The difference between Classes Objects and Instances

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What is a class  an object and an instance in Java

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A class is basically a definition  and contains the object s code  An object is an instance of a class  for example if you say   String word   new String      the class is the String class  which describes the object  instance  word   When a class is declared  no memory is allocated so class is just a template   When the object of the class is declared  memory is allocated

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Class is Data Type You use this type to create object    Instance is Logical but object is Physical means occupies some memory  We can create an instance for abstract class as well as for interface  but we cannot create an object for those  Object is instance of class and instance means representative of class i e object  Instance refers to Reference of an object  Object is actually pointing to memory address of that instance  You can   t pass instance over the layers but you can pass the object over the layers You can   t store an instance but you can store an object A single object can have more than one instance  Instance will have the both class definition and the object definition where as in object it will have only the object definition    Syntax of Object    Classname var new Classname      But for instance creation it returns only a pointer refering to an object  syntax is     Classname varname

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Class   Structure  Object   Physical Manifestation  Instance   each object created from class  Reference   Address of Object

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The definition  Object is an instance of a class   is conceptually wrong  but correct as per implementation  Actually the object oriented features are taken from the real life  for focusing the mind of programmer from more to less  In real life classes are designed to manage the object For eg- we human beings have a caste  religion nationality and much more  These casts  religion  nationality are the classes and have no existence without human beings  But in implementation there is no existence of objects without classes  Object- Object is an discrete entity having some well defined attribute  Here discrete mean something that makes it unique from other  Well defined attribute make sense in some context   Class- Classification of object having some common behaviour or objects of some common type

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The concept behind classes and objects is to encapsulate logic into single programming unit  Classes are the blueprints of which objects are created   Here an example of a class representing a Car   public class Car        int currentSpeed      String name       public void accelerate                  public void park                public void printCurrentSpeed               You can create instances of the object Car like this   Car audi   new Car    Car toyota   new Car      I have taken the example from this tutorial

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Any kind of data your computer stores and processes is in its most basic representation a row of bits  The way those bits are interpreted is done through data types  Data types can be primitive or complex  Primitive data types are - for instance - int or double  They have a specific length and a specific way of being interpreted  In the case of an integer  usually the first bit is used for the sign  the others are used for the value   Complex data types can be combinations of primitive and other complex data types and are called  Class  in Java   You can define the complex data type PeopleName consisting of two Strings called first and last name  Each String in Java is another complex data type  Strings in return are  probably  implemented using the primitive data type char for which Java knows how many bits they take to store and how to interpret them   When you create an instance of a data type  you get an object and your computers reserves some memory for it and remembers its location and the name of that instance  An instance of PeopleName in memory will take up the space of the two String variables plus a bit more for bookkeeping  An integer takes up 32 bits in Java   Complex data types can have methods assigned to them  Methods can perform actions on their arguments or on the instance of the data type you call this method from  If you have two instances of PeopleName called p1 and p2 and you call a method p1 getFirstName    it usually returns the first name of the first person but not the second person s

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In java  the objects are spawned on heap memory  These require reference to be pointed and used in our application  The reference has the memory location of the object with which we can use the objects in our application  A reference in short is nothing but a name of the variable which stores the address of the object instantiated on a memory location   An instance is a general term for object  FYI  Object is a class   For Example   Class A      A ref   new A      For the above code snippet  ref is the reference for an object of class A generated on heap

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A class is a blueprint that is needed to make an object   instance    The difference between an object and an instance is  an object is a thing and an instance is a relation   In other words  instance describes the relation of an object to the class that the object was made from

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Java  and any other programming language  is modeled in terms of types and values   At the theoretical level  a value is a representation for some quantum of information  and a type is a set of values   When we say value X is an instance of type Y  we are simply saying that X is a member of the set of values that is the type Y   So that s what the term  instance  really means  it describes a relationship not a thing   The type system of the Java programming language supports two kinds of types  primitive types and reference types   The reference types are further divided into the classes and array types   A Java object is an instance of a reference type      An object is a class instance or an array   JLS 4 3 1    That s the type theoretic view     In practice  most Java developers treat the words  instance  and  object  as synonyms    And that includes me then I m trying to explain something quickly    And most developers use the word  value  rather than  instance  to refer to an instance of a primitive type

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If you have a program that models cars you have a class to represent cars  so in Code you could say   Car someCar   new Car      someCar is now an instance of the class Car  If the program is used at a repairshop and the someCar represents your car in their system  then your car is the object   So Car is a class that can represent any real world car someCar is an instance of the Car class and someCare represents one real life object  your car   however instance and object is very often used interchangably when it comes to discussing coding

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I like Jesper s explanation in layman terms  By improvising examples from Jesper s answer   class House      blue print for House Objects    class Car      blue print for Instances of Class Car     House myHouse   new House    Car myCar   new Car      myHouse and myCar are objects   myHouse is an instance of House  relates Object-myHouse to its Class-House   myCar is an instance of Car  in short   myHouse is an instance of Class House  which is same as saying  myHouse is an Object of type House

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Class   It has logical existence  i e  no memory space is allocated when it is created  It is a set of objects  A class may be regarded as a blueprint to create objects     It is created using class keyword A class defines the methods and data members that will be possessed by Objects        Object   It has physical existence  i e  memory space is allocated when it is created  It is an instance of a class  An object is a unique entity which contains data members and member functions together in OOP language    It is created using new keyword An object specifies the implementations of the methods and the values that will be possessed by the data members in the class

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Honestly  I feel more comfortable with Alfred blog definitions   Object  real world objects shares 2 main characteristics  state and behavior  Human have state  name  age  and behavior  running  sleeping   Car have state  current speed  current gear  and behavior  applying brake  changing gear   Software objects are conceptually similar to real-world objects  they too consist of state and related behavior  An object stores its state in fields and exposes its behavior through methods   Class  is a    template         blueprint    that is used to create objects  Basically  a class will consists of field  static field  method  static method and constructor  Field is used to hold the state of the class  eg  name of Student object   Method is used to represent the behavior of the class  eg  how a Student object going to stand-up   Constructor is used to create a new Instance of the Class   Instance  An instance is a unique copy of a Class that representing an Object  When a new instance of a class is created  the JVM will allocate a room of memory for that class instance   Given the next example   public class Person       private int id      private String name      private int age       public Person  int id  String name  int age            this id   id          this name   name          this age   age             public int hashCode             final int prime   31          int result   1          result   prime   result   id          return result             public boolean equals Object obj            if  this    obj              return true          if  obj    null              return false          if  getClass      obj getClass                return false          Person other    Person  obj          if  id    other id              return false          return true             public static void main String   args              case 1         Person p1   new Person 1   Carlos   20           Person p2   new Person 1   Carlos   20              case 2         Person p3   new Person 2   John   15           Person p4   new Person 3   Mary   17             For case 1  there are two instances of the class Person  but both instances represent the same object   For case 2  there are two instances of the class Person  but each instance represent a different object   So class  object and instance are different things  Object and instance are not synonyms as is suggested in the answer selected as right answer

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While the above answers are correct  another way of thinking about Classes and Objects would be to use real world examples  A class named Animal might contain objects like Cat  Dog or Fish  An object with a title of Bible would be of class Book  etc  Classes are general  objects are specific  This thought example helped me when I was learning Java

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A class is a blueprint which you use to create objects  An object is an instance of a class - it s a concrete  thing  that you made using a specific class  So   object  and  instance  are the same thing  but the word  instance  indicates the relationship of an object to its class   This is easy to understand if you look at an example  For example  suppose you have a class House  Your own house is an object and is an instance of class House  Your sister s house is another object  another instance of class House       Class House describes what a house is class House                    You can use class House to create objects  instances of class House  House myHouse   new House    House sistersHouse   new House      The class House describes the concept of what a house is  and there are specific  concrete houses which are objects and instances of class House   Note  This is exactly the same in Java as in all object oriented programming languages

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Class is a template or type  An object is an instance of the class   For example   public class Tweet       Tweet newTweet   new Tweet      Tweet is a class and newTweet is an object of the class

[java] The difference between Classes, Objects, and Instances

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