Android运行异常，看了一天了。。。

Question

06-07 17:12:42.750: E/AndroidRuntime(2237): FATAL EXCEPTION: AsyncTask #3
06-07 17:12:42.750: E/AndroidRuntime(2237): Process: com.example.clonetelegram, PID: 2237
06-07 17:12:42.750: E/AndroidRuntime(2237): java.lang.RuntimeException: An error occured while executing doInBackground()
06-07 17:12:42.750: E/AndroidRuntime(2237): at android.os.AsyncTask$3.done(AsyncTask.java:300)
06-07 17:12:42.750: E/AndroidRuntime(2237): at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
06-07 17:12:42.750: E/AndroidRuntime(2237): at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
06-07 17:12:42.750: E/AndroidRuntime(2237): at java.util.concurrent.FutureTask.run(FutureTask.java:242)
06-07 17:12:42.750: E/AndroidRuntime(2237): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
06-07 17:12:42.750: E/AndroidRuntime(2237): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
06-07 17:12:42.750: E/AndroidRuntime(2237): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
06-07 17:12:42.750: E/AndroidRuntime(2237): at java.lang.Thread.run(Thread.java:841)
06-07 17:12:42.750: E/AndroidRuntime(2237): Caused by: java.lang.NullPointerException
06-07 17:12:42.750: E/AndroidRuntime(2237): at com.example.rr.DownLoaderTask.download(DownLoaderTask.java:81)
06-07 17:12:42.750: E/AndroidRuntime(2237): at com.example.rr.DownLoaderTask.doInBackground(DownLoaderTask.java:73)
06-07 17:12:42.750: E/AndroidRuntime(2237): at com.example.rr.DownLoaderTask.doInBackground(DownLoaderTask.java:1)
06-07 17:12:42.750: E/AndroidRuntime(2237): at android.os.AsyncTask$2.call(AsyncTask.java:288)
06-07 17:12:42.750: E/AndroidRuntime(2237): at java.util.concurrent.FutureTask.run(FutureTask.java:237)
06-07 17:12:42.750: E/AndroidRuntime(2237): ... 4 more

Answer 1

Caused by: java.lang.NullPointerException空指针异常啊