我如何正确使用布尔值来使用方法的不同部分

Question

I have a MP3Player class which has a method pressStop. I want to check if the method was used before reset the playlist and if not just pause the song. I've tried with booleans using them in different parts but to no avail. I will appreciate an explanation Code:

public void pressStop() {
        boolean isStopped = false;
        if(isStopped) {
            currentSong = 0;
            System.out.println("Songs are stopped");

        }
        if(!isStopped){
            System.out.println("Song " + currentSong + " is stopped");
            isStopped = true;
        }
    }

Answer 1

This code just won't work.

isStopped will always be false, so the first conditional will never be entered

---

One way you could handle these actions is to use callback listeners, like any popular GUI framework will provide on button clicks.

The idea here - you have one class that maintains the "state" of the player, and any events acting on that state are injected from outside the class.

public class MP3Player {

    private boolean playing; // defaults to false
    private MP3Player.StopButtonListener stopListener;

    public MP3Listener(StopButtonListener stopListener) {
        this.stopListener = stopListener;
        // TODO: Add listeners for other buttons, like start/next/prev
    }

    public interface StopButtonListener {
        public void onStopClicked();
    }

    public void stop() {
        System.out.println("stopping player!");
        this.playing = false;
        if (this.stopListener != null) {
            this.stopListener.onStopClicked();
        }
    }

    // TODO: add start

}

With this, you can add actions to whenever the mp3 player is stopped.

public class MP3Demo {

    public static void main(String[] args) {

        MP3Player.StopButtonListener l = new MP3Player.StopButtonListener() {
            @Override
            public void onStopClicked() {
                System.out.println("got stop event for player!");
                // TODO: Reset the player, update some other information in your app, etc.
            }
        };
        final MP3Player p = new MP3Player(l);
        p.stop();
    }

}

Answer 2

You're setting the isStopped variable at the start of the method. It is always false .

You can try :

public void pressStop() {
    // boolean isStopped = false; 
    if (isStopped) {
        currentSong = 0;
        System.out.println("Songs are stopped");
    } else {
        System.out.println("Song " + currentSong + " is stopped");
    }
    isStopped = true;
}

public void pressPlay() {
    // set the currentSong and do whatever
    isStopped = false;
}