lucene/core/src/java/org/apache/lucene/util/FilterIterator.java

/*
 * 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.lucene.util;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * An {@link Iterator} implementation that filters elements with a boolean predicate.
 *
 * @param <T> generic parameter for this iterator instance: this iterator implements {@link Iterator Iterator&lt;T&gt;}
 * @param <InnerT> generic parameter of the wrapped iterator, must be <tt>T</tt> or extend <tt>T</tt>
 * @see #predicateFunction
 * @lucene.internal
 */
public abstract class FilterIterator<T, InnerT extends T> implements Iterator<T> {
  
  private final Iterator<InnerT> iterator;
  private T next = null;
  private boolean nextIsSet = false;
  
  /** returns true, if this element should be returned by {@link #next()}. */
  protected abstract boolean predicateFunction(InnerT object);
  
  public FilterIterator(Iterator<InnerT> baseIterator) {
    this.iterator = baseIterator;
  }
  
  @Override
  public final boolean hasNext() {
    return nextIsSet || setNext();
  }
  
  @Override
  public final T next() {
    if (!hasNext()) {
      throw new NoSuchElementException();
    }
    assert nextIsSet;
    try {
      return next;
    } finally {
      nextIsSet = false;
      next = null;
    }
  }
  
  @Override
  public final void remove() {
    throw new UnsupportedOperationException();
  }
  
  private boolean setNext() {
    while (iterator.hasNext()) {
      final InnerT object = iterator.next();
      if (predicateFunction(object)) {
        next = object;
        nextIsSet = true;
        return true;
      }
    }
    return false;
  }
}