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Java Notes: Example - NanoEdit

This program is the simplest text editor you can imagine. It's used as an example of menus, JFileChooser, and reading and writing text files.

NanoEdit source code

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// File   : nanoedit/NanoEdit.java
// Purpose: Display file in text area.  Allow open/save file.
//          Illustrates menus, JFileChooser, read/write text files.
//          This combines everything into one class,
//          main, the GUI, and the model.  There really 
//          isn't much model because it's only the string
//          in the JTextArea.
// Bugs   : Doesn't write system dependent newlines.
//          Error handling is nonexistent.
// Author : Fred Swartz
// Date   : 2005-12-01

package nanoedit;

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.io.*;
import java.util.*;

public class NanoEdit extends JFrame {
    //================================================= instance variables
    //... GUI variables references at runtime.
    private JTextArea    m_textTA      = new JTextArea(25, 80);
    private JFileChooser m_fileChooser = new JFileChooser();
    private JMenuItem    m_openItem    = new JMenuItem("Open...");
    private JMenuItem    m_saveItem    = new JMenuItem("Save...");
    private JMenuItem    m_quitItem    = new JMenuItem("Quit");
    
    //======================================================== constructor
    NanoEdit() {
        JButton openButton = new JButton("Open");
        JButton saveButton = new JButton("Save");
        
        //... Layout main content panel
        JPanel content = new JPanel();
        content.setLayout(new BorderLayout());
        content.add(new JScrollPane(m_textTA), BorderLayout.CENTER);
        
        //... Set window characteristics
        this.setTitle("Nano Editor");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        this.setJMenuBar(createMenubar());
        this.setContentPane(content);
        this.pack();
    }
    //==================================================== createMenubar
    // It's not uncommon to write a method that produces the menubar.
    // It gives a little better organization to the program
    // and doesn't mix content pane and menu construction, but
    // there is no real need to separate it.

    private JMenuBar createMenubar() {
        //... Menubar, menus, menu items.  Use indentation to show structure.
        JMenuBar menubar = new JMenuBar();     // declare and create new menu bar
            JMenu fileMenu = new JMenu("File");// declare and create new menu
                menubar.add(fileMenu);         // add the menu to the menubar
                fileMenu.add(m_openItem);      // add menu item to the menu
                fileMenu.add(m_saveItem);      // add menu item to the menu
                fileMenu.addSeparator();       // add separator line to menu
                fileMenu.add(m_quitItem);
                
        //... Attach listeners.
        m_openItem.addActionListener(new OpenAction());
        m_saveItem.addActionListener(new SaveAction());
        m_quitItem.addActionListener(new QuitAction());
        
        return menubar;
    }

    
    //////////////////////////////////// inner listener class - OpenAction
    private class OpenAction implements ActionListener {
        public void actionPerformed(ActionEvent ae) {
            int retval = m_fileChooser.showOpenDialog(NanoEdit.this);
            if (retval == JFileChooser.APPROVE_OPTION) {
                File textFile = m_fileChooser.getSelectedFile();
                m_textTA.setText(readFile(textFile));
            }
        }
    }
    
    //////////////////////////////////// inner listener class - SaveAction
    private class SaveAction implements ActionListener {
        public void actionPerformed(ActionEvent ae) {
            int retval = m_fileChooser.showSaveDialog(NanoEdit.this);
            if (retval == JFileChooser.APPROVE_OPTION) {
                File textFile = m_fileChooser.getSelectedFile();
                writeFile(textFile, m_textTA.getText());
            }
        }
    }

    //////////////////////////////////// inner listener class - QuitAction
    private class QuitAction implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            System.exit(0);      // Terminate the program
        }
    }
    
    
    //======================================================= readFile
    // This reads a file and returns the contents as a string,
    // with ends of lines uniformly marked with \n.
    // It would probably be faster to read directly with
    // BufferedReader, but I decided to test Scanner this
    // time.  Seems to work fine.
    private String readFile(File f) {
        StringBuilder text = new StringBuilder(4000);
        try {
            Scanner wordScanner = new Scanner(f);
            while (wordScanner.hasNextLine()) {
                text.append(wordScanner.nextLine());
                text.append('\n');
            }
        } catch (FileNotFoundException fnfex) {
            System.out.println("Impossible?\n" + fnfex);
        }
        
        return text.toString();
    }
    
    //====================================================== writeFile
    // Should be fixed to get the system end of line string.
    private void writeFile(File f, String txt) {
        try {
            //... Create writer for text files.
            BufferedWriter writer = new BufferedWriter(new FileWriter(f));
            writer.write(txt);
            writer.close();  // Close to unlock and flush to disk.

        } catch (IOException e) {
            System.err.println(e);
            System.exit(1);
        }
    }
    
    //=========================================================== main
    public static void main(String[] args) {
        JFrame window = new NanoEdit();
        window.setVisible(true);
    }
}
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