How to create an instance of a generic class in Java?

Question

I have this big problem on my code that I can't create an instance of my generic class :( My goal was a server, that can handle different kinds of Client-Sockets.

public class Server<T extends AClientSocket> implements Runnable{
//... some other code
private void newClient() throws Exception
{
    System.out.println("[SERVER] Auf neuen Client warten...");
    Socket clientSocket = serverSocket.accept();
    clients.add(new T(clientSocket)); //Compile Error :(
    new Thread((clients.get(clients.size()-1))).start();
}
//... some more code
public static void main(String[] args) throws Exception
{
    Server<ClientSocketHTTP> server = new Server<ClientSocketHTTP>(8000);
    //... different code
}
}

"AClientSocket" is an abstract class with a defined constructor.

The Solution:

//... some code
private final Function<Socket, T> clientCreator;
public Server(int port, Function<Socket, T> clientCreator) throws Exception
{
    PORT = port;
    serverSocket = new ServerSocket(PORT);
    sockets = new ArrayList<T>();
    this.clientCreator = clientCreator;
}
//... some code
private void newClient() throws Exception
{
    System.out.println("[SERVER] Auf neuen Client warten...");
    Socket clientSocket = serverSocket.accept();
    System.out.println("[SERVER] Client gefunden...");
    sockets.add(clientCreator.apply(clientSocket));
    new Thread((sockets.get(sockets.size()-1))).start();
    System.out.println("[SERVER] Client hinzugefügt...");
}
//... some code
public static void main(String[] args) throws Exception
{
    Server<ClientSocketHTTP> server = new Server<ClientSocketHTTP>(8000, clientSocket -> {
        try {
            return new ClientSocketHTTP(clientSocket);
        } catch (Exception e1) {
            System.exit(0);
            return null;
        }
    });
//... code

chrylis -cautiouslyoptimistic- · Accepted Answer · 2020-12-29 15:06:11Z

6

You don't, and newing up instances like this is the source of all sorts of design headaches. Instead, pass a Function<Socket, T> to the class, and have newClient() call clientCreator.apply(clientSocket). If the implementation is as simple as creating a new instance, you can just pass ClientSocketHttp::new.

edited Dec 29, 2020 at 15:06

answered Dec 26, 2020 at 19:55

chrylis -cautiouslyoptimistic-

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11 Comments

Louis Wasserman Over a year ago

Given the OP's code, it looks like they want a Function<Socket, T>, not a Supplier<T>.

Alwin Brauns Over a year ago

Thanks for the help! What do you mean with "clientCreator.apply(clientSocket)"? Could you explain me more about that? Would appreciate! :)

chrylis -cautiouslyoptimistic- Over a year ago

@AlwinBrauns If the answer solved your problem, mark it as "accepted" by clicking the green checkmark on the left and optionally clicking the up arrow.

Alwin Brauns Over a year ago

I will give it a try tomorrow and if it worked I will do that! Thanks

Alwin Brauns Over a year ago

@chrylis-cautiouslyoptimistic- private class ClientSocketCreator implements Function<Socket, T> { @Override public T apply(Socket arg0) { return null; // How??? I want (new T(arg0)) } } private void newClient() throws Exception { System.out.println("[SERVER] Auf neuen Client warten..."); Socket client = serverSocket.accept(); clientSocket.add(clientCreator.apply(client)); new Thread((clientSocket.get(clientSocket.size()-1))).start(); } Sorry for the formatting im new here xD

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