### Question:
ELConstantsFunctions { public static String concat(String s1, String s2) { StringBuilder sb = new StringBuilder(); if (s1 != null) { sb.append(s1); } if (s2 != null) { sb.append(s2); } return sb.toString(); }  static Object firstNotNull(Object o1, Object o2); static String concat(String s1, String s2); static String replaceAll(String src, String regex, String replacement); static String appendAll(String src, String append, String delimeter); static String trim(String input); static String timestamp(); static String urlEncode(String input); static String toJsonStr(Map<String, String> map); static String toPropertiesStr(Map<String, String> map); static String toConfigurationStr(Map<String, String> map); static final long KB; static final long MB; static final long GB; static final long TB; static final long PB; static final int SUBMIT_MINUTES; static final int SUBMIT_HOURS; static final int SUBMIT_DAYS; }### Answer:
@Test public void testConcat() { assertEquals("a", ELConstantsFunctions.concat("a", null)); assertEquals("b", ELConstantsFunctions.concat(null, "b")); assertEquals("ab", ELConstantsFunctions.concat("a", "b")); assertEquals("", ELConstantsFunctions.concat(null, null)); }