What exactly is an instance in Java

Question

What is the difference between an object  instance  and reference  They say that they have to create an instance to their application  What does that mean

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When you use the keyword new for example JFrame j   new JFrame    you are creating an instance of the class JFrame         The new operator instantiates a   class by allocating memory for a new   object and returning a reference to   that memory    Note  The phrase  instantiating a class  means the same thing as    creating an object   When you create   an object  you are creating an    instance  of a class  therefore    instantiating  a class    Take a look here Creating Objects       The types of the Java programming   language are divided into two   categories  primitive types and   reference types    The reference types   are class types  interface types  and   array types    There is also a special   null type    An object is a   dynamically created instance of a   class type or a dynamically created   array    The values of a reference   type are references to objects    Refer Types  Values  and Variables for more information

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An object and an instance are the same thing   Personally I prefer to use the word  instance  when referring to a specific object of a specific type  for example  an instance of type Foo   But when talking about objects in general I would say  objects  rather than  instances     A reference either refers to a specific object or else it can be a null reference        They say that they have to create an instance to their application  What does it mean    They probably mean you have to write something like this   Foo foo   new Foo      If you are unsure what type you should instantiate you should contact the developers of the application and ask for a more complete example

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basically object and instance are the two words used interchangeably  A class is template for an object and an object is an instance of a class

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instance to an application  means nothing    object  and  instance  are the same thing  There is a  class  that defines structure  and instances of that class  obtained with new ClassName     For example there is the class Car  and there are instance with different properties like mileage  max speed  horse-power  brand  etc   Reference is  in the Java context  a variable  - it is something pointing to an object instance  For example  String s   null  - s is a reference  that currently references no instance  but can reference an instance of the String class    Jon Skeet made a note about the difference between a variable and a reference  See his comment  It is an important distinction about how Java works when you invoke a method - pass-by-value       The value of s is a reference  It s very important to distinguish between variables and values  and objects and references

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I think that Object   Instance  Reference is a  link  to an Object   Car c   new Car      variable c stores a reference to an object of type Car

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Computer c  new Computer     Here an object is created from the Computer class  A reference named c allows the programmer to access the object

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creating an instance of a class   how about   you are taking a class and making a new variable of that class that WILL change depending on an input that changes   Class in the library called Nacho  variable Libre to hold the  instance  that will change  Nacho Libre   new Nacho Variable  Scanner Input  or whatever goes here  This is the place that accepts the changes then puts the value in  Libre  on the left side of the equals sign  you know  Nacho Libre   new Nacho Scanner in     Nacho Libre  is on the left of the    that s not tech talk  that s my way of explaining it   I think that is better than saying  instance of type  or  instance of class    Really the point is it just needs to be detailed out more      instance of type or class  is not good enough for the beginner      wow  its like a tongue twister and your brain cannot focus on tongue twisters very well     that  instance  word is very annoying and the mere sound of it drives me nuts     it begs for more detail     it begs to be broken down better   I had to google what  instance  meant just to get my bearings straight      try saying  instance of class  to your grandma     yikes

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The Literal meaning of instance is  an example or single occurrence of something   which is very closer to the Instance in Java terminology   Java follows dynamic loading  which is not like C language where the all code is copied into the RAM at runtime  Lets capture this with an example      class A           int x 0      public static void main String    args                 int y 0      y y 1      x x 1                     Let us compile and run this code   step 1  javac A class    class file is generated which is byte code     step 2  java A     class file is converted into executable code   During the step 2 The main method and the static elements are loaded into the RAM for execution  In the above scenario  No issue until the line y y 1  But whenever x x 1 is executed  the run time error will be thrown as the JVM does not know what the x is which is declared outside the main method non-static    So If by some means the content of  class file is available in the memory for CPU to execute  there is no more issue    This is done through creating the Object and the keyword NEW does this Job    The concept of reserving memory in the RAM for the contents of hard disk  here  class file  at runtime is called Instance    The Object is also called the instance of the class

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Objects  which are also called instances  are self-contained elements of a program with related features and data  For the most part  you use the class merely to create instances and then work with those instances         -Definition taken from the book  Sams Teach Yourself Java in 21 days     Say you have 2 Classes  public class MainClass and public class Class 2 and you want to make an instance of Class 2 in MainClass    This is a very simple and basic way to do it   public MainClass                this is the constructor of MainClass              Class 2 nameyouwant   new Class 2         I hope this helps

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The main differnece is when you say ClassName obj   null  you are just creating an object for that class  It s not an instance of that class   This statement will just allot memory for the static meber variables  not for the normal member variables   But when you say ClassName obj   new ClassName    you are creating an instance of the class  This staement will allot memory all member variables

[java] What exactly is an instance in Java?

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