import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;

import javax.swing.JButton;
import javax.swing.JEditorPane;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;

public class DisplayHTML {

    private JEditorPane editorPane;
    private JTextField field;
    private JButton button;
    private JPanel buttPanel;
    private String str;
    private URL url;

    public DisplayHTML() {
        init();
    }

    private void init() {
        JFrame frame = new JFrame("HTML-Seite darstellen");
        field = new JTextField("file://");
        frame.add(field, BorderLayout.NORTH);

        button = new JButton("laden");
        button.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                setURL();
                loadPage();
            }
        });
        buttPanel = new JPanel(new FlowLayout());
        buttPanel.add(button);
        frame.add(buttPanel, BorderLayout.SOUTH);

        editorPane = new JEditorPane();
        editorPane.setEditable(false);
        frame.add(editorPane, BorderLayout.CENTER);
        setURL();
        loadPage();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 200);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    /**
     * loads the asked page to the JEditorPane and displays its source in the
     * console
     */
    public void loadPage() {
        // stop the program if there is no URL-Object
        if (url == null) {
            return;
        }
        try {
            // load the page
            editorPane.setPage(url);
            // get the source
            InputStream in = url.openStream();
            BufferedReader buff = new BufferedReader(new InputStreamReader(in));
            String s;
            // display source
            while ((s = buff.readLine()) != null) {
                System.out.println(s);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void setURL() {
        // check the URL-String
        if (!verifyStr())
            return;
        if (str.startsWith("file://") || str.startsWith("http://") || str.startsWith("https://")) {
            // create an URL-Object
            try {
                url = new URL(str);
            } catch (MalformedURLException e) {
                url = null;
                System.err.println("Falsches URL-Format!");
                e.printStackTrace();
            }
        }
    }

    private boolean verifyStr() {
        str = field.getText();
        if (str.equals("") || str.length() < 8) {
            return false;
        }
        if (str.startsWith("http://") || str.startsWith("https://")) {
            return true;
        }
        if (!str.startsWith("http://") && !str.startsWith("file://")) {
            str = "file://" + str;
        }
        if (str.startsWith("file://")) {
            // check if file is valid and can be read
            String s = str.substring(7);
            File file = new File(s);
            if (file.isFile() && file.canRead()) {
                return true;
            }
        }
        return false;
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new DisplayHTML());
    }
}