So I am looking at a past exam paper which I am studying for my exams coming up and I am really confused regarding this question, can someone give details on the state of the memory for the following:

class Q2 {
   private static int num = 0;
   private String prefix;

   public Q2 (String p)
    { prefix = p; }

   public String Msg (String str) {
      String n;
      n = num.ToString();
     return n + ” - ” + prefix + str;

Using an appropriate diagram, describe the state of memory after all of the following statements have been executed.

Q2 var1, var2;
   var1 = new Q2(“Question 2 ”);
   var2 = new Q2 (“Another view ”);

Now consider the method call.

… var1.Msg (“hello again”) …

Explain in detail the following aspects of the execution of this statement; use diagrams where appropriate.

The referencing and calling of the method.
The execution and return of the method.

Discuss the memory management issues that will arise with the following infinite loop.

while (true) {
      Var1 = new Q2 (“Part c ”);
      String s = Var1.Msg (“round and round”);
      … … …

I have been trying to draw diagrams to represent the stack and heap for this example, but I am confused due to the way the objects are created, I appreciate any help on this.


You must understand that the class Q2 has 2 parts
1) The static part is shared between al the instances of the class
2) The non static part, is owned exclusively by the instance

In this class, the static part is represented by the private static int num initialized by default to 0. Each time is executed the sentence num++; the value is incremented and is visible for each instance.

The non static part is represented by the private string prefix; .
This will mean that each instance will have his own prefix, wich is loaded during the class instantiation new Q2("Question 2") or Q2 (“Another view ”) .

After doing the

Q2 var1, var2;
var1 = new Q2(“Question 2 ”);
var2 = new Q2 (“Another view ”);

you have a unique num variable, with a value of 0, and two prefix variables (one for var1 and another for var2) with the "Question 2" and the "Another view" texts.

Each call of the var1.Msg() function, will increase the value of num by 1. Then will return a string containing the current value of the num variebla, a - sign, the prefix value for the current instance and the string passed to the Msg function.

The infinite loop has the trick that always use the same naming for the instance, but is creating a new instance each time, so the stack vill increase having as many instances as iterations done, and the num value will increase also by 1 at each Msg call.

Hope this helps

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