Class Name

• The first letter should be uppercase.

• A class name can contain:

  • Alphabets (A–Z, a–z)
  • Digits (0–9)
  • Special characters _ (underscore) and $ (dollar sign)

• Digits are allowed, but:

  • ❌ Not at the beginning
  • ✅ Allowed in the middle or at the end

Valid class names

class MobileShop
class Mobile_Shop
class Mobile$Shop
class MobileShop2

Invalid class names

class 2MobileShop   // starts with a number
class Mobile-Shop   // hyphen not allowed

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2. Object in Java

What is an Object?

• An object is a real‑world (physical) entity.
• An object is an instance of a class because it is a real thing created using the class blueprint and stored in memory.
• Object occupies memory.

=> Object Syntax

ClassName referenceName = new ClassName();

Example:

MobileShop mobile = new MobileShop();

Object Characteristics

• An object is a combination of:

  • State → variables / data
  • Behaviour → methods / actions

• new Keyword

  • new is a Java keyword.

• When we use new:

  • JVM allocates memory in the Heap area at runtime.
  • A new object is created.

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• Real‑World Example

  • Class → MobileShop (blueprint)
  • Object → Actual mobile shop (real instance)

We can create multiple objects from a single class.

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• Summary

  • Class → Blueprint, logical, no memory
  • Object → Real entity, physical, occupies memory
  • new keyword → Creates object and allocates heap memory