Why does this code sometimes throw a NullPointerException?

Question

Consider the following Java source:

if( agents != null ) {
  for( Iterator iter = agents.keySet().iterator(); iter.hasNext(); ) {
    // Code that uses iter.         


        

Answer 1

Shouldn't that be a while loop?

if (agents != null) {
    Iterator iter = agents.keyset().iterator();
    while (iter.hasNext()) {
        //some stuffs here
    }
}

or a for each?

if (agents != null) {
    //Assuming the key is a String
    for (String key : agents.keyset()) {
        //some stuffs here
    }
}