免费构建，但在Android Studio中点击音乐播放器时崩溃

Question

I'm new to programming and have been assigned to code a simple music player, there were slides to help guide and I have slightly altered them (can't use the exact same variables).

So now it's error free but when I click on the play button it crashes, no idea why. Here's my code for the page where I select the library:

package sg.edu.tp.project1;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;


import sg.edu.tp.project1.util.AppUtil;

public class myMusic extends AppCompatActivity {
private String[] Musicsong = new String[6];






    @Override
    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_my_music);
    //store values in array of the first song
    Musicsong[0] = "s1001";
    Musicsong[1] = "The Way You Look Tonight";
    Musicsong[2] = "ed sheeran";
    Musicsong[3] = "a5b8972e764025020625bbf9c1c2bbb06e394a60?cid=null";
    Musicsong[4] = "4.39";
    Musicsong[5] = "music to play";


}
public void sendDataToActiviy(String[] song)
{
    //1.create intent and specify destination
    Intent intent = new Intent(this, PlayMusicActivity.class);

    //2.store song info in intent to send to destination
    intent.putExtra("id", song[0]);
    intent.putExtra("title",song[1]);
    intent.putExtra("artist",song[2]);
    intent.putExtra("fileLink",song[3]);
    intent.putExtra("coverArt",song[5]);

    //3. launch desntination activiy
    startActivity(intent);


}
public void handleSelection(View view)
{

    // 1. get id of selected song
    String resourceId = AppUtil.getResourceId(this, view);

    //2. Search for the selected song based on the ID so that
    // all infomation of the song can be retreived

    String[] selectedSong =searchById(resourceId);
    //3.popup to show tittle of song
    AppUtil.popMessage(this, "Streaming song:" + selectedSong[1]);

    //4. send song data to player screen
    sendDataToActiviy(selectedSong);
}
private Object[] songs = {Musicsong};
public String[] searchById(String id) {
    //temporary empty array
    String[] song = null;
    //for loop to get song
    for (int index = 0; index < songs.length; index++) {
        //3. store each song item to song array.
        song = (String[]) songs[index];

        //4. match song id to see if its the one i want
        if (song[0].equals(id)) {
            return song;

        }

    }
    //if song not found in array empty array will be returned
    return song;
}

}

This part where the player is:

package sg.edu.tp.project1;

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

import java.io.IOException;

import sg.edu.tp.project1.util.AppUtil;

public class PlayMusicActivity extends AppCompatActivity
{

//streaming website
private static final String BASE_URL = "https://p.scdn.co/mp3-preview/";

private String songId ="";
private String title = "";
private String artist = "";
private String fileLink = "";
private String coverArt = "";
private String url = "";

//builtin media player
private MediaPlayer player = null;

//position  of song in playback
private int musicPosition = 0;

//button variable to link to play btn
private Button btnPlayPause = null;

private String[] Musicsong = {

        "s1001",
        "The Way You Look Tonight",
        "ed sheeran",
        "a5b8972e764025020625bbf9c1c2bbb06e394a60?cid=null",
        "4.39",
        "music to play"};



private Object[] songs = {Musicsong};


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

    btnPlayPause = (Button) findViewById(R.id.btnPlayPause);

    retrieveData();

    displaySong(title,artist,coverArt);

}
private void retrieveData() {
    Bundle songData = this.getIntent().getExtras();

    songId = songData.getString("id");
    title = songData.getString("title");
    artist = songData.getString("artist");
    fileLink = songData.getString("fileLink");
    coverArt = songData.getString("coverArt");

    url = BASE_URL + fileLink;
}
private void displaySong(String title, String artist, String coverArt)
{
    //retrieve song title
    TextView txtTitle = (TextView) findViewById(R.id.txtSongTitle);

    //set text of song title
    txtTitle.setText(title);

    //retrieve artist
    TextView txtArtist = (TextView) findViewById(R.id.txtArtist);

    //set text of artist
    txtArtist.setText(artist);

    //get id of coverart
    int imageId = AppUtil.getImageIdFromDrawable(this,coverArt);

    //retrieve coverart
    ImageView ivCoverArt = (ImageView) findViewById(R.id.imgCoverArt);

    ivCoverArt.setImageResource(imageId);

}






public void playOrPauseMusic(View view)
{// start player
    player.start();

    //2.
    btnPlayPause.setText("PAUSE");

    //3.set the heading title of played song
    setTitle("Now Playing:" + title + "-" + artist);

     }


private  void preparePlayer() { //create a new player
    player = new MediaPlayer();

    try {
        //set stream type to music
        player.setAudioStreamType(AudioManager.STREAM_MUSIC);
//set source of music
        player.setDataSource(url);
        //prepare player for playback
        player.prepare();
    } catch (IOException e) {
        e.printStackTrace();
    }


}}

Logs:

08-10 16:04:02.478 2567-2567/sg.edu.tp.project1 I/art: Not late-enabling -Xcheck:jni (already on)
08-10 16:04:02.478 2567-2567/sg.edu.tp.project1 W/art: Unexpected CPU variant for X86 using defaults: x86
08-10 16:04:02.535 2567-2567/sg.edu.tp.project1 W/System: ClassLoader referenced unknown path: /data/app/sg.edu.tp.project1-2/lib/x86
08-10 16:04:02.539 2567-2567/sg.edu.tp.project1 I/InstantRun: Starting Instant Run Server for sg.edu.tp.project1
08-10 16:04:02.913 2567-2567/sg.edu.tp.project1 W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
08-10 16:04:03.399 2567-2632/sg.edu.tp.project1 I/OpenGLRenderer: Initialized EGL, version 1.4
08-10 16:04:03.400 2567-2632/sg.edu.tp.project1 D/OpenGLRenderer: Swap behavior 1
08-10 16:04:03.400 2567-2632/sg.edu.tp.project1 W/OpenGLRenderer: Failed to choose config with EGL_SWAP_BEHAVIOR_PRESERVED, retrying without...
08-10 16:04:03.400 2567-2632/sg.edu.tp.project1 D/OpenGLRenderer: Swap behavior 0
08-10 16:04:03.490 2567-2567/sg.edu.tp.project1 W/art: Before Android 4.1, method int android.support.v7.widget.ListViewCompat.lookForSelectablePosition(int, boolean) would have incorrectly overridden the package-private method in android.widget.ListView
08-10 16:04:14.759 2567-2632/sg.edu.tp.project1 D/OpenGLRenderer: endAllActiveAnimators on 0x8d2e9b00 (RippleDrawable) with handle 0x8d2ff830
08-10 16:04:27.207 2567-2570/sg.edu.tp.project1 I/art: Do partial code cache collection, code=29KB, data=27KB
08-10 16:04:27.207 2567-2570/sg.edu.tp.project1 I/art: After code cache collection, code=29KB, data=27KB
08-10 16:04:27.207 2567-2570/sg.edu.tp.project1 I/art: Increasing code cache capacity to 128KB
08-10 16:04:27.463 2567-2632/sg.edu.tp.project1 D/OpenGLRenderer: endAllActiveAnimators on 0x8c313a00 (RippleDrawable) with handle 0x8d2ff420
08-10 16:04:29.695 2567-2567/sg.edu.tp.project1 D/AndroidRuntime: Shutting down VM

Beginning of crash:

08-10 16:04:29.696 2567-2567/sg.edu.tp.project1 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                  Process: sg.edu.tp.project1, PID: 2567
                                                                  java.lang.IllegalStateException: Could not execute method for android:onClick
                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                                                                      at android.view.View.performClick(View.java:5637)
                                                                      at android.view.View$PerformClick.run(View.java:22429)
                                                                      at android.os.Handler.handleCallback(Handler.java:751)
                                                                      at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                      at android.os.Looper.loop(Looper.java:154)
                                                                      at android.app.ActivityThread.main(ActivityThread.java:6119)
                                                                      at java.lang.reflect.Method.invoke(Native Method)
                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
                                                                   Caused by: java.lang.reflect.InvocationTargetException
                                                                      at java.lang.reflect.Method.invoke(Native Method)
                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                                                                      at android.view.View.performClick(View.java:5637) 
                                                                      at android.view.View$PerformClick.run(View.java:22429) 
                                                                      at android.os.Handler.handleCallback(Handler.java:751) 
                                                                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                      at android.os.Looper.loop(Looper.java:154) 
                                                                      at android.app.ActivityThread.main(ActivityThread.java:6119) 
                                                                      at java.lang.reflect.Method.invoke(Native Method) 
                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 
                                                                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
                                                                      at sg.edu.tp.project1.PlayMusicActivity.playOrPauseMusic(PlayMusicActivity.java:106)
                                                                      at java.lang.reflect.Method.invoke(Native Method) 
                                                                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                                                                      at android.view.View.performClick(View.java:5637) 
                                                                      at android.view.View$PerformClick.run(View.java:22429) 
                                                                      at android.os.Handler.handleCallback(Handler.java:751) 
                                                                      at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                      at android.os.Looper.loop(Looper.java:154) 
                                                                      at android.app.ActivityThread.main(ActivityThread.java:6119) 
                                                                      at java.lang.reflect.Method.invoke(Native Method) 
                                                                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
                                                                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

Answer 1

The exception message is very clear:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference 
    at sg.edu.tp.project1.PlayMusicActivity.playOrPauseMusic(PlayMusicActivity.java:106)

So your player field is null because you've never called PlayMusicActivity 's preparePlayer() before.