ArrayList<String> str = new ArrayList<>();
if (str.size()>0)

List<String> someStrings = new ArrayList<String>();
...
if (someStrings.size() > 0) {
    System.out.println("First element of list is: " + someStrings.get(0));
} else {
    System.out.println("List is empty");
}

List<String> str;

List<String> str = new Arraylist<String>();

str.isEmpty()

str.add("hello")

str.get(0)