Invoque el editor por defecto para editar un archivo

import java.awt.Desktop;
import java.io.File;
import java.io.IOException;

public class Test {
public static void main(String[] a) {
try {
Desktop desktop = null;
if (Desktop.isDesktopSupported()) {
desktop = Desktop.getDesktop();
}
desktop.edit(new File("c:\\a.txt"));
} catch (IOException ioe) {
ioe.printStackTrace();
}
}
}