### Question:
Strings { public static String subStringUntilFirst(String str, String s) { if (isNullOrEmpty(str) || isNullOrEmpty(s)) { return str; } final int pos = str.indexOf(s); if (pos < 0) { return str; } else { return str.substring(0, pos); } } 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 testSubstringUntilFirst() { assertEquals(subStringUntilFirst("hello", "el"), "h"); assertEquals(subStringUntilFirst("hellohello", "el"), "h"); assertEquals(subStringUntilFirst("hello", "hello"), ""); assertEquals(subStringUntilFirst("hello", ""), "hello"); assertEquals(subStringUntilFirst("hello", null), "hello"); assertEquals(subStringUntilFirst("", "el"), ""); assertEquals(subStringUntilFirst(null, "el"), null); }