Change DataStructure (or maybe just a symbol) from JLink/Java program

I've tested this code with Mathematica's default JDK (version 11.0.12, Mathematica 13.0.1).

Assume I've run an echo server on 127.0.0.1:8000.

Let's import JLink and have a variable to store our result:

Needs["JLink`"];

result = {};

Import Java WebSocket's jar file:

AddToClassPath["C:\\Java-WebSocket-1.5.2.jar"];

Now, you need to define your extended class in Java, like:

import com.wolfram.jlink.*;

import java.net.URI;
import java.util.Map;
import org.java_websocket.client.WebSocketClient;
import org.java_websocket.drafts.Draft;
import org.java_websocket.handshake.ServerHandshake;

public class ExampleClient extends WebSocketClient {

  // implement other methods
  // ...

  // send a string on opening
  @Override
  public void onOpen(ServerHandshake handshakedata) {
    send("Hello from java");
    System.out.println("opened connection");
  }

  // manipulate Wolfram variable on receiving message
  @Override
  public void onMessage(String message) {
    KernelLink link = StdLink.getLink();
    if (link != null) {
        try {
            link.evaluate("AppendTo[result,\""+message+"\"]");
        } catch (MathLinkException e) {
            System.out.println("Failed");
            link.clearError();
            link.newPacket();
        }
    }else{
        System.out.println("Null");
    }
  }
}

I couldn't find an easy way to access a defined class, so apply ExternalEvaluate to the above string (" needs to be escaped) and save the output in a variable, like:

temp = ExternalEvaluate["Java", ... ];

Now, the following code will manipulate the result variable in the Wolfram session:

client = JavaNew[temp, JavaNew["java.net.URI", "ws://127.0.0.1:8000"]]
client@connect[]

Checking our variable:

result

(* Out: {"Hello from java"} *)

For better debugging (seeing System.out.print), show the console with:

ShowJavaConsole[]

For further information read Requesting Evaluations by the Wolfram Language.