@Test
public void format() throws Exception {

  exampleData.write("jiwan@taobao.com");
  assertThat(exampleData.getDataVersion() == 1, is(true));
  assertThat(exampleData.read().equals("jiwan@taobao.com"), is(true));
  exampleData.format();
  assertThat(exampleData.getDataVersion() == 0, is(true));
  assertThat(exampleData.read().isEmpty(), is(true));
}