What is the functional difference between an instance and an object?

Question

I know that in OOP, instance=object. So if we have class like this:

public class something
{
    public static void main(String args[])
    {
    }
}

Would this line in main method create new object instance?

something instance=new something();

My second question is similar: if we create Thread object - Thread t1=new Thread(); does it actually mean we have created an instance of class Thread, from which we can statically call methods? (e.g sleep()).

+1 for asking a simple question, in a clear and concise way. You'be surprised how many people on here ask simple questions in a mind blowingly complicated way. — christopher, Aug 07 '13 at 14:44
I'm always extremely mindful of that (half of my family are't English natives, and have you ever tried telling a load of Arabs they're wrong? It's scary stuff). However, there are people on here who **are** english natives and still try to make themselves sound more intelligent, or over complicate the question with things like an `XY Problem`. I figured, while I see a good example of a simple question, might as well commend OP! — christopher, Aug 07 '13 at 15:32

score 12 · Accepted Answer · edited May 23 '17 at 12:32

if we create Thread object - Thread t1=new Thread(); does it actually mean we have created instance of class Thread, from which we can staticly call methods? (e.g sleep()).

When you call a static method, you do not call it from the object. That's why it's static. There is no need for an instance to execute a static method.

Example

Thread t1 = new Thread();
t1.checkAccess();  // <--- This is an instance method.

Thread.activeCount(); // <-- This is a static method.

When you see the new keyword, it means that a new object is being created. In this case, it was an instance of class Thread, as you quite rightly said.

How do you tell them apart?

Well, it's simple. If it's an instance method, it will be called from the context of an object.

String str = new String("hello");
str = str.replaceAll("o", "");

As you can see, you have to create an instance to use an instance method.

With a static method, it's even easier. They will be called with nothing but the name of the class.

String.copyValueOf(new char[] {'a', 'b', 'c'});

There is no need to create a new String instance. Simply use the name of the class.

NOTE: As pointed out in the comments, a static method can be called from an instance object, but this isn't common practice. If you're ever unsure, documentation is your best friend. Or you can simply test by trying to call the same method from a static context.

When to use a static method instead of an instance method ?

Well this is answered, very well, here: Java: when to use static methods and I see no sense in repeating it :)

It's probably worth noting that although you don't need an instance to execute a static method you can do it. It is not a syntax error to write t1.activeCount() — Mike, Aug 07 '13 at 14:43
No it's not, but I'm pretty sure most IDE's will complain that you're calling a static method from an instance object, so most of the time a static method will be (or at least should be) called from the context of the class. — christopher, Aug 07 '13 at 14:45
You are correct, the IDE's generally issue a warning. I just think it would make a nice addition to your answer as it's a confusing language feature for new programmers. — Mike, Aug 07 '13 at 14:46

score 2 · Answer 2 · answered Aug 07 '13 at 14:37

An object is created in both cases you cite:

something instance=new something();
Thread t1=new Thread();

However, in the second case, although a Thread object is created, a thread is not started until you start it.

does it actually mean we have created instance of class Thread, from which we can staticly call methods? (e.g sleep()).

You don't need an object to call static methods. A static method applies to the entire class, rather than a specific instance. You can call Thread.sleep() like this:

Thread.sleep( 500 );

score 1 · Answer 3 · answered Aug 07 '13 at 14:39

To your first question, yes, that would create an instance of something in the variable instance (not a great variable name, by the way).

To your second question, yes, that code would create an instance of Thread. However, you wouldn't call static methods with it (like t1.sleep()). Static methods are called with the class name, e.g., Thread.sleep. You don't have to create an instance for those.

score 0 · Answer 4 · answered Nov 20 '13 at 04:47

instance is nothing but a dynamic memory allocation.where in java 'new' operator is used to create an instance but object is the dynamic memory allocation for the members of the class or you can say object is the instance of the class. for example malloc(),calloc() are dynamic memory allocating function so we can say they are creating instance but we cant say that they are creating object.

What is the functional difference between an instance and an object?

4 Answers4