DataCursor


1   package jimm.datavision.source;
2   import java.util.List  ;
3   
4   /**
5    * Represents a row of data. Provides the interface needed by
6    * <code>Report</code>, no more. When using JDBC, this is a wrapper around
7    * a <code>ResultSet</code>.
8    * <p>
9    * The only method subclasses <em>must</em> implement is
10   * <code>readRowData()</code>.
11   *
12   * @author Jim Menard, <a HREF="mailto:jimm@io.com">jimm@io.com</a>
13   */
14  public abstract class DataCursor {
15  
16  protected int currRowNumber;    // Starts at 1
17  protected List   prevRowData;
18  protected List   currRowData;
19  protected List   nextRowData;
20  protected List   lastRowData;
21  protected int lastRowNumber;
22  
23  public boolean isFirst() { return currRowNumber == 1; }
24  public boolean isLast() {
25      if (nextRowData != null)    // We already have next row cached
26      return false;
27      nextRowData = readRowData(); // Read next row
28      return nextRowData == null; // If it's null, we are at the last row
29  }
30  
31  public boolean next() {
32      if (nextRowData == null)    // If we have no cached data, read the next row
33      nextRowData = readRowData();
34  
35      if (nextRowData == null) {  // When no more data, curr row is the last row
36      lastRowData = currRowData;
37      lastRowNumber = currRowNumber;
38      }
39  
40      prevRowData = currRowData;
41      currRowData = nextRowData;
42      nextRowData = null;
43  
44      ++currRowNumber;
45      return currRowData != null;
46  }
47  
48  public boolean previous() {
49      if (currRowNumber <= 1) // Not same as isFirst()
50      return false;
51  
52      nextRowData = currRowData;
53      currRowData = prevRowData;
54      prevRowData = null;
55      --currRowNumber;
56      return true;
57  }
58  
59  public boolean last() {
60      while (lastRowData == null && next())
61      ;
62      currRowData = lastRowData;
63      currRowNumber = lastRowNumber;
64      return true;
65  }
66  
67  public int getRow() { return currRowNumber; }
68  
69  public void close() {}
70  
71  /**
72   * Returns the object in the specified column. <var>index</var> starts
73   * at 1.
74   * <p>
75   * Even when running a report, <var>currRowData</var> is not always
76   * defined. For example, when a query returns zero rows but a column field
77   * is in some header or footer, this method is called but
78   * <var>currRowData</var> is <code>null</code>. If it is, we return
79   * <code>null</code>.
80   *
81   * @return the object in the <var>index</var>'th column, or <code>null</code>
82   * if no data has yet been read
83   */
84  public Object   getObject(int index) {
85      return currRowData == null ? null : currRowData.get(index - 1);
86  }
87  
88  protected abstract List   readRowData();
89  
90  }
91
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