Question

我需要一种方法来向我展示ListView中存储的所有.txt文件。

所以，我写了这个函数但应用程序崩溃了。为什么？

主要活动：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    lv = (ListView) findViewById(R.id.lvPlaylist);

    if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) ||  Environment.MEDIA_MOUNTED_READ_ONLY.equals(Environment.getExternalStorageState())) {
        ArrayList<File> mySongs = findTxT(Environment.getExternalStorageDirectory());
    }
}

public ArrayList<File> findTxT(File root){

    ArrayList<File> al = new ArrayList<File>();

    File[] files = root.listFiles();
    for(File singleFile : files){
       if(singleFile.isDirectory()){

       }else{
           if(singleFile.getName().endsWith(".txt")){
               al.add(singleFile);

           }

       }
    }

    return al;
}

activity_main.xml中：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/lvPlaylist"
        android:layout_alignParentTop="true"
        android:choiceMode="singleChoice" />

</RelativeLayout>

清单：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.shon.rodry.elegantxt" >

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/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>
    </application>

</manifest>

堆栈跟踪：

10-20 09:38:50.320    3405-3405/? I/art﹕ Late-enabling -Xcheck:jni
10-20 09:38:50.387    3405-3405/com.shon.rodry.elegantxt W/System﹕ ClassLoader referenced unknown path: /data/app/com.shon.rodry.elegantxt-1/lib/arm
10-20 09:38:50.620    3405-3405/com.shon.rodry.elegantxt D/AndroidRuntime﹕ Shutting down VM
10-20 09:38:50.625    3405-3405/com.shon.rodry.elegantxt E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.shon.rodry.elegantxt, PID: 3405
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.shon.rodry.elegantxt/com.shon.rodry.elegantxt.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.shon.rodry.elegantxt.MainActivity.findTxT(MainActivity.java:34)
            at com.shon.rodry.elegantxt.MainActivity.onCreate(MainActivity.java:25)
            at android.app.Activity.performCreate(Activity.java:6245)
            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)

Answer 1

该方法应如下所示：

public ArrayList<File> findTxT(File root){
    ArrayList<File> al = new ArrayList<File>();
    File[] files = root.listFiles();
    if(files == null) //This check will fix the crash
        return al;
    for(File singleFile : files){
       if(singleFile.isDirectory()){

       }else{
           if(singleFile.getName().endsWith(".txt")){
               al.add(singleFile);
           }
       }
    }

    return al;
}

您仍然需要找出数组为空的原因。您可能要么传递错误的根文件，要么可能是权限问题（权限未添加到清单和/或api 23 +上不允许的权限）。

简单的功能，显示存储的.txt文件

1 个答案: