Question

I have a question that, Iam xoring the first 2^21 bytes of a video file with an another byte array, but it gives a Null Pointer Exception while Xoring the bytes, I don’t know why? Please suggest me the right answer of the same.

Thanks in advance.

Error Stack:

11-04 12:28:49.283: ERROR/AndroidRuntime(506): FATAL EXCEPTION: main
11-04 12:28:49.283: ERROR/AndroidRuntime(506): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example/com.example.TestCryptoActivity}: java.lang.NullPointerException
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.app.ActivityThread.access$2300(ActivityThread.java:125)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.os.Handler.dispatchMessage(Handler.java:99)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.os.Looper.loop(Looper.java:123)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.app.ActivityThread.main(ActivityThread.java:4627)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at java.lang.reflect.Method.invokeNative(Native Method)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at java.lang.reflect.Method.invoke(Method.java:521)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at dalvik.system.NativeStart.main(Native Method)
11-04 12:28:49.283: ERROR/AndroidRuntime(506): Caused by: java.lang.NullPointerException
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at com.example.TestCryptoActivity.onCreate(TestCryptoActivity.java:132)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627)
11-04 12:28:49.283: ERROR/AndroidRuntime(506):     ... 11 more

Code:

String str;
    StringBuffer strBuf = new StringBuffer();
    String episode="E0022505.mp4";
    int bytesRead;

    InputStream is;
    byte[] bytesafterXor;
    byte[] bytes;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        File file = new File("/sdcard/E0022505.mp4");
        try {
               is = new FileInputStream(file);
        } catch (FileNotFoundException e2) {
               e2.printStackTrace();
        }
        byte[] fileData = new byte[2097152];
        int read = 0;
        while(read != fileData.length) {
           try {
            read += is.read(fileData, read, fileData.length - read);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        }
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        bos.write(fileData,0,read);
        bytes = bos.toByteArray();
        try {
            String byteString = new String(bytes,"UTF-8");
            System.out.println("the bytes array of video:"+byteString);
        } catch (UnsupportedEncodingException e1) {
            // TODO Auto-generated catch block
            e1.printStackTrace();
        }
        String data = "xyzzy734499639E0022505@a2+;%d3-";


        try {
            str = getHashCode(data);
            strBuf.append(str);
        } catch (NoSuchAlgorithmException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }catch(Exception e)
        {
            e.printStackTrace();
        }


        for(int i=1;i<31;i++)
        {
            if(i<10)
            {
                str = String.valueOf(30)+String.valueOf(30+i)+str;
                try {
                    str = new String(getHashCode(str));
                } catch (NoSuchAlgorithmException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }else
            {
                char[] num = String.valueOf(i).toCharArray();
                String firstIndex = String.valueOf(num[0]);
                String secondIndex = String.valueOf(num[1]);
                str = firstIndex + secondIndex+ str;
                try {
                    str = new String(getHashCode(str));
                } catch (NoSuchAlgorithmException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            strBuf.append(str);
            System.out.println("The final string after hashing:"+strBuf.toString());
        }
        for(int j=0;j<bytes.length;j+=1024)
        {
            String str = strBuf.toString();
            byte[] newByte = str.getBytes();
            for(int k=0;k<str.getBytes().length;k++)
            {
                bytesafterXor[k] = (byte)((newByte[k]^bytes[k])& 0x000000ff);// this is the error position
                //System.out.println("The result after Xoring:"+bytesafterXor[k]);
            }
        }
    }

Answer 1

EDIT: Okay, I’ve just seen the comment saying “this is the error position” – it was mostly hidden, which made it harder to see.

The answer is simple – you’ve never allocated bytesafterXor. You need something like:

bytesafterXor = new byte[newByte.length];

just before the k loop.

However, you should also read the rest of this answer to correct other bad practices.

---

You’ve posted a lot of code without showing where the stack trace is pointing, but I suspect the problem could well be due to one of the several blocks you’ve got like this:

try {
       is = new FileInputStream(file);
} catch (FileNotFoundException e2) {
       e2.printStackTrace();
}

If the file isn’t found, is is still going to be null afterwards… leading to a NullPointerException when you try to use it. Basically whenever you run into an exception, you’re currently just trying to carry on regardless, with no attempt to take account of the fact that the previous operation has just failed.

I suggest you create a separate method which is declared to throw IOException (and anything else which might be thrown) and call that from your onCreate method. Then have just one catch block (in onCreate) so that if anything goes wrong in the “inner” method, you don’t keep trying to carry on with it regardless.

Additionally:

• I see no reason for is (etc) to be instance variables rather than local variables
• You should close all your streams and any other unmanaged resources in finally blocks
• Your code would be considerably easier to understand and debug if you refactored it into small methods.
• I’m not an Android developer, but I doubt that doing all of this work in an onCreate method is good practice anyway – shouldn’t you be doing this in a background thread?

• Using String.getBytes() without specifying an encoding is generally a bad idea, and this:

  for(int k=0;k<str.getBytes().length;k++)
  

  … is going to create a new byte array on every iteration. You’ve created a byte array (newByte) – why aren’t you using newByte.length?

• It’s not clear what this loop is meant to achieve:

  for(int j=0;j<bytes.length;j+=1024)
  

  given that you’re not using the value of j within the loop…