应用程序当试图打开文件夹

Question

我越来越Android Studio中，指出错误，指出使用Android Studio崩溃创建尝试获取空数组 代码的长度是在这里：

public class Main3Activity extends AppCompatActivity { 

Button buttonOpenDialog; 
Button buttonUp; 
TextView textFolder; 

String KEY_TEXTPSS ="TEXTPSS"; 
static final int CUSTOM_DIALOG_ID = 0; 
ListView dialog_ListView; 

File root; 
File curFolder; 

private List<String> fileList = new ArrayList<String>(); 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main3); 

    buttonOpenDialog = (Button) findViewById(R.id.findMusic); 
    buttonOpenDialog.setOnClickListener(new View.OnClickListener() { 
     @Override 
     public void onClick(View v) { 
      showDialog(CUSTOM_DIALOG_ID); 
     } 
    }); 

    root = new File(Environment.getExternalStorageDirectory().getAbsolutePath()); 
    curFolder = root; 


} 
@Override 
protected Dialog onCreateDialog(int id) { 

    Dialog dialog = null; 

    switch (id) { 
     case CUSTOM_DIALOG_ID: 
      dialog = new Dialog(Main3Activity.this); 
      dialog.setContentView(R.layout.dialoguelayout); 
      dialog.setTitle("Custom Dialog"); 
      dialog.setCancelable(true); 
      dialog.setCanceledOnTouchOutside(true); 

      textFolder = (TextView) dialog.findViewById(R.id.folder); 
      buttonUp = (Button) dialog.findViewById(R.id.up); 
      buttonUp.setOnClickListener(new View.OnClickListener() { 
       @Override 
       public void onClick(View v) { 
        ListDir(curFolder.getParentFile()); 
       } 
      }); 

      dialog_ListView = (ListView) dialog.findViewById(R.id.dialoglist); 
      dialog_ListView.setOnItemClickListener(new AdapterView.OnItemClickListener() { 
       @Override 
       public void onItemClick(AdapterView<?> parent, View view, int position, long id) { 
        File selected = new File(fileList.get(position)); 
        if(selected.isDirectory()) { 
         ListDir(selected); 
        } else { 
         Toast.makeText(Main3Activity.this, selected.toString() + " selected", 
           Toast.LENGTH_LONG).show(); 
         dismissDialog(CUSTOM_DIALOG_ID); 
        } 
       } 
      }); 

      break; 
    } 
    return dialog; 
} 

@Override 
protected void onPrepareDialog(int id, Dialog dialog) { 
    super.onPrepareDialog(id, dialog); 
    switch (id) { 
     case CUSTOM_DIALOG_ID: 
      ListDir(curFolder); 
      break; 
    } 
} 

void ListDir(File f) { 
    if(f.equals(root)) { 
     buttonUp.setEnabled(false); 
    } else { 
     buttonUp.setEnabled(true); 
    } 

    curFolder = f; 
    textFolder.setText(f.getPath()); 

    File[] files = f.listFiles(); 
    fileList.clear(); 

    for(File file : files) { 
     fileList.add(file.getPath()); 

    } 

    ArrayAdapter<String> directoryList = new ArrayAdapter<String>(this, 
      android.R.layout.simple_list_item_1, fileList); 
    dialog_ListView.setAdapter(directoryList); 
} 
} 

以下是错误：

08-16 15:20:05.085 30981-30981/com.example.letwixtor.myapplication2 E/AndroidRuntime: FATAL EXCEPTION: main 
                        Process: com.example.letwixtor.myapplication2, PID: 30981 
                        java.lang.NullPointerException: Attempt to get length of null array 
                         at com.example.letwixtor.myapplication2.Main3Activity.ListDir(Main3Activity.java:117) 
                         at com.example.letwixtor.myapplication2.Main3Activity.onPrepareDialog(Main3Activity.java:99) 
                         at android.app.Activity.onPrepareDialog(Activity.java:3380) 
                         at android.app.Activity.showDialog(Activity.java:3444) 
                         at android.app.Activity.showDialog(Activity.java:3394) 
                         at com.example.letwixtor.myapplication2.Main3Activity$1.onClick(Main3Activity.java:43) 
                         at android.view.View.performClick(View.java:5198) 
                         at android.view.View$PerformClick.run(View.java:21147) 
                         at android.os.Handler.handleCallback(Handler.java:739) 
                         at android.os.Handler.dispatchMessage(Handler.java:95) 
                         at android.os.Looper.loop(Looper.java:148) 
                         at android.app.ActivityThread.main(ActivityThread.java:5417) 
                         at java.lang.reflect.Method.invoke(Native Method) 
                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

我相信来自因为它不能返回空的“listFiles”功能的错误（即它一直没有找到的目录）我不太明白为什么它找不到目录aft er root = new File(Environment.getExternalStorageDirectory().getAbsolutePath());

当运行模拟器和应用程序时，应用程序在尝试打开目录时崩溃。不知道如何解决这个问题。 在此先感谢。

Answer 1

确保您在清单中添加了<uses-permission android:name="androi‌​d.permission.READ_EXT‌​ERNAL_STORAGE" />。