Question about the Stack and the Heap

[Tim Eapen]

Hello everybody:

I have a question about the following mock exam question on page 263 of Sierra/Bates SCJP book:

    
Which is true?  (Choose all that apply.)
 
C. Some reference variable live on the stack, and some live on the heap?

Here is the answer:

    
[B]C[/B] and [B]E[/B] are correct.  When an object has a reference variable, the reference variable lives inside the object, on the heap.

I don't understand the answer. I thought that ALL objects lived on the heap, along with instance variables.

Thanks,

Tim

[Sandeep Kulkarni]

Hi Tim,

All objects lived on heap but the question is specifically about object references which can be on stack or on the heap. For eg.

class Test {
String string = null // Object Reference on heap

public void m1() {
String localString = null; // Obj Ref on stack
}
}

I hope this will clear your doubt.

[Chandra Bhatt]

See the code:

class Dog{}
 
class Person {
    Dog dog; //Line #1 ref variable dog on the heap
    Person(Dog dog) {
       this.dog =dog;
    }
 
public static void main(String... args) {
    Dog d1 = new Dog();
    Person p1 = new Person(d1);
}
}

reference variable d1 and p1 will be on the stack. But the person object
encapsulate Dog reference so therefore, the ref variable dog will be on
the heap.


Thanks,

[Louis Moloney]

Tim

what you need to remember is objects ALWAYS live on the heap.

But you can have a reference/pointer on the stack that points to an object on the heap.

In the class below i have 2 String refs (one on heap s1 and one on stack s2 ). The 2 String objects they point to are on the heap.

 class SomeClass 
{
        String s1 = new String("hello");
public static void main(String[] args) 
{
        String s2 = new String("test");
}
}

[Raghavan Muthu]

In short, can we say like this?

The Reference variables which are in method-level-scope live in Stack whereas the reference variables which are in instance or class-level-scope live in heap.

All the previous examples seems to conclude this hint.

Am i right?
[ June 03, 2007: Message edited by: Raghavan Muthu ]