JTextArea textComp = new JTextArea();

// Highlight the occurrences of the word "public"
highlight(textComp, "public");

// Creates highlights around all occurrences of pattern in textComp
public void highlight(JTextComponent textComp, String pattern) {
    // First remove all old highlights
    removeHighlights(textComp);

    try {
        Highlighter hilite = textComp.getHighlighter();
        Document doc = textComp.getDocument();
        String text = doc.getText(0, doc.getLength());
        int pos = 0;

        // Search for pattern
        while ((pos = text.indexOf(pattern, pos)) >= 0) {
            // Create highlighter using private painter and apply around pattern
            hilite.addHighlight(pos, pos+pattern.length(), myHighlightPainter);
            pos += pattern.length();
        }
    } catch (BadLocationException e) {
    }
}

// Removes only our private highlights
public void removeHighlights(JTextComponent textComp) {
    Highlighter hilite = textComp.getHighlighter();
    Highlighter.Highlight[] hilites = hilite.getHighlights();

    for (int i=0; i<hilites.length; i++) {
        if (hilites[i].getPainter() instanceof MyHighlightPainter) {
            hilite.removeHighlight(hilites[i]);
        }
    }
}

// An instance of the private subclass of the default highlight painter
Highlighter.HighlightPainter myHighlightPainter = new MyHighlightPainter(Color.red);

// A private subclass of the default highlight painter
class MyHighlightPainter extends DefaultHighlighter.DefaultHighlightPainter {
    public MyHighlightPainter(Color color) {
        super(color);
    }
}