@Test
  public void testEmptyResultIsReadonly() {
    Result emptyResult = Result.EMPTY_RESULT;
    Result otherResult = new Result();

    try {
      emptyResult.copyFrom(otherResult);
      fail("UnsupportedOperationException should have been thrown!");
    } catch (UnsupportedOperationException ex) {
      LOG.debug("As expected: " + ex.getMessage());
    }
    try {
      emptyResult.setExists(true);
      fail("UnsupportedOperationException should have been thrown!");
    } catch (UnsupportedOperationException ex) {
      LOG.debug("As expected: " + ex.getMessage());
    }
  }
Verifies that one can't modify instance of EMPTY_RESULT.
hbase-server/src/test/java/org/apache/hadoop/hbase/client/TestResult.java