public void startsWith(@Nullable String string) {
    checkNotNull(string);
    if (actual == null) {
      failWithActual("expected a string that starts with", string);
    } else if (!actual.startsWith(string)) {
      failWithActual("expected to start with", string);
    }
  }
@Test
  public void stringStartsWith() {
    assertThat("abc").startsWith("ab");
  }