how do i change the opacity of a jbutton

1private float opacity;
2//......
3public setOpacity(float opacity) {
4    this.opacity = opacity;
5}
6
7public void getOpacity(){
8    return this.opacity
9}

1MyJButton myJbtn = new MyJButton();
2myJbtn.setOpacity(0.5);
3myJbtn.repaint();