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[英]Android - java.lang.NullPointerException when loading files for the second time
[英]Android loading files into array NullPointerException
嗨,我正在嘗試將一組“列表”加載到String數組中,並且列表只是.txt文檔,我想使用文件名作為顯示器上的列表名稱,因此,我需要獲取所有我創建的文件夾中名為“列表”的文件,然后將它們顯示在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";
}
根據堆棧跟蹤的相關代碼行(第108行)
if(lists.length> 0){
堆棧跟蹤
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)
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() );
}
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();
}
}
您需要記錄files.getFileList()
是否真的返回任何東西:
也
改變這個:
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
數組最初為null
,則無法獲得長度。
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