Class, Object, Instance

What is a Class? #

A blueprint or template for creating objects.
A collection of related variables and methods.

What is an Object? #

The target to be implemented in the software world.
An entity created exactly as declared in a class.

Characteristics

Also referred to as an instance of a class.
An object has a comprehensive meaning, representing all instances.
From an OOP perspective, it is called an 'object' when declared as a type of class.

What is an Instance? #

A concrete entity implemented in the software world based on a blueprint.
- In other words, when an object is materialized in software, it is called an 'instance'.
- A materialized instance is allocated in memory.

Characteristics

An instance can be considered to be included within an object.
From an OOP perspective, it is called an 'instance' when an object is allocated in memory and actually used.
Used when focusing on the relationship between an abstract concept (or specification) and a concrete object.
- Used in the form of an instance of ~.
- An object is an instance of a class.
- A link between objects is an instance of an association between classes.
- A running process is an instance of a program.

Example #

// 클래스
public class Animal{
  ...
}

// 객체와 인스턴스
public class Main{
  public static void main(String[] args){
    Animal cat, dog; // 객체
  }
  //인스턴스화
  cat = new Animal(); // cat은 Animal 클래스의 인스턴스
  dog = new Animal(); // dog은 Animal 클래스의 인스턴스
}

Differences between Class, Object, and Instance #

Class vs. Object #

A class is a blueprint, and an object refers to all entities implemented from that blueprint.

Object vs. Instance #

It is called an object when declared as a type of class, and it is called an instance when that object is allocated in memory and actually used.
An object is closer to the real world, while an instance is closer to the software world.
An object focuses on 'entity', while an instance focuses on 'relationship'.
- An object is also referred to as an instance of a class.