### Question:
ModelUtils { static boolean isWholePositiveNumber(@Nullable String value) { return value != null && TextUtils.isDigitsOnly(value); }    }### Answer:
@Test public void wholePositiveNumberShouldPassWithLeadingZero() { assertTrue(ModelUtils.isWholePositiveNumber("000")); }
@Test public void wholePositiveNumberShouldFailIfNegative() { assertFalse(ModelUtils.isWholePositiveNumber("-1")); }
@Test public void wholePositiveNumberShouldFailIfLetters() { assertFalse(ModelUtils.isWholePositiveNumber("1a")); }
@Test public void wholePositiveNumberShouldFailNull() { assertFalse(ModelUtils.isWholePositiveNumber(null)); }
@Test public void wholePositiveNumberShouldPassIfEmpty() { assertTrue(ModelUtils.isWholePositiveNumber("")); }
@Test public void wholePositiveNumberShouldPass() { assertTrue(ModelUtils.isWholePositiveNumber("123")); }