### Question:
Strings { public static String whiteSpacesToSingleSpace(String str) { if (str == null) { return null; } if (str.isEmpty()) { return str; } return MULTI_SPACE.matcher(WHITE_SPACE_EXCEPT_SPACE.matcher(str).replaceAll(" ")) .replaceAll(" "); } private  Strings(); static boolean isNullOrEmpty(String str); static boolean hasText(String s); static boolean allHasText(String... strings); static boolean allNullOrEmpty(String... strings); static String leftTrim(String s); static String rightTrim(String str); static boolean containsNone(String str, String invalidCharsStr); static boolean containsOnly(String str, String allowedChars); static String repeat(char c, int count); static String repeat(String str, int count); static String reverse(String str); static String insertFromLeft(String str, int interval, String stringToInsert); static String insertFromRight(String str, int interval, String stringToInsert); static String rightPad(String str, int size); static String rightPad(String str, int size, char padChar); static String rightPad(String str, int size, String padStr); static String leftPad(String str, int size); static String leftPad(String str, int size, char padChar); static String subStringUntilFirst(String str, String s); static String subStringUntilLast(String str, String s); static String subStringAfterFirst(String str, String s); static String subStringAfterLast(String str, String s); static String leftPad(String str, int size, String padStr); static String whiteSpacesToSingleSpace(String str); static String eliminateWhiteSpaces(String str); static String[] separateGrams(String word, int gramSize); static final String EMPTY_STRING; static final String[] EMPTY_STRING_ARRAY; }### Answer:
@Test public void testWhiteSpacesToSingleSpace() { assertEquals(whiteSpacesToSingleSpace(null), null); assertEquals(whiteSpacesToSingleSpace(""), ""); assertEquals(whiteSpacesToSingleSpace("asd"), "asd"); assertEquals(whiteSpacesToSingleSpace("a a"), "a a"); assertEquals(whiteSpacesToSingleSpace(" "), " "); assertEquals(whiteSpacesToSingleSpace("\t"), " "); assertEquals(whiteSpacesToSingleSpace("\n"), " "); assertEquals(whiteSpacesToSingleSpace("\t \n"), " "); assertEquals(whiteSpacesToSingleSpace(" \t \n\r \f"), " "); assertEquals(whiteSpacesToSingleSpace(" a\t a\r\fa"), " a a a"); }