In Java, what’s the difference between an object and a class?

 

This kind of question is a test of your knowledge of the terminology associated with object oriented programming. You should note that this question could just as well be asked in the context of a C++ programmer interview, or any programming position that requires object oriented programming for that matter.

The terms ‘class’ and ‘object’ are definitely related to one another, but each term holds its own distinct meaning. Let’s start out by explaining what the term “class” means in the context of OOP. The term ‘class’ refers to the actual written piece of code which is used to define the behavior of any given class. So, a class is a static piece of code that consists of attributes which don’t change during the execution of a program – like the method definitions within a class.

An object is an instance of a class

The term ‘object’, however, refers to an actual instance of a class. Every object must belong to a class. Objects are created and eventually destroyed – so they only live in the program for a limited time. While objects are ‘living’ their properties may also be changed signficantly.

An example will help clarify what we’ve said. Suppose we have a class called ‘Animal’. All Animals have bodies and brains – and these could be the attributes of our fictional Animal class. We can also add some methods that would be common to all Animals – like “movement”, because all animals can move (maybe you can think of a better example for methods, but hopefully you get the point). So, the idea you really want to enforce in your own mind is that this very general ‘template’ of an Animal does not change – it’s simply just some lines of code that define the Animal class.


An instance of the Animal class would be a specific animal – like a lion, a cat, or a zebra. These instances of the Animal class would be called objects. Whereas the Animal class is a general concept, the instances of that class – the lions, cats, etc – take that general concept and create a real instance of it.

That is why programmers define constructors for their classes – so that when someone wants to create an object of the class, he/she can just pass in the actual properties that he/she wants the object to have – like what kind of animal it is, the name, the weight, etc. So, you can think of a constructor as something that brings the class to life – which is why it is called a constructor, because it constructs a specific instance of a class.

Objects have a lifespan but classes do not

And, as our Animal example clearly shows, every object has a lifespan associated with it – a cat or zebra can not live forever. And, the properties of those objects can change as well while they ‘live'; if we have a ‘size’ variable defined in the class that would of course change as the cat object grows bigger.

Object versus class summary

So, we can say that whereas a class is a general concept (like an Animal), an object is a very specific embodiment of that class, with a limited lifespan (like a lion, cat, or a zebra). Another way of thinking about the difference between a class and an object is that a class provides a template for something more specific that the programmer has to define, which he/she will do when creating an object of that class.

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21 Responses to “Difference between object and class”

  1. Eddy Ekofo says:

    Thank you for that! God bless

  2. Yawn says:

    ‘Cat’ is a class but ‘a cat’ is an object.

  3. Josh says:

    Great ! I have a silly doubt to ask –

    If object Lion belongs to class Animal, will the specific characteristics of Lion (i.e. animal with fur or an animal known as the king of jungle) be defined for the object (Lion) or in the generic class template (Animal)

    b) Is it right to call the attributes that we assign to the class or object as methods (instructions) ?

    Thank you.

  4. Jain says:

    Thanks. Easy to understand

  5. bufflowbill says:

    thank you. taking a class now and that helped

  6. faizan says:

    easy to understand the concept of class and object thanks

  7. hriday dubey says:

    Class is a template for creating objects which defines its state and behavior. A class contains field and method to define the state and behavior of its object.
    for more visit here http://thatsjavainfo.com/java/java-class-object/

  8. Abdelouahab Abdenour Aliane says:

    thank you ^_^
    note that in python, object are classes!

  9. Gustav Streicher says:

    You will probably first create the class, which exists in your code, and then write the instantiation instruction code of the object which will also exist in your code. When your program is executed the instantiation instructions with the properties of the object that you have defined will be sent to the class and then an object with those properties will be created in your computers memory and removed at the point where you ‘coded’ it to be removed thereby completing its lifetime.

  10. Sudha says:

    I have a doubt.. Will u please help me for clarifying? Is class first or object? what executes first?

  11. Michael says:

    But some objects can also be classes some times, i.e a lion is an object of the animal class, but also a lion class. isn’t so?

  12. Sunil Rawat says:

    oopsall.blogspot.com

  13. Shubham says:

    Beautifully explained, thanks.

  14. Matt says:

    Woops I mean instantiated not initialized.

  15. Matt says:

    You’re right. Cat, Lion and Zebra are sub-classes of the Animal class. Only when the class is initialized does it become an object (An actual instance of a class). I would describe a class as a blueprint of an object.

  16. vamsi says:

    good one

  17. Robert says:

    Surely Cat, Lion, Zebra etc. are classes, with a common ancestor of Animal related through the OO concept of inheritance.
    Specific instances (objects) would be individual animals — e.g. that cat on the chair, who is called Fluffy, is four years old, weighs 10 kg and had fish for dinner.

  18. anonymous says:

    Good explanation and easy to understand..

  19. incognito says:

    Nicely explained with simple example, just the way I like it 😉

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