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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 org.apache.hadoop.classification.InterfaceAudience;
22  import org.apache.hadoop.classification.InterfaceStability;
23  import org.apache.hadoop.hbase.Cell;
24  
25  /**
26   * An interface for iterating through a sequence of cells. Similar to Java's Iterator, but without
27   * the hasNext() or remove() methods. The hasNext() method is problematic because it may require
28   * actually loading the next object, which in turn requires storing the previous object somewhere.
29   *
30   * <p>The core data block decoder should be as fast as possible, so we push the complexity and
31   * performance expense of concurrently tracking multiple cells to layers above the CellScanner.
32   * <p>
33   * The {@link #current()} method will return a reference to a Cell implementation. This reference
34   * may or may not point to a reusable cell implementation, so users of the CellScanner should not,
35   * for example, accumulate a List of Cells. All of the references may point to the same object,
36   * which would be the latest state of the underlying Cell. In short, the Cell is mutable.
37   * <p/>
38   * Typical usage:
39   *
40   * <pre>
41   * while (scanner.next()) {
42   *   Cell cell = scanner.get();
43   *   // do something
44   * }
45   * </pre>
46   * <p>Often used reading {@link org.apache.hadoop.hbase.Cell}s written by
47   * {@link org.apache.hadoop.hbase.io.CellOutputStream}.
48   */
49  @InterfaceAudience.Private
50  @InterfaceStability.Unstable
51  public interface CellScanner {
52    /**
53     * @return the current Cell which may be mutable
54     */
55    Cell current();
56  
57    /**
58     * Advance the scanner 1 cell.
59     * @return true if the next cell is found and {@link #current()} will return a valid Cell
60     */
61    boolean advance();
62  }