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.bookkeeper.metadata.etcd.helpers;
  
  import static org.apache.bookkeeper.common.concurrent.FutureUtils.result;
  
  import java.util.List;
  import java.util.NoSuchElementException;
  import java.util.concurrent.CompletableFuture;
  
  /**
   * Iterator over a range of key/value pairs.
   */
  public class KeyIterator<T> {
  
      private final KeyStream<T> stream;
      private CompletableFuture<List<T>> readFuture = null;
      private boolean hasNext = true;
      private List<T> keys = null;
  
      public KeyIterator(KeyStream<T> stream) {
          this.stream = stream;
      }
  
      public synchronized boolean hasNext() throws Exception {
          if (hasNext) {
              if (null == readFuture) {
                  readFuture = stream.readNext();
              }
              keys = result(readFuture);
              if (keys.isEmpty()) {
                  hasNext = false;
              }
              return hasNext;
          } else {
              return false;
          }
      }
  
      public synchronized List<T> next() throws Exception {
          try {
              if (!hasNext()) {
                  throw new NoSuchElementException("Reach end of key stream");
              }
              return keys;
          } finally {
              // read next
              readFuture = stream.readNext();
          }
      }
  
  }
  