Avoid Enumeration For Loop Control

ID

java.loop_legacy_enumeration

Severity

info

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Best Practice

Language

Java

Tags

efficiency

Description

Reports the use of java.util.Enumeration as the driver of a while or do-while loop — the classic while (e.hasMoreElements()) { e.nextElement(); } pattern.

Rationale

Enumeration predates the Collections framework and has been superseded in practical code by Iterator and Iterable:

  • it is not Iterable, so it cannot be used with the enhanced for-each loop;

  • it does not support element removal (Iterator.remove());

  • method names (hasMoreElements, nextElement) are verbose compared with the idiomatic hasNext/next.

In practice the Enumeration almost always comes out of an API whose signature the caller cannot change — ServletContext.getInitParameterNames(), HttpServletRequest.getHeaderNames(), ResourceBundle.getKeys(), ZipFile.entries(). Changing the type is therefore not the remediation; wrapping the enumeration in a List at the call site is, and it works for every one of those APIs.

// Bad — legacy Enumeration drives the loop
Enumeration<String> e = vector.elements();
while (e.hasMoreElements()) {
    process(e.nextElement());
}

// Good — enhanced for-each over the collection
for (String s : vector) {
    process(s);
}

// Good — Iterator when removal is needed
Iterator<String> it = vector.iterator();
while (it.hasNext()) {
    if (isStale(it.next())) {
        it.remove();
    }
}

Remediation

Wrap the enumeration with Collections.list(…​) and iterate the resulting List with for-each — this works whoever owns the API:

for (String name : Collections.list(request.getHeaderNames())) {
    process(name);
}

When the receiver is a collection you control, take an Iterator from it instead (Collection.iterator()), or iterate it directly with for-each.

The receiver must resolve to java.util.Enumeration for the rule to fire; a hasMoreElements() on an unresolved or unrelated type is not reported.

References

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