content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to you under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.calcite.util;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  import static org.apache.calcite.linq4j.Nullness.castNonNull;
  
  /**
   * Filtered iterator class: an iterator that includes only elements that are
   * instanceof a specified class.
   *
   * <p>Apologies for the dorky name.
   *
   * @see Util#cast(java.util.List, Class)
   * @see Util#cast(Iterator, Class)
   *
   * @param <E> Element type
   */
  public class Filterator<E extends Object> implements Iterator<E> {
    //~ Instance fields --------------------------------------------------------
  
    final Class<E> includeFilter;
    final Iterator<?> iterator;
    @Nullable E lookAhead;
    boolean ready;
  
    //~ Constructors -----------------------------------------------------------
  
    public Filterator(Iterator<?> iterator, Class<E> includeFilter) {
      this.iterator = iterator;
      this.includeFilter = includeFilter;
    }
  
    //~ Methods ----------------------------------------------------------------
  
    @Override public boolean hasNext() {
      if (ready) {
        // Allow hasNext() to be called repeatedly.
        return true;
      }
  
      // look ahead to see if there are any additional elements
      try {
        lookAhead = next();
        ready = true;
        return true;
      } catch (NoSuchElementException e) {
        ready = false;
        return false;
      }
    }
  
    @Override public E next() {
      if (ready) {
        E o = lookAhead;
        ready = false;
        return castNonNull(o);
      }
  
      while (iterator.hasNext()) {
        Object o = iterator.next();
        if (includeFilter.isInstance(o)) {
          return includeFilter.cast(o);
        }
      }
      throw new NoSuchElementException();
    }
  
    @Override public void remove() {
      iterator.remove();
    }
  }
  