Object and Classes

What is an Object in Java?

An object is a fundamental building block in Java representing an entity that has:

• State: The attributes or data values of the object (e.g., color, size).
• Behavior: The actions or methods the object can perform (e.g., start, stop, deposit).
• Identity: A unique identifier used internally by the JVM to distinguish objects.

Objects can represent real-world tangible things like a chair, bike, or pen, or logical concepts like a banking system or customer account. An object is an instance of a class. If the class is the recipe, the object is the actual cake.

Example

Consider a pen as an object:

• State: Name (Reynolds), color (white)
• Behavior: Writing capability

Object Facts

• An object is a runtime entity.
• It is an instance of a class (class is the blueprint).
• Each object has unique data and behavior based on the class it is created from.

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What is a Class in Java?

A class is a blueprint or template that defines properties (fields) and behaviors (methods) common to all objects of that type.

• It is a logical entity, not physical.
• A class contains:
  • Fields (variables)
  • Methods (functions)
  • Constructors
  • Initialization blocks
  • Nested classes/interfaces

Example Code

class Student {  
  // Fields (instance variables)  
  int id;  
  String name;  
  
  // Main method to create and access objects  
  public static void main(String args[]) {  
    // Creating an object of Student class  
    Student s1 = new Student();  
    
    // Accessing default field values  
    System.out.println(s1.id);   // Output: 0  
    System.out.println(s1.name); // Output: null  
  }  
}

How Classes and Objects Relate

• A class is a blueprint.
• An object is an instance created from that blueprint.
• You can create multiple objects from one class, each with its own state.

In-depth Explanation

• When you declare a class, no memory is allocated for fields or methods.
• Memory is allocated only when an object is instantiated using the new keyword.
• Constructors initialize object data when the object is created.
• Each object stores its own copy of fields (instance variables).
• Objects communicate by invoking methods on each other.
• this keyword refers to the current object instance.
• Java uses garbage collection to automatically free memory from unused objects.

Simple Diagram of Class and Object

Class:       Student  
Blueprint:   defines id, name, methods  

Object 1:    s1 -> id=0, name=null  
Object 2:    s2 -> id=101, name="Alice"