SwingJShell.java

import jdk.jshell.tool.JavaShellToolBuilder;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.io.*;
import java.nio.charset.StandardCharsets;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

public class REPL {

    public static void main(String[] args) throws Exception {
        REPL repl = new REPL();
        SwingUtilities.invokeLater(repl::openFrame);
    }

    private BlockingQueue<Integer> queue = new ArrayBlockingQueue<>(10000);
    private JTextField input ;
    private JTextArea output ;

    private InputStream inStream = new InputStream() {
        @Override
        public int read() throws IOException {
            try {
                return queue.take();
            } catch (InterruptedException e) {
                e.printStackTrace();
                return -1;
            }
        }
    };

    private OutputStream outStream = new OutputStream() {
        @Override
        public void write(byte[] b) throws IOException {
            output.append(new String(b, StandardCharsets.UTF_8));
        }
        @Override
        public void write(int b) throws IOException {
            output.append(String.valueOf((char)b));
        }
    };

    private void openFrame() {
        JFrame f = new JFrame("REPL");
        JPanel root = new JPanel(new BorderLayout());
        f.getContentPane().add(root);
        input = new JTextField();
        output = new JTextArea();
        output.setFont(Font.decode("monospaced"));
        root.add(new JScrollPane(output), BorderLayout.CENTER);
        root.add(input, BorderLayout.SOUTH);
        input.addActionListener(this::evaluateInput);
        Runnable shellRunnable = () -> {
            try {
                JavaShellToolBuilder.builder().in(inStream, null).out(new PrintStream(outStream)).run();
            } catch (Exception e) {
                e.printStackTrace();
            }
        };
        new Thread(shellRunnable).start();
        f.setSize(400, 300);
        f.setVisible(true);
    }

    private void evaluateInput(ActionEvent event) {
        input.getText().chars().forEachOrdered(queue::add);
        queue.add((int)'\n');
        input.setText("");
    }

}