Concatenates the content of two enumerations into one.
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import java.util.ArrayList;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.NoSuchElementException;
/**
* @since 4.37
* @author Jaroslav Tulach
*/
public class Utils {
/**
* Concatenates the content of two enumerations into one.
* Until the
* end of <code>en1</code> is reached its elements are being served.
* As soon as the <code>en1</code> has no more elements, the content
* of <code>en2</code> is being returned.
*
* @param en1 first enumeration
* @param en2 second enumeration
* @return enumeration
*/
public static <T> Enumeration<T> concat(Enumeration<? extends T> en1, Enumeration<? extends T> en2) {
ArrayList<Enumeration<? extends T>> two = new ArrayList<Enumeration<? extends T>>();
two.add(en1);
two.add(en2);
return new SeqEn<T>(Collections.enumeration(two));
}
}
class SeqEn<T> extends Object implements Enumeration<T> {
/** enumeration of Enumerations */
private Enumeration<? extends Enumeration<? extends T>> en;
/** current enumeration */
private Enumeration<? extends T> current;
/** is {@link #current} up-to-date and has more elements?
* The combination <CODE>current == null</CODE> and
* <CODE>checked == true means there are no more elements
* in this enumeration.
*/
private boolean checked = false;
/** Constructs new enumeration from already existing. The elements
* of <CODE>en</CODE> should be also enumerations. The resulting
* enumeration contains elements of such enumerations.
*
* @param en enumeration of Enumerations that should be sequenced
*/
public SeqEn(Enumeration<? extends Enumeration <? extends T>> en) {
this.en = en;
}
/** Ensures that current enumeration is set. If there aren't more
* elements in the Enumerations, sets the field <CODE>current</CODE> to null.
*/
private void ensureCurrent() {
while ((current == null) || !current.hasMoreElements()) {
if (en.hasMoreElements()) {
current = en.nextElement();
} else {
// no next valid enumeration
current = null;
return;
}
}
}
/** @return true if we have more elements */
public boolean hasMoreElements() {
if (!checked) {
ensureCurrent();
checked = true;
}
return current != null;
}
/** @return next element
* @exception NoSuchElementException if there is no next element
*/
public T nextElement() {
if (!checked) {
ensureCurrent();
}
if (current != null) {
checked = false;
return current.nextElement();
} else {
checked = true;
throw new java.util.NoSuchElementException();
}
}
}
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