As I understand , in Java, Stack memory holds primitives and method invocations and heap memory is used to store objects .

Suppose I have a class

class A {
       int a ;
       String b;
       //getters and setters
}

1. where will the primitive 'a' in class A be stored ?

2. Why does heap memory exist at all ? Why cant we store everything in stack ?

3. When the object gets garbage collected , is the stack associated with the objected destroyed ?