how to cast various objects from an objectInputStream

Question

I'm practicing socket communication with java, especially exchanging objects that the server and client know. if I only ever send one class type (Message), I can easily cast and rebuild it:

while (!socket.isClosed()) {
    try {
        Message message = (Message) objectInputStream.readObject();
        System.out.println(data.getValue);
    } catch (IOException | ClassNotFoundException ex) {
        disconnected(ex);
    }
}

Now I need to send different object types and I can't find the right way to implement it. someone suggests sending two messages: the first indicates the type and prepares the server for casting the second;

while (!socket.isClosed()) {
    String type = null;
    try {
        if (type == null){
            Message message = (Message) objectInputStream.readObject();
            type = message.getType();
            System.out.println(data.getValue);
        } else {
            //switch statement with possible types I can handle
            //where I can have different casting
            X message = (X) objectInputStream.readObject();
        }

    } catch (IOException | ClassNotFoundException ex) {
        disconnected(ex);
    }
}

In other cases I have seen the use of generics, but they cause quite a few problems because the type or cast is assumed as unsafe.

I would like a more versatile version that can dynamically convert objects coming from client and viceversa, something like this:

while (!socket.isClosed()) {
    try {
        Object data = objectInputStream.readObject();
        //if object type is Message cast into a Message
        //if object type is X cast into a X object
    } catch (IOException | ClassNotFoundException ex) {
        disconnected(ex);
    }
}

Maybe I should go for something completely different, do you have any suggestions?

Answer 1

Object data = objectInputStream.readObject();
//if object type is Message cast into a Message
//if object type is X cast into a X object

You're looking for instanceof. Example:

Object data = objectInputStream.readObject();
if(data instanceof Message) {
    Message m = (Message)data;
    System.out.println("message type: "+m.type);
    receivedMessage(m);
} else if(data instanceof X) {
    X x = (X)data;
    receivedX(m);
}

Since Java 14 there is also a shorthand syntax:

Object data = objectInputStream.readObject();
if(data instanceof Message m) { // <- note the name m
    System.out.println("message type: "+m.type);
    receivedMessage(m);
} else if(data instanceof X x) { // <- and here
    receivedX(x);
}

If the type is totally unknown, you can also use getClass() to get the type, which returns a Class object and you can get information about it. (Beware that null.getClass() will throw NPE)

Object data = objectInputStream.readObject();
if(data instanceof Message) {
    receivedMessage((Message)data);
} else if(data == null) {
    System.err.println("received null");
} else {
    // don't know what it is, but it's not a Message
    System.err.println("received unknown class "+data.getClass().getName());
}