Create a typesafe filter of an unchecked enumeration. : Generic Collections « Generics « Java Tutorial

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import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

 * @since 4.37
 * @author Jaroslav Tulach

public class Utils {
   * Create a typesafe filter of an unchecked enumeration.
   * @param rawEnum an unchecked enumeration
   * @param type the desired enumeration type
   * @param strict if false, elements which are not null but not assignable to the requested type are omitted;
   *               if true, {@link ClassCastException} may be thrown from an enumeration operation
   * @return an enumeration guaranteed to contain only objects of the requested type (or null)
  public static <E> Enumeration<E> checkedEnumerationByFilter(Enumeration rawEnum, final Class<E> type, final boolean strict) {
      Enumeration<Object> _rawEnum = rawEnum;
      return Enumerations.<Object,E>filter(_rawEnum, new Enumerations.Processor<Object,E>() {
          public E process(Object o, Collection<Object> ignore) {
              if (o == null) {
                  return null;
              } else {
                  try {
                      return type.cast(o);
                  } catch (ClassCastException x) {
                      if (strict) {
                          throw x;
                      } else {
                          return null;


abstract class CheckedIterator<E> implements Iterator<E> {

  static final Object WAITING = new Object();

  private final Iterator it;

  private Object next = WAITING;

  public CheckedIterator(Iterator it) { = it;

  protected abstract boolean accept(Object o);

  public boolean hasNext() {
    if (next != WAITING) {
      return true;
    while (it.hasNext()) {
      next =;
      if (accept(next)) {
        return true;
    next = WAITING;
    return false;

  public E next() {
    if (next == WAITING && !hasNext()) {
      throw new NoSuchElementException();
    assert next != WAITING;
    // type-checking is done by accept()
    E x = (E) next;
    next = WAITING;
    return x;

  public void remove() {


12.2.Generic Collections
12.2.1.Generics and Collections: ArrayList
12.2.2.Arrays: Storing class objects in Array as data items
12.2.3.Using Generic Comparable interface
12.2.4.A generic first-in, first-out bounded collection of objects
12.2.5.A list declared to hold objects of a type T can also hold objects that extend from T.
12.2.6.Utilities for generic ArrayList
12.2.7.Your own tree with generic user object
12.2.8.Generic to list
12.2.9.Create a typesafe copy of a raw list.
12.2.10.Create a typesafe copy of a raw map.
12.2.11.Create a typesafe filter of an unchecked iterator.
12.2.12.Create a typesafe view over an underlying raw set.
12.2.13.Create a typesafe view over an underlying raw map.
12.2.14.Create a typesafe filter of an unchecked enumeration.