Why does JShell have an echo in my shell demo?

Question

I am writing a simple shell demo that highlights a problem I am having with JShell. When I pass JShell as an argument to /bin/zsh -c it works fine except for the fact that my Process echoes back whatever I send it.

Here is the code to my shell demo.

import java.io.*;

public class ShellDemo {

    private String[] cmd = new String[] {"/bin/zsh", "-c", "jshell"};
    private Process process;

    public ShellDemo() {}

    public void init() {
        try {
            ProcessBuilder pb = new ProcessBuilder(cmd);
            process = pb.start();
            read();
            listen();
        } catch (IOException ex) {
            System.err.println(ex);
        }
    }

    private void read() {
        Thread thread = new Thread(() -> {
            try (BufferedReader reader = process.inputReader()) {
                char[] buf = new char[10000];
                while (reader.ready() || process.isAlive()) {
                    int count = reader.read(buf, 0, 10000);
                    System.out.print(new String(buf, 0, count));
                }      
                int exitVal = process.waitFor();
                System.out.println("Process exited with value " + exitVal + "\n");
            } catch (IOException | InterruptedException ex) {
                System.err.println(ex);
            }
        });
        thread.start();
    }

    private void listen() {
        Thread thread = new Thread(() -> {
            try (
                BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
                BufferedWriter writer = process.outputWriter();
            ) {
                while (process.isAlive()) {
                    String line = reader.readLine();
                    writer.write(line);
                    writer.newLine();
                    writer.flush();
                }
            } catch (IOException ex) {
                System.err.println(ex);
            }
        });
        thread.start();
    }

    public static void main(String[] args) {
        ShellDemo demo = new ShellDemo();
        demo.init();
    }
}

Then I compile and run my shell demo, and you'll see the echo in the output.

% javac ShellDemo.java
% java ShellDemo      
|  Welcome to JShell -- Version 17.0.1
|  For an introduction type: /help intro

jshell> System.out.println("hello world");
System.out.println("hello world")hello world

jshell> 

See how System.out.println("hello world") gets echoed before the "hello world" message is read by the inputReader? Why does zsh or JShell echo what I send it?

Is there a way to avoid the echo? What's the best solution?