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According to the Java API \[[API 06\|AA. Java References#API 06]\], Interface {{Enumeration}} 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 has been is used below to display the contents of a Vector.

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Unfortunately, a {{Vector}} and an {{Enumeration}} may not always work well together, as willis be demonstrated in the noncompliant code example. In fact, the Java API \[[API 06|AA. Java References#API 06]\] itself recommends, "New implementations should consider using Iterator in preference to Enumeration."

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This noncompliant example implements a BankOperations class with a removeAccounts() method that is 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 1 one account. The remove() method attempts to iterate through all the vector entries comparing each entry with the name "Harry".

Upon encountering the first "Harry", it successfully removes the entry and the size of the vector diminishes to 3three. Awkwardly, the index of the Enumeration does not decrease by 1 one leading the program to use "Tom" for the next (now final) comparison. As a result, the second "Harry" continues to remain in the vector unscathed, having shifted to the second position in the vector.

Code Block

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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"); 
  }
}

Compliant Solution

Wiki Markup
According to the Java API \[[API 06\|AA. Java References#API 06]\], Interfaceinterface {{Iterator}} Documentationdocumentation:

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.

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Code Block

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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"); 
  }
}

Risk Assessment

Using Enumerations while when performing remove operations on a vector might result in may cause unexpected program behavior.

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