After asking my previous working experiences, the interviewer just asked one single coding problem. No any other skills has been asked. I.e., completely depends on the coding! Coding Problem: Coding hasNext() and next() methods for a class that contains a Collection of Collections. (main collection must not be null or empty, sub collections must not be null.) Answer: I got several hints from the interviewer. I think I didn't get the offer because I didn't finish the coding by myself! Even we discussed nicely. public class CollectionOfCollections { private Iterator mainIter; private Iterator subIter; CollectionOfCollections(Collection col) { mainIter = col.iterator(); sunIter = mainIter.next().iterator(); } public boolean hasNext() { if (subIter.hasNext()) { return true; } while (mainIter.hasNext()) { subIter = mainIter.next().iterator(); if (subIter.hasNext()) { return true; } } } public Object next() throws NoSuchElementException { if (hasNext()) { return subIter.next(); } throws new NoSuchElementException(); } }