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for (Enumeration e = v.elements() ; e.hasMoreElements() ;) { System.out.println(e.nextElement()); } |
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Unfortunately, a {{Vector}} and an {{Enumeration}} may not always work well together, as is demonstrated in the noncompliant code example. In fact, the Java API \[[API 06|AA. Java References#API 06]\] recommends, "New implementations should consider using {{Iterator}} in preference to {{Enumeration}}." |
Noncompliant Code Example
This noncompliant code 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 one account. The remove() method attempts to iterate through all the vector entries comparing each entry with the name "Harry".
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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");
}
}
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Compliant Solution
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Iteratortakes the place ofEnumerationin 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.
The This compliant solution remedies the aforementioned problem described in the noncompliant code example and confirms the advantages of using an Iterator over an Enumeration.
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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");
}
}
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