Create an instance field opacity
in your button class, then create setter and getters:
private float opacity;
//......
public setOpacity(float opacity) {
this.opacity = opacity;
}
public void getOpacity(){
return this.opacity
}
Then class repaint after setting any opacity to the button:
MyJButton myJbtn = new MyJButton();
myJbtn.setOpacity(0.5);
myJbtn.repaint();