Q:

How to check the state of an JCheckBox? 

I'd like to mute my music player player using player.mute() when the JCheckBox mutemusic_box is checked. 
If its unchecked and the player is muted, the command player.mute() should execute.  How to get this working?  My code currently does not work (may because of the ActionEvent). 
The check if it's checked or not is in actionPerformed. 
private JCheckBox pwremember_box, mutemusic_checkbox;

//GUI setup

public void actionPerformed(ActionEvent ae) {

    if (mutemusic_checkbox.isSelected()) {
        System.out.println("Muted!"); 
        player.mute();
    }
    else {
      System.out.println("Unmuted!"); 
      player.mute();
    } 
}

A:

First this check is not required : 
if (ae.getSource().equals(mutemusic_box)) {

The ActionListener should be associated to the JCheckBox button. 
So, there is not any ambiguity. 
Then in order to know if the checkbox is checked, use simply the isSelected() method that is designed for. 
Besides mute() is not a good named method to toggle the sound of the Player. 
toggleSound() could make more sense. 
And as explained in my comment, you have to attach first the listener to the JCheckBox. 
Here is a sample code :
checkBoxMute.addActionListener(
   new ActionListener(){
        public void actionPerformed(ActionEvent ae) {   
          if (checkBoxMute.isSelected()) {
              System.out.println("Muted!"); 
              player.toggleSound();
          }
          else {
              System.out.println("Unmuted!"); 
              player.toggleSound();
          } 
       }
   });

And here the same thing with a lambda instead of creating an anonymous class of ActionListener :
checkBoxMute.addActionListener(
    ae -> {
        if (checkBoxMute.isSelected()) {
          System.out.println("Muted!"); 
          player.toggleSound();
        } else {
          System.out.println("Unmuted!"); 
          player.toggleSound();
        }
    });
}

