### Question:
StringUtils { public static String join(String joiner, String... parts) { final StringBuilder builder = new StringBuilder(); for (int i = 0; i < parts.length; i++) { builder.append(parts[i]); if (i < parts.length - 1) { builder.append(joiner); } } return builder.toString(); }  static Integer toInteger(StringBuilder value); static String toString(StringBuilder value); static Boolean toBoolean(StringBuilder value); static String fromInteger(Integer value); static String fromLong(Long value); static String fromString(String value); static String fromBoolean(Boolean value); static String fromBigInteger(BigInteger value); static String fromBigDecimal(BigDecimal value); static BigInteger toBigInteger(String s); static BigDecimal toBigDecimal(String s); static String fromFloat(Float value); static String fromDate(Date value); static String fromDouble(Double d); static String fromByte(Byte b); static String fromByteBuffer(ByteBuffer byteBuffer); static String replace(String originalString, String partToMatch, String replacement); static String join(String joiner, String... parts); static String lowerCase(String str); static String upperCase(String str); @SuppressWarnings("checkstyle:innerassignment") static boolean isBlank(final CharSequence cs); static final Charset UTF8; }### Answer:
@Test public void testJoiner() { final String part1 = "one"; final String part2 = "two"; final String part3 = "one"; final String part4 = "four"; final String join1 = StringUtils.join("+", part1, part2, part3); assertEquals(StringUtils.join("=", join1, part4), "one+two+one=four"); }