### Question:
Constants { public static String constructTokensString(List<String> tokens) { StringBuilder sb = new StringBuilder(); if (tokens != null && !tokens.isEmpty()) { for (String token : tokens) { sb.append(token).append(" "); } sb.deleteCharAt(sb.length() - 1); } return sb.toString(); } private  Constants(); static List<String> parseTokens(String tokens); static String constructTokensString(List<String> tokens); static Map<String, String> parseRFC2617(String header); static String constructRFC2617(Map<String, String> params); static List<String> filterHeaders(List<String> headers); static final String SCHEME; static final String CHALLENGE; static final String REALM; static final String KEY_ID; @Deprecated
    static final String HEADER_REQUEST_LINE; static final String HEADER_REQUEST_TARGET; static final String HEADER_DATE; static final List<String> DEFAULT_HEADERS; static final List<String> IGNORE_HEADERS; static final String AUTHORIZATION; static final String HEADERS; static final String SIGNATURE; static final String ALGORITHMS; static final String ALGORITHM; static final Charset CHARSET; static final Pattern RFC2617_PARAM; static final KeyId DEFAULT_KEY_IDENTIFIER; static final Collection<Algorithm> ALL_SUPPORTED_ALGORITHMS; static final Challenge PREEMPTIVE_CHALLENGE; }### Answer:
@Test public void testConstructTokensString() { assertEquals("headers should be empty if provided with a null list", "", Constants.constructTokensString(null)); assertEquals("headers should be empty if provided with an empty list", "", Constants.constructTokensString(Collections.<String>emptyList())); assertEquals("should be a single token with no leading or trailing whitespace", "date", Constants.constructTokensString(Arrays.asList("date"))); assertEquals("headers should be separated by one space with no trailing spaces", "date content-type", Constants.constructTokensString(Arrays.asList("date", "content-type"))); }