import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

class MyClass implements ChangeListener{
  JSlider slider;

  public void init(){
    slider = new Slider(SwingConstants.HORIZONTAL, 1, 20, 3); //orientation, min, max, defaultValue
    slider.addChangeListener(this);
  }

  @Override
  public void stateChanged(ChangeEvent e){
    if (e.getSource() == slider){
      //....
    }
  }
};