Java : recursive constructor call and stackoverflow error

Question

Please help to understand why does the following code

public class HeapQn1 {

    /**
     * @param args
     */
    public HeapQn1() {
        new HeapQn1();
    }

    static HeapQn1 n = new HeapQn1();

    public static void main(String[] args) {

    }

}

results in

java.lang.StackOverflowError
    at com.rg.test.interview.HeapQn1.<init>(HeapQn1.java:8)
    at com.rg.test.interview.HeapQn1.<init>(HeapQn1.java:9)
    ...

As per my understanding the memory allocation for an object happens in the heap memory and I was expecting an OutOfMemoryError as at some point the heap memory will be full because of repetitive object creation.

On research , I came across that a java constructor is considered a method and that explained the StackOverflowError , until I read the following thread.

When does the Constructor gets called in java?

which says

3. The object is fully constructed/created when the constructor returns.

From what I could gather , the constructor is a method and since the heap memory is much larger than stack memory , the recursive constructor call resulted in StackOverflowError . Is this correct ?

Since no object in the give code will get completely created , will stack frame allocation for constructor actually happen ?

--edit-- For the duplicates pointed out , I do understand what StackoverflowError is . I have mentioned in the question "On research , I came across that a java constructor is considered a method and that explained the StackOverflowError". My question is to understand if a constructor gets a stack frame allocated just like other methods as the object creation is not complete until the constructor returns. Hope this clarifies.

Answer 1

Whenever the constructor is called, its return address is pushed onto the stack . As the stack is finite and smaller than the heap memory, you are getting error like StackOverflowError rather than OutOfMemoryError .

The constructor is a method and since the heap memory is much larger than the stack memory, the recursive constructor call resulted in StackOverflowError. Is this correct ?

Yeah, your wild guess is completely correct. Cheers!

Answer 2

The constructor is a method, aka a function. Every time you call it a chunk of memory is allocated to the stack , to store the variables of the function.

Your code creates calls to the constructor function indefenitely, allocating memory to the stack until the memory finishes.

You are obtaining a StackOverflowError and not an OutOfMemoryError , because the quantity of memory dedicated to the stack is smaller than the quantity of memory dedicated to the heap.

EDIT: I have done some test using your code. I've specified a heap memory space of 8M ( -Xms8M -Xmx8M ) and a stack memory space of 100M ( -Xss100M ). The computation result is always the error StackOverflowError .

Then, this could mean that no memory is allocated to the heap in this case . As you stated in your question:

The object is fully constructed/created when the constructor returns.

Answer 3

你是对的：堆栈比堆小得多，并且不会完全创建任何对象。

Answer 4

基本上你所说的是正确的，堆栈空间在堆空间之前耗尽。

Answer 5

根据java，所有引用变量都存储在堆栈内存空间中，堆栈内存空间小于堆空间，这就是我们得到的堆栈空间溢出异常。