MSC61-JG. Prefer using Iterators over Enumerations

According to the Java API [API 2011], Interface Enumeration<E> documentation,

An object that implements the Enumeration interface generates a series of elements, one at a time. Successive calls to the nextElement method return successive elements of the series.

As an example, an Enumeration is used in the following code to display the contents of a Vector.

for (Enumeration e = vector.elements(); e.hasMoreElements();) {
  System.out.println(e.nextElement());
}

Unfortunately, a Vector and an Enumeration may not always work well together. In fact, the Java API [API 2011] recommends, "New implementations should consider using Iterator in preference to Enumeration.". Consequently, iterable collections should prefer the use of iterators over enumerations.

Noncompliant Code Example

This noncompliant code example implements a BankOperations class with a removeAccounts() method used to terminate all the accounts of a particular account holder, as identified by the name. Names can be repeated in the vector if a person has more than one account. The remove() method attempts to iterate through all the vector entries comparing each entry with the name "Harry."

class BankOperations {
  private static void removeAccounts(Vector v, String name) {
    Enumeration e = v.elements();
		 
    while (e.hasMoreElements()) {
      String s = (String) e.nextElement();
      if (s.equals(name)) {
        v.remove("Harry"); // Second Harry is not removed
      }
    }

    // Display current account holders
    System.out.println("The names are:");
    e = v.elements();
    while (e.hasMoreElements()) {
      System.out.println(e.nextElement());  // Prints Dick, Harry, Tom	  
    }
  }
	 
  public static void main(String args[]) { 
    // List contains a sorted array of account holder names
    // Repeats are admissible 
    List list = new ArrayList(Arrays.asList(
      new String[] {"Dick", "Harry", "Harry", "Tom"}));
    Vector v = new Vector(list);
    removeAccount(v, "Harry"); 
  }
}

Upon encountering the first "Harry," it successfully removes the entry, and the size of the vector diminishes to three. However, the index of the Enumeration does not decrease by 1, causing the program to use "Tom" for the next (now final) comparison. As a result, the second "Harry" remains in the vector unscathed, having shifted to the second position in the vector.

Compliant Solution

According to the Java API [API 2011], Interface Iterator<E> documentation,

Iterator takes the place of Enumeration in the Java collections framework. Iterators differ from enumerations in two ways:

• Iterators allow the caller to remove elements from the underlying collection during the iteration with well-defined semantics.
• Method names have been improved.

This compliant solution remedies the problem described in the noncompliant code example and demonstrates the advantages of using an Iterator over an Enumeration.

class BankOperations {
  private static void removeAccounts(Vector v, String name) {
    Iterator i = v.iterator();
	 
    while (i.hasNext()) {
      String s = (String) i.next();
      if (s.equals(name)) {
        i.remove(); // Correctly removes all instances of the name Harry
      }
    }

    // Display current account holders
    System.out.println("The names are:");
    i = v.iterator();
    while (i.hasNext()) {
      System.out.println(i.next()); // Prints Dick, Tom only	 
    }
  }
	 
  public static void main(String args[]) {
    List list = new ArrayList(Arrays.asList(
      new String[] {"Dick", "Harry", "Harry", "Tom"}));
    Vector v = new Vector(list);
    remove(v, "Harry"); 
  }
}

Applicability

Using Enumeration when performing remove operations on a vector may cause unexpected program behavior.

The Coverity Prevent Version 5.0 ITERATOR checker can detect the instance where next() or previous() on an iterator is called that may not have a next or previous element.

Bibliography

[API 2011] Interfaces Enumeration<E> and Iterator<E>
[Daconta 2003] Item 21: Use Iteration over Enumeration