Question

我想创建一个播放列表，以便当用户点击歌曲时播放，当他点击下一首歌曲时，正在播放的歌曲将停止并恢复下一首歌曲。此外，我已经放置了一个停止按钮，也停止正在播放的歌曲。现在，这是我编写以下代码块时的问题，我的程序崩溃，我的错误日志显示java.lang.NullPointerException -

 public void playSong(int res)
    {
        if(mp.isPlaying()) {
            mp.reset();
            mp = MediaPlayer.create(getApplicationContext(), resid[res]);
            mp.start();
        }

        else {
            mp = MediaPlayer.create(getApplicationContext(), resid[res]);
            mp.start();
        }


    }

但是，如果我将代码更改为以下代码，它可以正常工作。我看到mp.reset（）是个问题。但是我不知道如何修复它，因为没有它，当我点击一首歌并且已经播放的歌曲时，两首歌曲同时播放.-

   public void playSong(int res)
    {
            mp = MediaPlayer.create(getApplicationContext(), resid[res]);
            mp.start();
     }

//以下是我的完整代码。请帮我修复bug。谢谢 -

package com.example.user.musicp;

import android.app.Activity;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;

public class MainActivity extends Activity {

    Button b;
    private MediaPlayer mp;
    private ListView v;
    private int [] resid = {R.raw.k,R.raw.u};
    private String [] name = {"k","u"};

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

        v = (ListView) findViewById(R.id.listView);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, name);

        v.setAdapter(adapter);
        b = (Button) findViewById(R.id.StopButton);

        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
              //  mp.stop();
              //  mp.release();
                onDestroy();

            }
        });

        v.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                playSong(i);
              //  mp = MediaPlayer.create(MainActivity.this, resid[i]);
              //  mp.start();
            }
        });

    }

        public void playSong(int res)
    {
        if(mp.isPlaying()) {
            mp.reset(); //this is where the issue begins
            mp = MediaPlayer.create(getApplicationContext(), resid[res]);
            mp.start();
        }

        else {
            mp = MediaPlayer.create(getApplicationContext(), resid[res]);
            mp.start();
        }


    }

    @Override

    public void onDestroy() {

        super.onDestroy();
        mp.release();

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

参考文献： - http://www.geeks.gallery/how-to-play-mp3-file-from-raw-folder-in-a-listview-android/

在一些用户提出一些有价值的建议之后，我编辑了我的代码，并且能够修复错误。但是当我点击一首歌并且有一首已经播放的歌曲时，我希望停止播放上一首歌并播放新歌，但不幸的是两首歌都在播放。我尝试了一切可能的方法来解决它，但我可能在某个地方犯了错误。这是我编辑的代码，当我实现以下内容时，我的程序崩溃了。让我帮忙 -

 public void playSong(int res)
    {



        boolean isplaying = mp.isPlaying();

         if(isplaying)
         {
             mp.stop();
             mp = MediaPlayer.create(getApplicationContext(), resid[res]);
             mp.start();
         }

         else
         {
             mp = MediaPlayer.create(getApplicationContext(), resid[res]);
             mp.start();
         }

}

以下是完整的工作代码，以防万一将来陷入困境。非常感谢StackOverflow用户帮助我。干杯！

package com.example.user.musicp;

import android.app.Activity;
import android.content.Intent;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;

import java.io.IOException;

public class MainActivity extends Activity {

    Button b,c;
    private MediaPlayer mp;
    private ListView v;
    private int [] resid = {R.raw.k,R.raw.u};
    private String [] name = {"k","u"};

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

        v = (ListView) findViewById(R.id.listView);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, name);

        v.setAdapter(adapter);
        b = (Button) findViewById(R.id.StopButton);
        c = (Button) findViewById(R.id.next);


        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                mp.stop();


            }
        });

        c.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent in = new Intent("com.example.user.musicp.SPLASH");
                startActivity(in);
            }
        });
        v.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                playSong(i);
              //  mp = MediaPlayer.create(MainActivity.this, resid[i]);
              //  mp.start();
            }
        });

    }

        public void playSong(int res)
    {
        mp.reset();// stops any current playing song

        mp = MediaPlayer.create(getApplicationContext(), resid[res]);// create's
        mp.start(); // starting mediaplayer






    }

    @Override

    public void onDestroy() {

        super.onDestroy();
        mp.release();

    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Answer 1

if(mp.isPlaying()) { // mp only declared (still null)
    mp.reset();
    mp = MediaPlayer.create(getApplicationContext(), resid[res]);
    mp.start();     }

else {
    mp = MediaPlayer.create(getApplicationContext(), resid[res]);
    mp.start();
}

我想在if语句mp的那一刻才宣布。这就是为什么你不能mp.isPlaying()因为mp仍然是null的原因。

这应该解释为什么在这种情况下它可以正常工作

public void playSong(int res)
{
    // here mp is still null
    mp = MediaPlayer.create(getApplicationContext(), resid[res]); // From here on we can use mp
    mp.start();
 }

这里没有在初始化之前使用mp，因此没有空指针异常。

编辑：在playSong方法中创建媒体播放器有点奇怪。这意味着每次调用playSong时mp都会被重新分配。一个更好的地方可能是在类的构造函数中。这样做可以确保mp在null被调用时不会playSong。

编辑2：您仍在mp重新分配playSong。我建议这样做：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mp = MediaPlayer.create(getApplicationContext(), resid[res]);
    // ...
}

我会像这样实施playSong：

public void playSong(int songIndex)
{
    if(mp.isPlaying()) {
        mp.pause(); // Pause the current track
    }

    mp.selectTrack(songIndex); // Select the requested track
    mp.start();
}

如何在AndroidStudio中从原始文件夹播放mp3文件时修复java.lang.NullPointerException？

1 个答案: