Question

Enumeration doesn’t throw ConcurrentModificationException , why?

See below code .

public static void main(String[] args) {

    Vector<String> v=new Vector<String>();
    v.add("Amit");
    v.add("Raj");
    v.add("Pathak");
    v.add("Sumit");
    v.add("Aron");
    v.add("Trek");

    Enumeration<String> en=v.elements();

    while(en.hasMoreElements())
    {
        String value=(String) en.nextElement();
        System.out.println(value);
        v.remove(value);

    }

}

It only prints :

Amit
Pathak
Aron

Why is this such behavior . Can we say that Enumerator is thread safe.

Edit: When working with Iterator it throws ConcurrentModificationException in single thread application.

public static void main(String[] args) {

    Vector<String> v=new Vector<String>();
    v.add("Amit");
    v.add("Raj");
    v.add("Pathak");
    v.add("Sumit");
    v.add("Aron");
    v.add("Trek");

    Iterator<String> it=v.iterator();
    while(it.hasNext())
    {
        String value=(String) it.next();
        System.out.println(value);
        v.remove(value);
    }
}

Please check.

Answer 1

Enumeration doesn’t throw ConcurrentModificationException , why?

Because there is no path in the code being invoked which throws this exception. Edit: I am referring to the implementation provided by the Vector class, not about the Enumeration interface in general.

Why is this such behavior . Can we say that Enumerator is thread safe.

It is thread-safe in a sense that the code executed is properly synchronized. However, I don’t think the result your loop yields is what you would except.

The reason for your output is that the Enumeration object maintains a counter, which is incremented after every invokation of nextElement(). This counter is not aware of your invokation of remove().