@Test
  public void testPreadReversedScan01() throws IOException {
    String[][] keysCases = new String[][] { { "d0", "d1", "d2", "d3" }, // all rowKeys fit in the
                                                                        // last region.
      { "a0", "a1", "a2", "a3" }, // all rowKeys fit in the first region.
      { "a0", "b1", "c2", "d3" }, // each region with a rowKey
    };

    for (int caseIndex = 0; caseIndex < keysCases.length; caseIndex++) {
      testPreadReversedScanInternal(keysCases[caseIndex]);
      TEST_UTIL.truncateTable(TABLE_NAME);
    }
  }
all rowKeys are fit in the last region.
hbase-server/src/test/java/org/apache/hadoop/hbase/client/TestPreadReversedScanner.java