NullPointerException - 尝试获取空数组的长度（readDirectory（））

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问题描述

我有这里所示的代码。
我的问题是files.length的一个NullPointerException

  for（int i = 0; i< files.length ; i ++）{

这是因为我有一个fail readDirectory（）errno = 13

  File [] files = f.listFiles（）;

但是，当路径好的时候为什么我的readDirectory失败？

  package com.example.androidexplorer; 
 
 import java.io.File; 
 import java.util.ArrayList; 
 import java.util.List; 
 
 import android.os.Bundle; 
 import android.os.Environment; 
 import android.app.AlertDialog; 
 import android.app.ListActivity; 
 import android.util.Log; 
 import android.view.View; 
 import android.widget.ArrayAdapter; 
 import android.widget.ListView; 
 import android.widget.TextView; 
 
 public class MainActivity extends ListActivity {
 
 private List< String> item = null; 
私人列表< String> path = null; 
 private String root; 
 private TextView myPath; 
 
 @Override 
 public void onCreate（Bundle savedInstanceState）{
 super.onCreate（savedInstanceState）; 
 setContentView（R.layout.activity_main）; 
 myPath =（TextView）findViewById（R.id.path）; 
 
 root = Environment.getExternalStorageDirectory（）。getPath（）; 
 getDir（root）; 
} 
 
 private void getDir（String dirPath）
 {
 myPath.setText（Location：+ dirPath）; 
 item = new ArrayList< String>（）; 
 path = new ArrayList 文件f =新建文件（dirPath）; 
 
 Log.v（Path：，dirPath）; 
 
 Log.v（BEFORE，阅读失败之前...）; 
文件[] files = f.listFiles（）; 
 Log.v（AFTER，阅读失败后）; 
 
 
 if（！dirPath.equals（root））
 {
 item.add（root）; 
 path.add（root）; 
 items.add（../）; 
 path.add（f.getParent（））; 
} 
 
 Log.v（CRASH，1线之前崩溃）; （int i = 0; i< files.length; i ++）{
 Log.v（AFTER CRASH，1 Line after crash）; 
 
文件文件=文件[i]; 
 
 if（！file.isHidden（）&& file.canRead（））{
 path.add（file.getPath（））; 
 if（file.isDirectory（））{
 item.add（file.getName（）+/）; 
} else {
 item.add（file.getName（））; 
} 
} 
} 
 
 ArrayAdapter< String> fileList = 
 new ArrayAdapter< String>（this，R.layout.row，item）; 
 setListAdapter（fileList）; 
} 
 
 @Override 
 protected void onListItemClick（ListView l，View v，int position，long id）{
 // TODO自动生成的方法stub 
文件文件= new File（path.get（position））; 
 
 if（file.isDirectory（））
 {
 if（file.canRead（））{
 getDir（path.get（position））; 
} else {
 new AlertDialog.Builder（this）
 .setIcon（R.drawable.ic_launcher）
 .setTitle（[+ file.getName（）+]文件夹无法读取！）
 .setPositiveButton（OK，null）.show（）; 
} 
} else {
 new AlertDialog.Builder（this）
 .setIcon（R.drawable.ic_launcher）
 .setTitle（[+ file.getName （）+]）
 .setPositiveButton（OK，null）.show（）; 
 
} 
} 
 
}

LogCat：

更新

我的清单中出现了权限错误。
开始时我有：

 <？xml version =1.0encoding =utf-8 >？; 
< manifest xmlns：android =http://schemas.android.com/apk/res/android
 package =com.spicysoftware.infoid
 android：versionCode = 1
 android：versionName =1.0> 
 
< uses-sdk 
 android：minSdkVersion =15
 android：targetSdkVersion =15/> 
 
< application 
 android：allowBackup =true
 android：icon =@ drawable / ic_launcher
 android：label =@ string / app_name 
 android：theme =@ style / AppTheme
 android：name =android.permission.READ_EXTERNAL_STORAGE> 
< activity 
 android：name =。MainActivity
 android：label =@ string / app_name> 
< intent-filter> 
< action android：name =android.intent.action.MAIN/> 
 
< category android：name =android.intent.category.LAUNCHER/> 
< / intent-filter> 
< / activity> 
< activity android：name =。sysinfo>< / activity> 
< activity android：name =。daten>< / activity> 
< activity android：name =。storage>< / activity> 
< activity android：name =org.achartengine.GraphicalActivity> < /活性GT; 
< / application> 
 
< / manifest>

并更新：

 <？xml version =1.0encoding =utf-8？> 
< manifest xmlns：android =http://schemas.android.com/apk/res/android
 package =com.spicysoftware.infoid
 android：versionCode = 1
 android：versionName =1.0> 
 
< uses-sdk 
 android：minSdkVersion =15
 android：targetSdkVersion =15/> 
 
< uses-permission android：name =android.permission.READ_EXTERNAL_STORAGE/> 
 
< application 
 android：allowBackup =true
 android：icon =@ drawable / ic_launcher
 android：label =@ string / app_name 
 android：theme =@ style / AppTheme> 
 
< activity 
 android：name =。MainActivity
 android：label =@ string / app_name> 
< intent-filter> 
< action android：name =android.intent.action.MAIN/> 
 
< category android：name =android.intent.category.LAUNCHER/> 
< / intent-filter> 
< / activity> 
< activity android：name =。sysinfo>< / activity> 
< activity android：name =。daten>< / activity> 
< activity android：name =。storage>< / activity> 
< activity android：name =org.achartengine.GraphicalActivity> < /活性GT; 
< / application> 
 
< / manifest>

解决方案

  f。 listFiles（）

如果路径不存在，则返回null。

请检查您的路径。

更新

Android还需要有权限读取某些文件。
您可能需要将其添加到您的清单中：

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

更多信息：

I have the code as shown here. My Problem is a NullPointerException at files.length

for(int i=0; i < files.length; i++){

It is caused because i have a "fail readDirectory() errno=13" at

File[] files = f.listFiles();

But why do I have a readDirectory fail when the path is good?

package com.example.androidexplorer;

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

import android.os.Bundle;
import android.os.Environment;
import android.app.AlertDialog;
import android.app.ListActivity;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;

public class MainActivity extends ListActivity {

    private List<String> item = null;
    private List<String> path = null;
    private String root;
    private TextView myPath;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myPath = (TextView)findViewById(R.id.path);

        root = Environment.getExternalStorageDirectory().getPath();
        getDir(root);
    }

    private void getDir(String dirPath)
    {
        myPath.setText("Location: " + dirPath);
        item = new ArrayList<String>();
        path = new ArrayList<String>();
        File f = new File(dirPath);

Log.v("Path: ", dirPath);

        Log.v("BEFORE", "Before Reading Fail...");
        File[] files = f.listFiles();
        Log.v("AFTER", "After Reading Fail...");


        if(!dirPath.equals(root))
        {
            item.add(root);
            path.add(root);
            item.add("../");
            path.add(f.getParent());    
        }

        Log.v("CRASH", "1 Line before crash");
        for(int i=0; i < files.length; i++){
            Log.v("AFTER CRASH", "1 Line after crash");
            File file = files[i];

            if(!file.isHidden() && file.canRead()){
                path.add(file.getPath());
                if(file.isDirectory()){
                    item.add(file.getName() + "/");
                }else{
                    item.add(file.getName());
                }
            }   
        }

        ArrayAdapter<String> fileList =
                new ArrayAdapter<String>(this, R.layout.row, item);
        setListAdapter(fileList);   
    }

    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        // TODO Auto-generated method stub
        File file = new File(path.get(position));

        if (file.isDirectory())
        {
            if(file.canRead()){
                getDir(path.get(position));
            }else{
                new AlertDialog.Builder(this)
                    .setIcon(R.drawable.ic_launcher)
                    .setTitle("[" + file.getName() + "] folder can't be read!")
                    .setPositiveButton("OK", null).show();  
            }   
        }else {
            new AlertDialog.Builder(this)
                    .setIcon(R.drawable.ic_launcher)
                    .setTitle("[" + file.getName() + "]")
                    .setPositiveButton("OK", null).show();

          }
    }

}

LogCat:

UPDATE

Error was in my manifest with the permissions. What I had at the beginning:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.spicysoftware.infoid"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="15"
        android:targetSdkVersion="15" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme"
         android:name="android.permission.READ_EXTERNAL_STORAGE">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".sysinfo"></activity>
        <activity android:name=".daten"></activity>
        <activity android:name=".storage"></activity>
        <activity android:name="org.achartengine.GraphicalActivity"> </activity>
    </application>

</manifest>

And Updated:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.spicysoftware.infoid"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="15"
        android:targetSdkVersion="15" />

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

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">

        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".sysinfo"></activity>
        <activity android:name=".daten"></activity>
        <activity android:name=".storage"></activity>
        <activity android:name="org.achartengine.GraphicalActivity"> </activity>
    </application>

</manifest>

解决方案

f.listFiles()

will return null if the path does not exist.

Please check your path.

Update

Android will also require a permission to read certain files.Maybe you need to add this to your manifest:

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

More information:

http://developer.android.com/reference/android/Manifest.permission.html

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