iterating through Enumeration of hastable keys throws NoSuchElementException error

问题

I am trying to iterate through a list of keys from a hash table using enumeration however I keep getting a NoSuchElementException at the last key in list?

Hashtable<String, String> vars = new Hashtable<String, String>();

vars.put("POSTCODE","TU1 3ZU");
vars.put("EMAIL","job.blogs@lumesse.com");
vars.put("DOB","02 Mar 1983");

Enumeration<String> e = vars.keys();

while(e.hasMoreElements()){

System.out.println(e.nextElement());
String param = (String) e.nextElement();
}

Console output:

EMAIL
POSTCODE

Exception in thread "main" java.util.NoSuchElementException: Hashtable Enumerator
    at java.util.Hashtable$Enumerator.nextElement(Unknown Source)
    at testscripts.webdrivertest.main(webdrivertest.java:47)

回答1:

You call nextElement() twice in your loop. This call moves the enumeration pointer forward. You should modify your code like the following:

while (e.hasMoreElements()) {
    String param = e.nextElement();
    System.out.println(param);
}

回答2:

for (String key : Collections.list(e))
    System.out.println(key);

回答3:

Every time you call e.nextElement() you take the next object from the iterator. You have to check e.hasMoreElement() between each call.

Example:

while(e.hasMoreElements()){
    String param = e.nextElement();
    System.out.println(param);
}

回答4:

You are calling nextElement twice. Refactor like this:

while(e.hasMoreElements()){


String param = (String) e.nextElement();
System.out.println(param);
}

回答5:

You're calling e.nextElement() twice inside your loop when you're only guaranteed that you can call it once without an exception. Rewrite the loop like so:

while(e.hasMoreElements()){
  String param = e.nextElement();
  System.out.println(param);
}

回答6:

You're calling nextElement twice in the loop. You should call it only once, else it moves ahead twice:

while(e.hasMoreElements()){
    String s = e.nextElement();
    System.out.println(s);
}

回答7:

Each time you do e.nextElement() you skip one. So you skip two elements in each iteration of your loop.

来源：https://stackoverflow.com/questions/7160568/iterating-through-enumeration-of-hastable-keys-throws-nosuchelementexception-err

标签

java

hashtable

enumeration

key