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1   /**
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10   *     http://www.apache.org/licenses/LICENSE-2.0
11   *
12   * Unless required by applicable law or agreed to in writing, software
13   * distributed under the License is distributed on an "AS IS" BASIS,
14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15   * See the License for the specific language governing permissions and
16   * limitations under the License.
17   */
18  
19  package org.apache.hadoop.hbase;
20  
21  import java.io.IOException;
22  
23  import org.apache.hadoop.hbase.classification.InterfaceAudience;
24  import org.apache.hadoop.hbase.classification.InterfaceStability;
25  
26  /**
27   * An interface for iterating through a sequence of cells. Similar to Java's Iterator, but without
28   * the hasNext() or remove() methods. The hasNext() method is problematic because it may require
29   * actually loading the next object, which in turn requires storing the previous object somewhere.
30   *
31   * <p>The core data block decoder should be as fast as possible, so we push the complexity and
32   * performance expense of concurrently tracking multiple cells to layers above the CellScanner.
33   * <p>
34   * The {@link #current()} method will return a reference to a Cell implementation. This reference
35   * may or may not point to a reusable cell implementation, so users of the CellScanner should not,
36   * for example, accumulate a List of Cells. All of the references may point to the same object,
37   * which would be the latest state of the underlying Cell. In short, the Cell is mutable.
38   * <p/>
39   * Typical usage:
40   *
41   * <pre>
42   * while (scanner.advance()) {
43   *   Cell cell = scanner.current();
44   *   // do something
45   * }
46   * </pre>
47   * <p>Often used reading {@link org.apache.hadoop.hbase.Cell}s written by
48   * {@link org.apache.hadoop.hbase.io.CellOutputStream}.
49   */
50  @InterfaceAudience.Private
51  @InterfaceStability.Unstable
52  public interface CellScanner {
53    /**
54     * @return the current Cell which may be mutable
55     */
56    Cell current();
57  
58    /**
59     * Advance the scanner 1 cell.
60     * @return true if the next cell is found and {@link #current()} will return a valid Cell
61     * @throws IOException
62     */
63    boolean advance() throws IOException;
64  }