public static void main(String[] args) {
    String str = "test";

    List<String> methods = (List<String>) str; // java.lang.String cannot be cast to java.util.List
}

修改之后

public static void main(String[] args) {
    String str = "test 921";

    List<String> methods = Arrays.asList(str);

    System.out.println(methods);
}