I do not know why "NullPointerException" will occur

Question

I am trying to write to the SD card using SAF on the android terminal. In my code, after creating a file on the SD card, I check the status of the file, but "NullPointerException" occurred at that time by some users. Despite being just after creating the file, I am struggling without knowing why it is null.

So is this, but now follows a code block

Error occurs at "if(newFile.canWrite()){".:

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
getContentResolver().takePersistableUriPermission(data.getData(),
                        Intent.FLAG_GRANT_READ_URI_PERMISSION | 
Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
}

try {
    countDownLatch = new CountDownLatch(1);
        new Thread(new Runnable() {
        @Override
            public void run() {
                try {
                    Thread.sleep(300);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                countDownLatch.countDown();
            }
        }).start();

    countDownLatch.await();

} catch (InterruptedException e) {
    e.printStackTrace();
}
target_filename= String.valueOf(System.currentTimeMillis());
treeUri = data.getData();
DocumentFile pickedDir = DocumentFile.fromTreeUri(this, treeUri);

// Create a new file and write into it
newFile = pickedDir.createFile("text", target_filename);
for(int i=0;i<10;i++){
   newFile=pickedDir.findFile(target_filename);
   int wait_sd=0;
   countDownLatch = new CountDownLatch(1);
   do{
   if(newFile.canWrite()){
               wait_sd=10;
                    }else{
                        try {
                            countDownLatch.await(500, TimeUnit.MILLISECONDS);
                            wait_sd++;
                        } catch (InterruptedException e) {
                            // TODO 自動生成された catch ブロック
                            e.printStackTrace();
                        }
                    }
                }while(wait_sd<10);

            }

score 0 · Answer 1 · answered Jan 21 '18 at 05:44

0

Are you sure from permission. I think it may be the permission

answered Jan 21 '18 at 05:44

Nima Mohammadi

345
2
9

Permission is acquired. This code is the processing after receiving the result of permission with activityresult. – funash Jan 21 '18 at 07:06

score 0 · Answer 2 · answered Jan 21 '18 at 06:00

0

Null pointer usually occurs when you forget to initialize a variable

or when you initialize it via a method that is returning null

in your case

newFile = pickedDir.createFile("text", target_filename); //returning null

It's returning null.

answered Jan 21 '18 at 06:00

Ratul Bin Tazul

2,050
1
13
22

Does this mean that "pickedDir.createFile" does not work well? – funash Jan 21 '18 at 07:03
In the code mentioned in the question, (String) targetfile is written just before createfile, but actually it is defined when clicking another button, after rewriting via OnActivityResult after getting permission to write to the SD card. Does this make targetfile null? – funash Jan 21 '18 at 07:16

I do not know why "NullPointerException" will occur

2 Answers2