Validators { public static IValidator<String> isNotEmpty() { return value -> value != null && !value.isBlank(); }  static IValidator<String> isNotEmpty(); static IValidator<String> isInteger(); static IValidator<String> isIntegerOrBlank(); static IValidator<String> isDouble();  }
@Test public void isNotEmptyWithNull() { assertFalse(isNotEmpty().isValid(null)); }
@Test public void isNotEmptyWithEmptyString() { assertFalse(isNotEmpty().isValid("")); }
@Test public void isNotEmptyWithOnlySpaces() { assertFalse(isNotEmpty().isValid(" ")); }
@Test public void isNotEmptyWithNonEmptyString() { assertTrue(isNotEmpty().isValid("Test")); }