應用程序中的NullPointerException

Question

我正在製作音樂播放器應用程序。

其在class.This給NullPointerException異常是player.java的代碼

package com.example.mudit.take6; 

import android.content.Intent; 
import android.os.Bundle; 
import android.os.Environment; 
    import android.support.v7.app.AppCompatActivity; 
    import android.support.v7.widget.Toolbar; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
    import android.widget.AdapterView; 
import android.widget.ArrayAdapter; 
import android.widget.ListView; 
    import android.widget.Toast; 

    import java.io.File; 
    import java.util.ArrayList; 


public class MainActivity extends AppCompatActivity { 
    ListView lv; 
    String[] items; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.content_main); 
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar); 
    setSupportActionBar(toolbar); 

    lv = (ListView) findViewById(R.id.lvplaylist); 

    final ArrayList<File> mySongs = findsongs(Environment.getExternalStorageDirectory()); 

    items = new String[mySongs.size()]; 

    for (int i = 0; i < mySongs.size(); i++) { 
     //toast(mySongs.get(i).getName().toString()); 
     items[i] = mySongs.get(i).getName().toString().replace(".mp3", "").replace("", ""); 
    } 

    ArrayAdapter<String> adp = new ArrayAdapter<String>(getApplicationContext(), R.layout.song_layout, R.id.textView, items); 
    lv.setAdapter(adp); 


    lv.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
            @Override 
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
             startActivity(new Intent(getApplicationContext(), Player.class).putExtra("pos", position).putExtra("SongList", mySongs)); 
            } 
           } 
    ); 

} 

public ArrayList<File> findsongs(File root) { 
    ArrayList<File> al = new ArrayList<File>(); 
    File[] files = root.listFiles(); 
    for (File SingleFiles : files) { 
     if (SingleFiles.isDirectory() && !SingleFiles.isHidden()) { 
      al.addAll(findsongs(SingleFiles)); 
     } else { 
      if (SingleFiles.getName().endsWith(".mp3")) { 
       al.add(SingleFiles); 

      } 
     } 
    } 
    return al; 
} 

public void toast(String text) { 
    Toast.makeText(getApplicationContext(), text, Toast.LENGTH_SHORT).show(); 

} 

@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); 
} 
} 

錯誤： -

04-16 01:00:56.575 17684-17684/? I/art: Late-enabling -Xcheck:jni 
04-16 01:00:56.640 17684-17684/com.example.mudit.take6 W/System:   ClassLoader referenced unknown path: /data/app/com.example.mudit.take6- 1/lib/arm 
04-16 01:00:56.822 17684-17684/com.example.mudit.take6 D/AndroidRuntime: Shutting down VM 
04-16 01:00:56.825 17684-17684/com.example.mudit.take6 E/AndroidRuntime: FATAL EXCEPTION: main 
                     Process: com.example.mudit.take6, PID: 17684 
                     java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mudit.take6/com.example.mudit.take6.MainActivity}: java.lang.NullPointerException: Attempt to get length of null array 
                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2426) 
                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490) 
                     at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354) 
                     at android.os.Handler.dispatchMessage(Handler.java:102) 
                     at android.os.Looper.loop(Looper.java:148) 
                     at android.app.ActivityThread.main(ActivityThread.java:5443) 
                     at java.lang.reflect.Method.invoke(Native Method) 
                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728) 
                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 
                     Caused by: java.lang.NullPointerException: Attempt to get length of null array 
                     at com.example.mudit.take6.MainActivity.findsongs(MainActivity.java:59) 
                     at com.example.mudit.take6.MainActivity.onCreate(MainActivity.java:33) 
                     at android.app.Activity.performCreate(Activity.java:6259) 
                     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1130) 
                     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379) 
                     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)  
                     at android.app.ActivityThread.-wrap11(ActivityThread.java)  
                     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)  
                     at android.os.Handler.dispatchMessage(Handler.java:102)  
                     at android.os.Looper.loop(Looper.java:148)  
                     at android.app.ActivityThread.main(ActivityThread.java:5443)  
                     at java.lang.reflect.Method.invoke(Native Method)  
                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)  
                       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)  

我怎樣才能成功地運行它。請告訴我是否做錯了方式。如果你能以正確的方式講述。 任何幫助將不勝感激。

Answer 1

當您試圖訪問外部存儲器時，您需要在Manifest文件中包含權限。 既然你只是讀取SD卡，這樣你就可以包括：只讀權限 -

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" /> 

如果任何你正在試圖修改SD卡中的內容應包括： -

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /> 

Answer 2

唯一的例外是正在拋出第59行，這是 for (File SingleFiles : files)，這裏的文件數組爲空，這是導致NullPointerException。

更改您的代碼以驗證文件數組是否爲空。

public ArrayList<File> findsongs(File root) { 
    ArrayList<File> al = new ArrayList<File>(); 
    File[] files = root.listFiles(); 
    if(files != null){ 
    for (File SingleFiles : files) { 
     if (SingleFiles.isDirectory() && !SingleFiles.isHidden()) { 
      al.addAll(findsongs(SingleFiles)); 
     } else { 
      if (SingleFiles.getName().endsWith(".mp3")) { 
       al.add(SingleFiles); 

     } 
    } 
    } 
} 
return al; 

}

Answer 3

它看起來像你的數組列表 「mySongs」 爲空。通過在代碼中添加類似內容來檢查它：

if(mySongs != null && mySongs.length > 0) { 
    ... 
}