Question

I am new to programming, and I’m trying to teach myself what a StackOverflow is caused by. I played around with loops and caused the error, but a code for a forkbomb I tested doesn’t cause the error. Why is this?

public class ForkBomb
{
  public static void main(String[] args) throws java.io.IOException 
  {
     while(true) 
    {
        Runtime.getRuntime().exec(new String[]{"java", "-cp", System.getProperty("java.class.path"), "ForkBomb"});
    }
  }
}

This doesn’t cause a stack overflow. Why? Yet this causes one:

public class WhileBomb
{
   public WhileBomb()
   {
      while (true)
      {
      new WhileBomb();
      }
   }

   public static void main(String[] args) throws java.io.IOException
   {
     WhileBomb goodBye = new WhileBomb();
   }
}

Answer 1

Each thread gets its own stack. When you create a new process you also get a new thread and a new stack. A stack overflow error occurs when a single stack is filled, but you are creating many different stacks, none of which ever get full.

In the second example you only have one thread and you have a recursive call. Each time you call a method some information is temporarily stored on the stack until the method returns. Because your method never returns the stack will be consumed until you get a stack overflow exception.

You don’t even need the while loop. This would also give a stack overflow exception:

public WhileBomb()
{
   new WhileBomb();
}