Java SE (Java Platform, Standard Edition)

Here is my code :
1.

DefaultHighlighter.DefaultHighlightPainter painter=new DefaultHighlighter.DefaultHighlightPainter( Color.red);
2. Pattern pattern=Pattern.compile("\\d");
3. Highlighter highlighter = textPane.getHighlighter();
4. HTMLDocument document=(HTMLDocument)textArea.getDocument();

at line 4 i'm gettting this exception :

Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: javax.swing.text.DefaultStyledDocument cannot be cast to javax.swing.text.html.HTMLDocument
        at apidocs.JavaLibraryInfo.jMenuItem2ActionPerformed(JavaLibraryInfo.java:224)
        at apidocs.JavaLibraryInfo.access$200(JavaLibraryInfo.java:42)
        at apidocs.JavaLibraryInfo$3.actionPerformed(JavaLibraryInfo.java:156)
        at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
        at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
        at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
        at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
        at javax.swing.AbstractButton.doClick(AbstractButton.java:357)
        at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:1223)

Why this error is occuring? Please Suggest.