public String nonNullValue(String key) {
    String value = value(key);
    if (value == null) {
      throw new IllegalArgumentException("Missing property: " + key);
    }
    return value;
  }
@Test
  @UseDataProvider("beforeAndAfterBlanks")
  public void nonNullValue(String blankBefore, String blankAfter) {
    Properties p = new Properties();
    p.setProperty("foo", blankBefore + "bar" + blankAfter);
    Props props = new Props(p);

    assertThat(props.nonNullValue("foo")).isEqualTo("bar");
  }