Convert List<String> to List<Integer> directly

No, you need to loop over the array

for(String s : strList) intList.add(Integer.valueOf(s));

Using Java8:

stringList.stream().map(Integer::parseInt).collect(Collectors.toList());

Using lambda:

strList.stream().map(org.apache.commons.lang3.math.NumberUtils::toInt).collect(Collectors.toList());