StackOverFlow Exception help

[mazzoty Laurence]

I have this tiny piece of 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);
}
}



I really don't get it why this throws StackOverflowException.
There is someone who can explain how this code works ?

Please help me !
Thank you in advice .

[Henry Wong]

mazzoty Laurence wrote:I have this tiny little piece of 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);
}
}



I really don't get it why this throws StackOverflowException.

Basically, in the main() method, it instantiates a Test object. During this process, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated. This variable is for a Test object, which is assigned to the test variable. During the instantiation of that Test object, there is an instance variable which gets instantiated..... etc. etc. etc........ Stack overflow.

Henry

[Anayonkar Shivalkar]

Nice explanation Henry Wong!

(Initially, I thought that either you pasted the same text by mistake, or there's some serious glitch at coderanch server side )

[mazzoty Laurence]

Great answer , i spotted the way it works .


Thank you so much

[Praveen Kumar M K]

Just to add, StackOverflowError is an Error(java.lang.StackOverflowError) and not an Exception...

However, I just re-checked the same as I was typing my post here and my Eclipse IDE threw up this interesting message

Exception in thread "main" java.lang.StackOverflowError
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)

Is the message here, misleading?

[Janki Shah]

Can any one explain little more? I am still confuse about what is happening here.

[Helen Ma]

When a Test object is created in the main method, this Test object also instantiates another new Test object, which also instantiates another new Test object and so on.

It runs out of memory.

[Praveen Kumar M K]

Janki Shah wrote:Can any one explain little more? I am still confuse about what is happening here.

Sure...

1) The original post spoke about StackOverflowException which I corrected in my post as StackOverflowError since it is a Error and not a Exception.

2) However when I simulated the same by a test program, the message that I got at the console was "Exception in thread "main" - java.lang.StackOverflowError". Here I queried whether the output of console was misleading as it mentioned "Exception".

Hope this helps.

[Kondwani Chipeta]

Shouldn't it be HeapOverflowError since Objects and instance variables live on the heap ?

[Matthew Brown]

Kondwani Chipeta wrote:Shouldn't it be HeapOverflowError since Objects and instance variables live on the heap ?

What actually happens is that the space allocated for keeping track of the stack of method calls runs out before the heap runs out. You create a Test object, which runs the Test constructor, which creates another, which runs the TestConstructor, etc.

You can potentially get the same error with any recursive method, regardless of whether it creates new objects or not. Try this:

public void willOverflowTheStack() {
    System.out.println("Still going...");
    willOverflowTheStack();
}

[Matthew Brown]

Praveen Kumar M K wrote:
Exception in thread "main" java.lang.StackOverflowError
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)
at com.tools.java.praveen.misc.ArrayAndStringLengths.<init>(ArrayAndStringLengths.java:5)

Is the message here, misleading?

Yes, it's not entirely accurate. Maybe it's OK if you interpret it as "exception" (as in exceptional condition), rather than Exception.

[Praveen Kumar M K]

Matthew Brown wrote:
What actually happens is that the space allocated for keeping track of the stack of method calls runs out before the heap runs out.

Yes, its actually like a race-condition...In case the heap runs out before the stack(lets say due to heavier objects), you get an OutOfMemoryError...

Matthew Brown wrote:Yes, it's not entirely accurate. Maybe it's OK if you interpret it as "exception" (as in exceptional condition), rather than Exception.

Sure, thanks for the clarification Matt