Android将文件加载到数组NullPointerException

Question

嗨，我正在尝试将一组“列表”加载到String数组中，并且列表只是.txt文档，我想使用文件名作为显示器上的列表名称，因此，我需要获取所有我创建的文件夹中名为“列表”的文件，然后将它们显示在arrayadapter中。

private String[] getListNames() {
    //generates the file containing the list names
    File file = new File(this.getFilesDir() +File.separator +"Lists"); 
    System.out.println(file.getAbsolutePath());
    File lists[] = file.listFiles();
    String names[] = {};


    if(lists.length >0){

    for(int i = 0; i < lists.length; i ++){
        names[i] = lists[i].getName();
    }
    }else{
        names[0] = "Create New List";
    }

根据堆栈跟踪的相关代码行（第108行）

if（lists.length> 0）{

堆栈跟踪

03-25 04:37:16.981: E/AndroidRuntime(2099): Caused by: java.lang.NullPointerException
03-25 04:37:16.981: E/AndroidRuntime(2099):     at dev.shaw.MyShoppingPlanner.List_Activity.getListNames(List_Activity.java:108)

Answer 1

private List<String> myList = new ArrayList<String>();   

String root_sd = Environment.getExternalStorageDirectory().toString();
file = new File( root_sd + "/Lists" ) ;       
File list[] = file.listFiles();

for( int i=0; i< list.length; i++)
{
        myList.add( list[i].getName() );
}

Answer 2

try this one.

//use two boolean variables.
 File file;
boolean mExternalStorageAvailable = false;
boolean mExternalStorageWriteable = false;

//check if the sdcard is enable or disable in your device

String state = Environment.getExternalStorageState();

        if (Environment.MEDIA_MOUNTED.equals(state)) {
            mExternalStorageAvailable = mExternalStorageWriteable = true;
        } else if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
            mExternalStorageAvailable = true;
            mExternalStorageWriteable = false;
        } else {
            mExternalStorageAvailable = mExternalStorageWriteable = false;
        }

    private String[] getListNames() {
 if (mExternalStorageAvailable) {
File path = new File(Environment.getExternalStorageDirectory().getAbsolutePath());
if (!path.exists()) {
path.mkdirs();
}                   
file=path.getAbsolutePath();

} 
else {
//use   ContextWrapper it a onw way to access internal storage.    
   ContextWrapper cw = new ContextWrapper(getApplicationContext());     
(or)
        //generates the file containing the list names
file = new File(this.getFilesDir() +File.separator +"Lists"); 
if (!file.exists()) {
file.mkdirs();
}
        System.out.println(file.getAbsolutePath());
   }

   File lists[] = file.listFiles();
     String names[] = {};                   
    try{
        if (lists.length==0)
    {
      System.out.print("There is no files in the Directory");
    }
      else{

        if(lists.length >0){

        for(int i = 0; i < lists.length; i ++){
            names[i] = lists[i].getName();
        }
        }else{
            names[0] = "Create New List";
        }
    catch(Exception e)
    {
    e.printStackTrace();
    }
    }

Answer 3

您需要记录files.getFileList()是否真的返回任何东西：

也

改变这个：

File lists[] = file.listFiles();
String names[] = {};


if(lists.length >0){

for(int i = 0; i < lists.length; i ++){
    names[i] = lists[i].getName();
}
}else{
    names[0] = "Create New List";
}

至

File lists[] = file.listFiles();
String names[] = {};

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

for(int i = 0; i < lists.length; i ++){
    names[i] = lists[i].getName();
}
}else{
    names[0] = "Create New List";
}

PS：如果lists数组最初为null ，则无法获得长度。