### Question:
OAuth2TokenHandler { @VisibleForTesting protected static String listToString(List<String> list) { if (list == null) return ""; return Joiner.on(' ').join(list); } OAuth2TokenHandler(OAuth2Service service); OAuth2NormalizedResponse handle(HttpServletRequest request,
      HttpServletResponse response);  }### Answer:
@Test public void listToStringEmptyIsEmpty() { assertEquals("", OAuth2TokenHandler.listToString( Collections.<String>emptyList())); }
@Test public void listToStringNullIsEmpty() { assertEquals("", OAuth2TokenHandler.listToString(null)); }
@Test public void listToStringSingleValueIsSingleValue() { assertEquals("foo", OAuth2TokenHandler.listToString( Collections.singletonList("foo"))); }
@Test public void listToStringMultipleValueIsCommaSeparatedAndOrdered() { assertEquals("foo bar", OAuth2TokenHandler.listToString( Arrays.asList("foo", "bar"))); }