Fluent API with inheritance and generics

Question

I\'m writing a fluent API to configure and instantiate a series of \"message\" objects. I have a hierarchy of message types.

To be able to access method of subclasse

Answer 1

If you change the signatures like this you should neither get any warnings nor do you need any casts:

abstract class Message> {

    public T withID(String id) {
        return self();
    }

    protected abstract T self();
}

abstract class CommandMessage> extends Message {

    public T withCommand(String command) {
        // do some work ...
        return self();
    }
}

class CommandWithParamsMessage extends CommandMessage {

    public static CommandWithParamsMessage newMessage() {
        return new CommandWithParamsMessage();
    }

    public CommandWithParamsMessage withParameter(String paramName, String paramValue) {
        // do some work ...
        return this;
    }

    @Override protected CommandWithParamsMessage self() {
        return this;
    }
}