What's wrong with my code, please help me

Question

MainActivity.java:

    import android.media.MediaPlayer;
    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.Button;

    public class MainActivity extends AppCompatActivity {

        int currentPosition;

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            MyListener myListener = new MyListener();

            Button startButton = (Button) findViewById(R.id.start);
            startButton.setOnClickListener(myListener);

            Button pauseButton = (Button) findViewById(R.id.pause);
            pauseButton.setOnClickListener(myListener);

        }
    }

MyListener.java:

    import android.media.MediaPlayer;
    import android.provider.MediaStore;
    import android.support.v7.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;

    public class MyListener extends AppCompatActivity implements View.OnClickListener{

        MediaPlayer musicPlayer = MediaPlayer.create(this, R.raw.sound_file);

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_my_listener);
        }

        @Override
        public void onClick(View v){

            switch(v.getId()){
                case R.id.start:
                    musicPlayer.start();
                    break;
                case R.id.pause:
                    musicPlayer.pause();
                    break;
            }
        }
    }

Logcat shows the following error:

    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.musicplayer/com.example.android.musicplayer.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference

I am trying to create an app that plays audio from my res/raw file, but there seems to be something wrong with my code as it is not working, please help me.

You cannot initialize `musicPlayer` before receiving `onCreate` call. — Pawel, Sep 28 '18 at 10:20
Your listener should not extend `AppCompatActivity`. They are special purpose classes. Only an Activity should extend it. In fact, you don't even need a different class for listener here. That code should go to `MainActivity` itself. — Rohit5k2, Sep 28 '18 at 10:24

Michael Dodd · Accepted Answer · 2018-09-28T11:12:49.163

Firstly, your MediaPlayer instance should reside within MainActivity, not MyListener, and MyListener should not extend an activity. In fact, you should move all of your code from MyListener into MainActivity, I don't really see a purpose for it in the snippet you've provided.

Secondly, You're creating your MediaPlayer outside of the Activity Lifecycle, while still trying to pass a Context to it:

public class MainActivity extends AppCompatActivity {

    MediaPlayer musicPlayer = MediaPlayer.create(this, R.raw.sound_file);
    ...
}

The activity has no Context (this) until the Activity Lifecycle has started, the way you've written it above is equivalent to defining musicPlayer in a constructor:

public class MainActivity extends AppCompatActivity {

    MediaPlayer musicPlayer; 

    public MyListener() {
        musicPlayer = MediaPlayer.create(this, R.raw.sound_file);
    }
    ...
}

This will also fail as the Activity Lifecycle has not yet started. What you need to do is declare musicPlayer as a member of the class, and then create an instance in onCreate() where the Context will have been initialsed:

public class MainActivity extends AppCompatActivity {

    MediaPlayer musicPlayer; 

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my_listener);
        musicPlayer = MediaPlayer.create(this, R.raw.sound_file);
    }
    ...
}

To address your comment, here's an example of how it could all fit in to your MainActivity using lambdas:

public class MainActivity extends AppCompatActivity {

    int currentPosition;
    MediaPlayer musicPlayer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        musicPlayer = MediaPlayer.create(this, R.raw.sound_file);

        Button startButton = (Button) findViewById(R.id.start);
        startButton.setOnClickListener(view -> musicPlayer.start());

        Button pauseButton = (Button) findViewById(R.id.pause);
        pauseButton.setOnClickListener(view -> musicPlayer.pause());
    }
}

Thank you for answering! I wanted to create a different class that implements listener because i think that would make my mainactivity less clustered, is that stupid? My listener class is working after adding the following public constructor| public MyListener(Context c) { musicPlayer = MediaPlayer.create(c, R.raw.sound_file); } should i just implement listener in my mainactivity and build my code there? — Jordan23, Sep 28 '18 at 11:07
Well, with lambda methods introduced in Java 8, there's no real need to define a full `OnClickListener`, I'll update my answer with an example — Michael Dodd, Sep 28 '18 at 11:09
But yes, your listener can still implement `OnClickListener` if you want to take that approach. The rest of the class will still work if you remove the extension on `AppCompatActivity` and pass in a Context via the constructor, like you've already done. — Michael Dodd, Sep 28 '18 at 11:16
Omggg i just got introduced to this, this is much more convenient than implementing listener or creating a anonymous class, thanks for your help!! — Jordan23, Sep 28 '18 at 11:42

What's wrong with my code, please help me

1 Answers1