Use System.in for your input and System.out for the output like in this example:

• https://repl.it/@KarimBonnaa/Java-Calculator-App

You can't use the swing GUI classes: JTextField or JTextArea in headless java repl.it.

As Matt mentioned you can use the special https://repl.it/languages/java_swing

This will at least compile but misses the file of course:

import java.io.File;
import java.util.Scanner;

import javax.swing.JFrame;
import javax.swing.JTextArea;
import javax.swing.JTextField;

public class Main extends JFrame {

    // Typing Area:
    private JTextField txtEnter = new JTextField();

    // Chat Area:
    private JTextArea txtChat = new JTextArea();

    public Main(String name) {
        // Frame Attributes:
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setSize(600, 600);
        this.setVisible(true);
        this.setResizable(false);
        this.setLayout(null);
        this.setTitle("name");

        // txtChat Attributes:
        txtChat.setLocation(15, 5);
        txtChat.setSize(560, 510);

        this.add(txtChat);

        try {
            File f = new File(name = ".txt");
            Scanner scan = new Scanner(f);
            txtChat.append("File name: " + f.getName() + "\n");
            txtChat.append("File Size in Bytes: " + f.length() + " bytes\n");
            txtChat.append("\nFile Contents:\n\n");

            while (scan.hasNext()) {
                txtChat.append(scan.nextLine() + "\n");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        // creating instance of JFrame
        Main f = new Main("test");

        f.setSize(400, 500);
        f.setLayout(null);
        // make the frame visible
        f.setVisible(true);
    }
}