在非活动类中获取原始资源（声音）

Question

I'm trying to create a singleton class that will be responsible for playing game sounds. I created a singleton class GameSounds with a method playSound() . In the res folder I have aa subfolder 'raw' with a file letter_found.mp3 .

This is the source code of the GameSounds class I wrote:

import android.app.Application;
import android.content.Context;
import android.media.MediaPlayer;

public class GameSounds extends Application {

    private static GameSounds gameSounds = new GameSounds();
    private static MediaPlayer soundPlayer;
    private static Context mContext;
    private static int mySoundId = R.raw.letter_found;

    private GameSounds() {
        mContext = this;
    }

    public static GameSounds getInstance() {
        return gameSounds;
    }

    public static void playSound() {
        soundPlayer = MediaPlayer.create(mContext, mySoundId);
        soundPlayer.start();
    }
}

This doesn't seem to work as I'm getting the following error message:

"java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference"

I don't understand why this is happening. I tried to search Stackoverflow but couldn't find a solution.

Any help/explanation is greatly appreciated.

Answer 1

You shouldn't inherit Application class unless you try to use Singleton pattern. Because Application is base class which contains all other components such as activities and services.

Instead, GameSound class should contain Context object and proper constructor.

Example)

public class GameSounds {

    private GameSounds gameSounds;
    private MediaPlayer soundPlayer;
    private WeakReference<Context> mContext;
    private int mySoundId = R.raw.letter_found;

    private GameSounds(Context context) {
        mContext = new WeakReference<>(context);
    }

    public GameSounds getInstance(Context context) {
        if (gameSounds == null) {
            gameSounds = new GameSounds(context);
        }

        return gameSounds;
    }

    public void playSound() {
        soundPlayer = MediaPlayer.create(mContext.get(), mySoundId);
        soundPlayer.start();
    }
}

In this code, there is WeakReference<Context> instead of Context. WeakReference is used to prevent memory leaks because memory leaks can occur if you have an instance outside the activity.

To play sound, execute GameSounds.getInstance(this).playSound(); is fine.

If Context can't provide when try to play sound, implement initialize methods and called in Application class can be ok.

public class GameSounds {

    private static GameSounds gameSounds;
    private MediaPlayer soundPlayer;
    private WeakReference<Context> mContext;
    private int mySoundId = R.raw.letter_found;

    private GameSounds(Application context) {
        mContext = new WeakReference<>(context);
    }

    public static void initialize(Application context) {
        if (gameSounds == null) {
            gameSounds = new GameSounds(context);
        }
    }

    public static GameSounds getInstance() {
        if (gameSounds == null) {
            throw new NullPointerException("You need to initialize this code by GameSound.initialize(this) in application class");
        }

        return gameSounds;
    }

    public void playSound() {
        soundPlayer = MediaPlayer.create(mContext.get(), mySoundId);
        soundPlayer.start();
    }
}

In this case, you should make Application class and initialize GameSound class by GameSound.initialize(this) in Application class.

To play sound, GameSound.getInstance().playSound() is fine.

Answer 2

You can have a Singleton holding an Application Context (NOT Activity context) but practically you have to set this context before you use your singleton which can be enforced by throwing exceptions. See below example code.

public class GameSounds {
    private static Context sContext;

    public static void setContext(Context context) {
        if (context == null) {
            throw new IllegalArgumentException("context cannot be null!");
        }

        // In order to avoid memory leak, you should use application context rather than the `activiy`
        context = context.getApplicationContext();
        if (context == null) {
            throw new IllegalArgumentException("context cannot be null!");
        }

        sContext = context;
    }

    private static Context getContext() {
        if (sContext != null) {
            return (Context)sContext;
        }
        throw new IllegalStateException("sContext was not set yet! Please call method setContext(Context context) first.");
    }

    // the rest of other methods. e.g. playSounds()
    private static GameSounds gameSounds = new GameSounds();
    private GameSounds() {

    }

    public static GameSounds getInstance() {
        return gameSounds;
    }


    public void playSound() {

        Context context = getContext();

        soundPlayer = MediaPlayer.create(context, mySoundId);
        soundPlayer.start();
    }
}