JavaFX layout containers and event handlers enable the construction of a responsive message client. The application utiilzes a BorderPane containing a TextArea for message history and a TextField for user input. Keyboard events on the input field difefrentiate between standard ENTER for regular messages and SHIFT+ENTER for echoed messages. File operations are delegated to a dedicated persistence handler.

Application Logic (ChatClientApp.java)

package app;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.control.*;
import javafx.scene.input.KeyCode;
import javafx.scene.input.KeyEvent;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import java.time.LocalTime;

public class ChatClientApp extends Application {

    private final TextArea messageDisplay = new TextArea();
    private final TextField messageInput = new TextField();
    private final DocumentStorageHandler storageHandler = new DocumentStorageHandler();

    @Override
    public void start(Stage window) {
        messageDisplay.setEditable(false);
        messageDisplay.setWrapText(true);

        VBox centerLayout = new VBox(10,
            new Label("Chat History:"),
            messageDisplay,
            new Label("Compose Message:"),
            messageInput
        );
        centerLayout.setPadding(new Insets(15));
        VBox.setVgrow(messageDisplay, javafx.scene.layout.Priority.ALWAYS);

        Button sendBtn = new Button("Send");
        Button exportBtn = new Button("Export Log");
        Button importBtn = new Button("Import Log");
        Button closeBtn = new Button("Close");

        sendBtn.setOnAction(evt -> transmitMessage(false));
        closeBtn.setOnAction(evt -> Platform.exit());
        exportBtn.setOnAction(evt -> {
            String timestamp = LocalTime.now().withNano(0).toString();
            storageHandler.writeToFile(timestamp + "\n" + messageDisplay.getText());
        });
        importBtn.setOnAction(evt -> {
            String fileContent = storageHandler.readFromFile();
            if (fileContent != null) {
                messageDisplay.clear();
                messageDisplay.setText(fileContent);
            }
        });

        messageInput.setOnKeyPressed(this::handleKeyPress);

        HBox bottomControls = new HBox(15, sendBtn, exportBtn, importBtn, closeBtn);
        bottomControls.setPadding(new Insets(10, 15, 15, 15));

        BorderPane root = new BorderPane();
        root.setCenter(centerLayout);
        root.setBottom(bottomControls);

        window.setScene(new Scene(root, 750, 500));
        window.setTitle("Message Client");
        window.show();
    }

    private void handleKeyPress(KeyEvent evt) {
        if (evt.getCode() == KeyCode.ENTER && !messageInput.getText().trim().isEmpty()) {
            boolean isShiftActive = evt.isShiftDown();
            transmitMessage(isShiftActive);
        }
    }

    private void transmitMessage(boolean isShiftHeld) {
        String text = messageInput.getText();
        if (isShiftHeld) {
            messageDisplay.appendText("[Echo] " + text + "\n");
        } else {
            messageDisplay.appendText(text + "\n");
        }
        messageInput.clear();
    }

    public static void main(String[] args) {
        launch(args);
    }
}

File I/O Handler (DocumentStorageHandler.java)

To manage file loading and saving, the DocumentStorageHandler utilizes java.nio.file.Files. It opens a FileChooser dialog to determine the target or source file, then writes the chat log with UTF-8 encoding or reads the file contenst line by line into the display area.

package app;

import javafx.stage.FileChooser;

import java.io.File;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.StandardOpenOption;
import java.util.List;

public class DocumentStorageHandler {

    public void writeToFile(String content) {
        FileChooser fileChooser = new FileChooser();
        fileChooser.setTitle("Save Chat Log");
        File targetFile = fileChooser.showSaveDialog(null);
        if (targetFile == null) return;

        try {
            Files.write(targetFile.toPath(), content.getBytes(StandardCharsets.UTF_8), StandardOpenOption.CREATE, StandardOpenOption.APPEND);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public String readFromFile() {
        FileChooser fileChooser = new FileChooser();
        fileChooser.setTitle("Open Chat Log");
        File sourceFile = fileChooser.showOpenDialog(null);
        if (sourceFile == null) return null;

        StringBuilder builder = new StringBuilder();
        try {
            List<String> lines = Files.readAllLines(sourceFile.toPath(), StandardCharsets.UTF_8);
            for (int i = 0; i < lines.size(); i++) {
                builder.append(lines.get(i));
                if (i < lines.size() - 1) {
                    builder.append("\n");
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return builder.toString();
    }
}