DotViewer.java

import regaut.FA;

import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.*;

public class DotViewer {
    private String dot;

    public DotViewer(FA fa) {
        this.dot = fa.toDot();
    }

    public void view() {
        try {
            File dotFile = File.createTempFile("finite-automaton", ".dot");
            dotFile.deleteOnExit();

            File imageFile = File.createTempFile("finite-automaton", ".jpg");
            imageFile.deleteOnExit();

            BufferedWriter dotFileWriter = new BufferedWriter(new FileWriter(dotFile));
            dotFileWriter.write(this.dot);
            dotFileWriter.close();

            // Virker sandsynligvis ikke på Windows. Virker måske med "cmd /c dot.exe ..."
            executeCommand("dot -Tjpg -o " + imageFile.getAbsolutePath() + " " + dotFile.getAbsolutePath());

            BufferedImage image = ImageIO.read(imageFile);

            JFrame frame = new JFrame();
            frame.setLayout(new FlowLayout());
            frame.setSize(image.getWidth() + 20,image.getHeight() + 30);

            JLabel imageLabel = new JLabel();
            imageLabel.setIcon(new ImageIcon(image));

            frame.add(imageLabel);
            frame.setLocationRelativeTo(null);
            frame.setVisible(true);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private InputStream executeCommand(String command) {
        final InputStream[] result = new InputStream[1];

        final Process p;
        try {
            p = Runtime.getRuntime().exec(command);

            new Thread(new Runnable() {
                public void run() {
                    result[0] = p.getInputStream();
                }
            }).start();

            p.waitFor();
        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        }

        return result[0];
    }
}

For at vise en FA:

FA fa = new FA();
...
DotViewer viewer = new DotViewer(fa);
viewer.view();