Question

Why does this java code produce StackOverflowError? I understand that this somehow connected with recursive generic type parameter. But I don’t understand clear the whole mechanism.

public class SomeClass<T extends SomeClass> {

    SomeClass() {
        new SomeClassKiller();
    }

    private class SomeClassKiller extends SomeClass<T> {
    }

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

Answer 1

The generic part doesn’t matter – nor does it really matter that the class is nested. Look at this mostly-equivalent pair of classes and it should be more obvious:

public class SuperClass
{
    public SuperClass()
    {
        new SubClass();
    }
}

public class SubClass extends SuperClass
{
    public SubClass()
    {
        super();
    }
}

So the subclass constructor calls the superclass constructor – which then creates a new subclass, which calls into the superclass constructor, which creates a new subclass, etc… bang!