How to extract class from Java generic class to satisfy compiler?

You can avoid unnecessary casts! See below:

public static class MessageType<T> {
    static MessageType<List<String>> STRINGS_LIST_TYPE1 = new MessageType<>(List.class);
    static MessageType<List<String>> STRINGS_LIST_TYPE2 = new MessageType<>();
    static MessageType<Double> DOUBLE_TYPE = new MessageType<>(Double.class);
    static MessageType<String> STRING_TYPE = new MessageType<>(String.class);

    List<T> messages = new ArrayList<>();
    Class<T> clazz;

    @SuppressWarnings({"rawtypes","unchecked"})
    MessageType(Class<? extends T> clazz) { this.clazz = (Class) clazz; }

    MessageType() { this.clazz = null; }

    void addMessage(T message) { messages.add(message); }
    T getMessage(int i) { return messages.get(i); }
    Class<T> getClazz() { return this.clazz; }

    public static void main(String[] args) {
        // MessageType<List<String>> STRINGS_LIST_TYPE1 = new MessageType<>(List.class);
        STRINGS_LIST_TYPE1.addMessage(new ArrayList<String>()); // no compile issues
        STRINGS_LIST_TYPE1.addMessage(new ArrayList<Integer>()); // compile error "not applicable  for the arguments (ArrayList<Integer>)"
        List<String> message1 = STRINGS_LIST_TYPE1.getMessage(0); // no warnings!
        Class<List<String>> clazz1 = STRINGS_LIST_TYPE1.getClazz(); // no warnings!

        // MessageType<List<String>> STRINGS_LIST_TYPE2 = new MessageType<>()
        STRINGS_LIST_TYPE2.addMessage(new ArrayList<String>()); // no compile issues
        STRINGS_LIST_TYPE2.addMessage(new ArrayList<Integer>()); // compile error "not applicable  for the arguments (ArrayList<Integer>)"
        List<String> message2 = STRINGS_LIST_TYPE2.getMessage(0); // no warnings!
        Class<List<String>> clazz2 = STRINGS_LIST_TYPE2.getClazz(); // no warnings! but returns null
    }
}