Create a typesafe view over an underlying raw set. : Generic Collections « Generics « Java Tutorial

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import java.io.Serializable;
import java.util.AbstractSet;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * @since 4.37
 * @author Jaroslav Tulach
 */

public class Utils {
  /**
   * Create a typesafe view over an underlying raw set.
   * Mutations affect the underlying set (this is not a copy).
   * {@link Set#clear} will make the view empty but may not clear the underlying set.
   * You may add elements only of the requested type.
   * {@link Set#contains} also performs a type check and will throw {@link ClassCastException}
   * for an illegal argument.
   * The view is serializable if the underlying set is.
   * @param rawSet an unchecked set
   * @param type the desired element type
   * @param strict if false, elements in the underlying set which are not null and which are not assignable
   *               to the requested type are omitted from the view;
   *               if true, a {@link ClassCastException} may arise during some set operation
   * @return a view over the raw set guaranteed to match the specified type
   */
  public static <E> Set<E> checkedSetByFilter(Set rawSet, Class<E> type, boolean strict) {
      return new CheckedSet<E>(rawSet, type, strict);
  }
  private static final class CheckedSet<E> extends AbstractSet<E> implements Serializable {

      private static final long serialVersionUID = 1L;

      private final Set rawSet;
      private final Class<E> type;
      private final boolean strict;

      public CheckedSet(Set rawSet, Class<E> type, boolean strict) {
          this.rawSet = rawSet;
          this.type = type;
          this.strict = strict;
      }

      private boolean acceptEntry(Object o) {
          if (o == null) {
              return true;
          } else if (type.isInstance(o)) {
              return true;
          } else if (strict) {
              throw new ClassCastException(o + " was not a " + type.getName()); // NOI18N
          } else {
              return false;
          }
      }

      @Override
      public Iterator<E> iterator() {
          return new CheckedIterator<E>(rawSet.iterator()) {
              @Override
              protected boolean accept(Object o) {
                  return acceptEntry(o);
              }
          };
      }

      @Override
      public int size() {
          int c = 0;
          Iterator it = rawSet.iterator();
          while (it.hasNext()) {
              if (acceptEntry(it.next())) {
                  c++;
              }
          }
          return c;
      }

      @Override
      @SuppressWarnings("unchecked") // complains about usage of raw set
      public boolean add(E o) {
          return rawSet.add(type.cast(o));
      }

      @Override
      public boolean contains(Object o) {
          return rawSet.contains(type.cast(o));
      }

  }

}

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) {
    this.it = it;
  }

  protected abstract boolean accept(Object o);

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

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

  public void remove() {
    it.remove();
  }

}