Concatenates the content of two enumerations into one. : Enumeration Interface « Collections « Java Tutorial

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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>>();
      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 true if we have more elements */
  public boolean hasMoreElements() {
      if (!checked) {
          checked = true;

      return current != null;

  /** @return next element
  * @exception NoSuchElementException if there is no next element
  public T nextElement() {
      if (!checked) {

      if (current != null) {
          checked = false;

          return current.nextElement();
      } else {
          checked = true;
          throw new java.util.NoSuchElementException();

9.35.Enumeration Interface
9.35.1.The Enumeration Interface
9.35.2.If you prefer a for-loop
9.35.3.Where do we get the enumeration from?
9.35.4.The SequenceInputStream Class
9.35.5.Concatenates the content of two enumerations into one.
9.35.6.Filters enumeration to contain each of the provided elements just once.
9.35.7.Filters some elements out from the input enumeration.
9.35.8.For each element of the input enumeration asks the Processor to provide a replacement
9.35.9.Removes all nulls from the input enumeration.
9.35.10.Returns an enumeration that iterates over provided array.
9.35.11.Support for breadth-first enumerating.
9.35.12.Serializable Enumeration
9.35.13.An enumeration that iterates over an array.
9.35.14.Creating Custom Enumerations