When object created on Heap and when on Stack

[ram gaurav]

Hi

Which all things are created on Memory Heap and which all things created on Stack.

Whats the basic rule behind this.

Thanks
Gaurav

[Sanjeev Singh]

Objects will always be created on heap.There is no other place of creation for objects.Local vaiables are created on stack.

[ram gaurav]

Thanks for the answer.

So whats the basic difference beteween stack and heap.
Why we are using 2 places to initialize the objects or variables.

Why we not use only 1.

Thanks
Gaurav

[Nandini Bhaduri]

The stack is used to store the volatile environment of methods. It's allocated when the method is called and released again when the method is left. It holds the variables and methods declared in the method.

A key feature of Java is its garbage-collected heap, which takes care of freeing dynamically allocated memory that is no longer referenced. This frees the programmer from having to keep track of when to free allocated memory, thereby preventing many potential bugs and headaches.

[ram gaurav]

Thanks for your clarification.

There is one question ,that, we have:

  String str = "ram";
        String temp = new String("ram");

Can you please tell me that where str and temp will be created.
Either on Heap or Stack.

Thanks
Gaurav

[Nandini Bhaduri]

Both str and temp will be created on the stack if they are declared in a method, otherwise on the heap.

[ram gaurav]

But in Javaranch i have read that String literals are never been garbage collected , then hows that possible that they are defined on heap and not been garbage collected.

[Nandini Bhaduri]

str and temp are references to strings and not the string itself. "ram" is stored in the literal pool.

[ram gaurav]

Thanks a lot for clarification.

[Chandra shekar M]

1. String str = "ram";
is not same as
2. String temp = new String("ram");

1. will not create any object on the heap it will store this in the literal pool if it is not already present if it is already present then it will not be created again insted the refrence is reused
in this case there is one refrence and one object in the literal pool.
irrespective of where it is written in the class.

But in

2. first it always create a new object on the heap area and then it will check in the literal pool if it is present then it wont add if not a object will be added to literal pool to. so in this case we HAVE ONE REFRENCE AND TWO OBJECTS BUT THE REFRENCE WILL BE POINTING TO HEAP OBJECT.

Thanks

[Nandini Bhaduri]

In the second case two objects will be created. First one associated with the "ram" literal and then a second one which is a clone of the first. Both objects are stored on the heap but the first is immediately released again. A reference to the second one is assigned to temp.

[Bert Bates]

I would recommend that when you start to study the stack and the heap, you leave String objects out of the mix - they are a special case! Get the concepts of how the stack and heap work down very clearly first. Then, and only then, you can look at the String constant pool (which, by the way is part of the heap.)

Also, on the 5.0 exam we really tried to minimize the importance of the String constant pool - some of you might get one question that relates to this pool - but most of you probably won't get any - so I'd advise that you focus more on the topics that are tricky AND that get a lot of coverage on the exam. For example: threads, GC (for non-String objects), generics, paths, serialization, collections, inner classes.

hth,

Bert