### Question:
Preconditions { public static String checkNotEmpty(String value) { if (value == null || value.length() == 0) { throw new IllegalArgumentException(); } return value; } private  Preconditions(); static String checkNotEmpty(String value); static T checkArgument(T reference, boolean expression); static void checkArgument(boolean expression); static void checkArgument(boolean expression, Object errorMessage); static void checkArgument(boolean expression,
                                     String errorMessageTemplate,
                                     Object... errorMessageArgs);  }### Answer:
@Test public void isNotEmptyString() { assertThat(checkNotEmpty(" "), is(" ") ); }
@Test public void isEmptyString() { assertThrows(IllegalArgumentException.class, () -> checkNotEmpty(null)); }