Doubt regarding StackOverflowError

[Mansukhdeep Thind]

Hi

Have a look at the code:

public class Test  
{
    Test test = new Test();
    String x="a";
    public Test()
    {
        x = "b";
    }
    public static void main(String[] args) 
    {
        System.out.println(new Test().x);  
    }
}

abstract class Abs 
{
    static
    {
        System.out.print("T");
    }
    public Abs()
    {
        System.out.print("W");
    }
}

When run, it throws an Error object of type StackOverflowError. Why? What is the recursion that is causing this to happen at run time? The eclipse stack trace points to the line 3. I always held the notion that when a class is loaded in the JVM, the static blocks load first, then instance blocks, then the constructors and finally, the methods. What is going on here?

[Joanne Neal]

Mansukhdeep Thind wrote:The eclipse stack trace points to the line 3.

When you create a new Test instance you initialise the test variable of that object with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance...

[Niraj Jha]

Here instantiation is recursive causing StackOverflowError.

[Joanne Neal]

Niraj Jha wrote:I executed the same piece of code , no error is coming.

Output:b

With lines 3 and 11 exactly the same as in the OP's code ?

[Mansukhdeep Thind]

@ Jo : Thanks a pile. Again.

@ Niraj : You may have missed out on adding line 3. Are you sure you ran the exact same code which I have pasted above?

[Niraj Jha]

@Mansukhdeep:
It is coming on the same line, earlier I forgot to change on line 3.

[Prabhakar Reddy Bokka]

Joanne Neal wrote:
When you create a new Test instance you initialise the test variable of that object with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance. This instance will initialise it's test field with another new Test instance...

Could you explain clearly... what is causing the error??

[Mansukhdeep Thind]

@ Prabhakar : When the main() thread starts its execution in its call stack inside the JVM, the following things happen (in order):

a) The statement

System.out.println(new Test().x);

invokes the constructor of the class.

b) Remember this, whenever a constructor is invoked, all the instance variables of the class are assigned their respective values(either default or through setters and getters) for this new object on the heap.

c) As a part of action in statement (b) above, since this instance variable assignment

Test test = new Test();

at line 3 is itself a part of the class's instance variables, it will again invoke the constructor of the class.

d) This second invocation of the constructor again tries to do the action mentioned in (c ) above. This is where the memory gets screwed up. This action will never complete since the instance variable assignment is calling the same constructor which initiated that assignment. And so the stack overflows its capacity.

Let me know in case you have any further doubts.

[Campbell Ritchie]

public class Foo
{
   private Foo f;

   public Foo()
   {
      f = new Foo();
   }

   public static void main(String[] args)
   {
      new Foo();
   }
}

Get a pencil, paper and eraser. Start from the new Foo(); call in line 12, and write down how many Foo objects there are.
Hint: Every new Foo() counts as 1.

[Prabhakar Reddy Bokka]

@ Mansukhdeep: Thank you !