Split camel cased/snake cased String
My first impression:


The implementation looks long and complicated for something that sounds simple
You've provided some example cases to verify correctness, which is a very good thing


My first reaction is to embody the example test cases in proper unit tests, and then replace the implementation with a different approach, and make the tests I broke work again.

The unit tests, straightforward from your examples:

@Test
public void testCamelCased() {
    assertEquals(Arrays.asList("I", "Some", "Camel", "Cased", "String"), split("ISomeCamelCasedString"));
}

@Test
public void testSnakeCased() {
    assertEquals(Arrays.asList("UNDERSCORED", "STRING"), split("UNDERSCORED_STRING"));
}

@Test
public void testMixed() {
    assertEquals(Arrays.asList("camel", "Cased", "and", "UNDERSCORED"), split("camelCased_and_UNDERSCORED"));
}


Next, I was thinking that this can be simplified by splitting on some clever regex. I'm no regex wiz, but there are a lot of those on Stack Overflow, and I found a suitable thread about splitting camelCase to its words, here. Adapting it to also split on underscores was relatively easy, though probably not perfect, resulting in this:

private static final String RE_CAMELCASE_OR_UNDERSCORE =
        "(?&lt;!(^|[A-Z]))(?=[A-Z])|(?&lt;!^)(?=[A-Z][a-z])|_";

public static List&lt;String&gt; split(String string) {
    List&lt;String&gt; words = new ArrayList&lt;String&gt;();
    for (String word : string.split(RE_CAMELCASE_OR_UNDERSCORE)) {
        if (!word.isEmpty()) {
            words.add(word);
        }
    }
    return words;
}


This is probably not perfect, but a lot shorter than the original, and easier to understand how it works. If somebody can figure out how to get RE_CAMELCASE_OR_UNDERSCORE so that it doesn't produce empty elements, then the method can be shortened to simply:

return Arrays.asList(string.split(RE_CAMELCASE_OR_UNDERSCORE));


PS: this_is_usually_called_snake_cased, not "underscored".