Java override method from another class without inheritance

I found a similar question here:
overriding methods without subclassing in Java

But mine a little different, I have two classes, 1 GUI based, and the other just methods to modify elements in first class. If it just editing the basic function, I meet no problem, but now I want to override a jbutton in first class method from the second class without inheriting it. Where do I have to start?

I have temporary solution which is that the second class extends JButton, override method I want to, and add that class to my GUI class (anonymous object or not, is doesn't matter). But I want to discover a way to find a way to my question, is it even possible? Thanks :)

Edit
Here's sample code:

First class, as it only a button in jframe, I only add these in constructor:
ButtonOverrider bo=new ButtonOverrider(); -> this is the overrider class
button=bo.overridePaintComponent(bo); //first try
button=bo.overridePaintComponent(); //second try
bo.overridePaintComponent(bo); //third try

And here's the ButtonOverrider method:

public JButton ButtonOverrider(JButton button) {
  button = new JButton() {
    @Override
    protected void paintComponent(Graphics g) {
      Graphics2D g2 = (Graphics2D) g.create();
      GradientPaint gp = new GradientPaint(0, 0,
      Color.blue.brighter().brighter(), 0, getHeight(),
      getBackground().darker().darker());

      g2.setPaint(gp);
      g2.fillRect(0, 0, getWidth(), getHeight());
      g2.dispose();

      super.paintComponent(g);
      super.setContentAreaFilled(false);
      super.setFocusPainted(false);
      super.setBorder(new LineBorder(Color.yellow, 2));
      super.setText("Shuro");
    }
  };
  return button;
}