public class Window extends JFrame implements ActionListener {
    private JButton findnext;
    private JButton replace;
    private JButton delete;
    private JButton upper;
    private JTextField from,to;
    private JTextArea textArea;
    final static Color found = Color.PINK;
    final Highlighter hilit;
    final Highlighter.HighlightPainter painter;

    public Window() {
        setTitle("Project 8");
        Toolkit tk = Toolkit.getDefaultToolkit();
        Dimension d = tk.getScreenSize();
        setSize((d.width/4)*3,d.height);
        textArea = new JTextArea ("The apple ate the apple.",8,40);
        textArea.setLineWrap(true);
        Container contentPane = getContentPane();
        addWindowListener(new Close());
        contentPane.add(textArea);
        JPanel panel = new JPanel();
        JButton findnext = new JButton("FindNext");
        panel.add(findnext);
        from = new JTextField(8);
        panel.add(from);
        findnext.addActionListener(this);
        JButton replace = new JButton("Replace");
        panel.add(replace);
        to = new JTextField(8);
        panel.add(to);
        findnext.addActionListener(this);
        JButton delete = new JButton("Delete");
        panel.add(delete);
        findnext.addActionListener(this);
        JButton upper = new JButton("Upper");
        panel.add(upper);
        findnext.addActionListener(this);
        contentPane.add(panel, "South");
        hilit = new DefaultHighlighter();
        painter = new DefaultHighlighter.DefaultHighlightPainter(found);
        textArea.setHighlighter(hilit);
    }

    public void actionPerformed(ActionEvent evt) {
        String f = from.getText();
        String t = to.getText();
        int n = textArea.getText().indexOf(f);
        Object source = evt.getSource();

        if (source == findnext) {
            hilit.removeAllHighlights();
            String text = textArea.getText();
            int index = text.indexOf(f,0);
            if (index>0) {
                try {
                    hilit.addHighlight(index, index+f.length(), DefaultHighlighter.DefaultPainter);
                }
                catch (BadLocationException e) {
                    ;
                }
            }else if (source == replace) {
                if (n>=0 && f.length() > 0) {
                    textArea.replaceRange(to.getText(),n,n+f.length());
                    ;
                }else if (source == delete) {
                    textArea.setText(" ");
                }else if (source == upper) {
                    f.toUpperCase() ;
                }
            }
        }
    }
}

private JButton findnext;
private JButton replace;
private JButton delete;
private JButton upper;

JButton findnext = new JButton("FindNext");
//...
JButton replace = new JButton("Replace");
//...
JButton delete = new JButton("Delete");
//...
JButton upper = new JButton("Upper");

if (source == findnext) {

JButton findnext = new JButton("FindNext");
//
JButton replace = new JButton("Replace");
//
JButton delete = new JButton("Delete");
//
JButton upper = new JButton("Upper");

findnext = new JButton("FindNext");
//
replace = new JButton("Replace");
//
delete = new JButton("Delete");
//
upper = new JButton("Upper");