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Converting int to byte[] in android

I am using the write() method in order to write in a file of the external storage. This method only accepts byte[] as an input. I have tried passing a String and I get an error message ("The method write(int) in the type FileOutputStream is not applicable for the arguments String"). If I pass an int, I don't get error but in the file nothing is written. The value I get from calling getNumSentPackets() is an int and I need to convert it to byte[]. I have been looking at other questions already answered here and I have tried the ByteBuffer option but the result I get in the file is not what I want, this means, I don't get the number of sent packets. Can anybody help me, please?
This is my code:
public void createFile(String name) {
try {
String filename = name;
File myFile = new File(Environment.getExternalStorageDirectory(), filename);
if (!myFile.exists())
myFile.createNewFile();
String title = "FLOODING RESULTS FILE\n\n";
String sent = "Number of sent packets\n";
FileOutputStream fos;
byte[] data = title.getBytes();
byte[] intSent = sent.getBytes();
int numSent = mSender.getNumSentPackets();
byte[] numSentBytes = ByteBuffer.allocate(10).putInt(numSent).array();
try{
fos = new FileOutputStream(myFile);
fos.write(data);
fos.write(intSent);
fos.write(numSentBytes);
fos.flush();
fos.close();
} catch (Exception e) {
e.printStackTrace();
}
} catch (IOException e) {
e.printStackTrace();
}
}
public static int getNumSentPackets() {
return nSentPackets;
}
The expected output file would be as follows:
FLOODING RESULTS FILE
Number of sent packets 200
200 is only an example, meaning with this that I would like to see there a number which would correspond to the total number of sent packets.
Thank you in advance.

As I am a lazy developer, I like to use the existing facilities in my languages of choice, for example, for java, a PrintWriter.
public void createFile(String filename) {
try {
File myFile = new File(Environment.getExternalStorageDirectory(), filename);
PrintWriter out = new PrintWriter(myFile); // this will create the file if necessary
out.println("FLOODING RESULTS FILE");
out.println();
out.print("Number of sent packets ");
out.println(mSender.getNumSentPackets());
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
This is much easier to read and maintain than your current approach, and looks more idiomatic.

ByteBuffer.allocate(capacity).putInt(yourInt).array();

The text representation of "200" requires you to write 3 characters. All files are just a bunch of bytes in the end so there needs to be a mapping from character to some byte value. Assuming ASCII(*) the data to write into the file would be
// '2','0','0'
byte[] textVersion = { 50, 48, 48 }
int on the other hand is a 32bit numeric value, i.e. has 4 bytes and 200 is equivalent to
byte[] intVersion = { 0, 0, 0, 200 }
When using a ByteBuffer, you'll get this. If you write that into a file and a text viewer tries to display that it would display something like ◻◻◻Č if you're lucky. A 0 is actually a non printable control character and therefore often either skipped when printing or replaced with strange looking character like boxes. The 200 would be equivalent to Č in Windows-CP1250. It has no meaning on it's own when interpreted as UTF8 - it's the start of a 2 byte sequence and so the next 2 byte are required to determine which character to display.
You could have used
String.valueOf(200).getBytes( /* you should specify which encoding to use here */ );
which will create the "200" string first, then return you the required bytes for those 3 characters.
You should however use Java's character based IO facility: The numerous (and confusing) Reader & Writer implementations. They all(*^2) wrap an InputStream or OutputStream in the end and do the text to byte conversion for you.
PrintWriter is probably the most convenient to use but not without flaw: https://stackoverflow.com/a/15803472/995891
FileWriter should be avoided because you can't specify the encoding
The longer alternative route would be
BufferedWriter writer = new BufferedWriter(
new OutputStreamWriter(
new FileOutputStream(file), encoding));
writer.write("Hello ");
writer.write(String.valueOf(200));
writer.newLine();
(*) most encodings are ASCII compatible for the first 127 characters which basically covers normal english text.
(*^2) nothing forces a Writer to output the characters into a stream, e.g. StringWriter. But they are used mostly that way.

Dynamically load a class as byte array with ClassLoader in Android

I Am trying to load a class as a byte array so I could send it over the network and execute it remotely via Reflection. This Class (Bubble in this case) is in the same package. The thing is that I can't get the resource using the getResourceAsStream(classpath) method.
The .getResourceAsStream(classpath) is always returning null. I've tested this code in a Java project and worked properly. I think the problem is the resource path, does Android load a .class file?
private void doSomething() {
Bubble b = new Bubble();
try {
//Try to retrieve the class byte array
byte[] classBytes = getBytes(b.getClass());
} catch (IOException e) {
e.printStackTrace(System.err);
}
...
}
private byte[] getBytes(Class c) throws IOException{
ByteArrayOutputStream out = new ByteArrayOutputStream();
byte b[] = new byte[1024];
String classpath = c.getCanonicalName().replace('.', File.pathSeparatorChar) + ".class";
//classpath is now, for example, com:myproject:Bubble.class
InputStream in = c.getClassLoader().getResourceAsStream(classpath);
int load;
while((load=in.read(b))>0){
out.write(b,0,load);
}
byte[] _r = out.toByteArray();
out.close();
in.close();
return _r;
}

Android uses dex file format for classes, and the best way would be to not send zipped (jarred) classes.dex, which contains all classes you need.

How to wrap Uri content with a nio.ByteBuffer on Android?

I'm trying to read content from a Uri on Android, and I need the final Object type passed to the underlying SDK to by a nio.ByteBuffer.
I can get my hands on an InputStream, via ContentResolver but didn't find a way to wrap it with an nio.ByteBuffer.
Is there a way to convert a Uri content to a nio.ByteBuffer on Android?

I've ended up downloading the content of the Uri locally and open it via other method to get the ByteBuffer

Suppose you are working on an Activity,
private ByteBuffer getByteBuffer(Uri uri){
try{
InputStream iStream = getContentResolver().openInputStream(uri);
if(iStream!=null){
//value of MAX_SIZE is up to your requirement
final int MAX_SIZE = 5000000;
byte[] byteArr = new byte[MAX_SIZE];
int arrSize = 0;
while(true){
int value = iStream.read(byteArr);
if(value == -1){
break;
}else{
arrSize += value;
}
}
iStream.close();
return ByteBuffer.wrap(byteArr, 0, arrSize);
}
}catch(IOException e){
//do something
}
return null;
}
Notes:
(i) InputStream.read(byte[] b) will return an Integer which indicate total number of bytes read into the byte array b at each time.
(ii) If InputStream.read(Byte[] b) returns -1, it indicates that it is the end of the inputStream.
(iii) arrSize stores the total number of bytes read, i.e. the length of byte[] b
(iv) ByteBuffer.wrap(byte[] b, int offset, int length) will wrap the byte array to give a ByteBuffer. You may check this reference
(v) ContentResolver.openInputStream(Uri uri) and InputStream.read(byte[] b) will throw IOException so you must handle it.
(vi) Caution: IndexOutOfBoundException might happen if arrSize > MAX_SIZE, you may need to add if-else clause to handle such issue.
Please feel free to comment or change the code if there is any mistake or if there is a faster way to do that. Happy coding

Byte Array to Bundle

I have a Bundle and I store it to the disk as a byte array. Now, when I retrieve it I take the byte array. How can I convert this again to Bundle?
byte fileContent[] = new byte[(int)file.length()];
int numerOfReturnedbytes = 0;
try {
//read the stream and set it into the byte array readFileByteArray
//and returns the numerOfReturnedbytes. If returns -1 means that
//that the end of the stream has been reached.
numerOfReturnedbytes = fis.read(fileContent);
fis.close();
} catch (IOException e) {
e.printStackTrace();
}
if (numerOfReturnedbytes == -1) {
return;
} else {
//creating empty parcel object
Parcel parcel = Parcel.obtain();
//un-marshalling the data contained into the byte array to the parcel
parcel.unmarshall(fileContent, 0, numerOfReturnedbytes);
}
The fileContent is the byte array. Any ideas on how can I solve my problem?

To convert Bundle to ByteArray
public byte[] bundleToBytes(#NonNull Bundle bundle) {
Parcel parcel = Parcel.obtain();
parcel.writeBundle(bundle);
byte[] bytes = parcel.marshall();
parcel.recycle();
return bytes;
}
To convert ByteArray to Bundle
#NonNull
public Bundle bytesToBundle(byte[] bytes) {
Parcel parcel = Parcel.obtain();
parcel.unmarshall(bytes, 0, bytes.length);
parcel.setDataPosition(0);
Bundle bundle = parcel.readBundle(ClassWithinProject.class.getClassLoader());
parcel.recycle();
return bundle;
}

Don't do that. From the Android documentation:
Parcel is not a general-purpose serialization mechanism. This class (and the corresponding Parcelable API for placing arbitrary objects into a Parcel) is designed as a high-performance IPC transport. As such, it is not appropriate to place any Parcel data in to persistent storage: changes in the underlying implementation of any of the data in the Parcel can render older data unreadable.
This means, that after an OS upgrade the data written by your application could become unreadable.

Would it be something like :
Bundle bundle = Bundle.CREATOR.createFromParcel(parcel);
once you have the parcel ?
Edit
or is it
Bundle bundle = parcel.readParcelable(null);
? I don't remember. I'd read the documentation, but you know ...
(actually, i really don't know what is best, they appear to do pretty much the same thing)
Edit 2
There is also
Bundle bundle = parcel.readBundle();
Amazing the quantity of information in the documentation thingy. I should go there more often.

Android byte array to string to byte array

All I need is convert byte[] to String. Then do something with that string and convert back to byte[] array. But in this testing I'm just convert byte[] to string and convert back to byte[] and the result is different.
to convert byte[] to string by using this:
byte[] byteEntity = EntityUtils.toByteArray(entity);
String s = new String(byteEntity,"UTF-8");
Then i tried:
byte[] byteTest = s.getBytes("UTF-8");
Then i complared it:
if (byteEntity.equals(byteTest) Log.i("test","equal");
else Log.i("test","diff");
So the result is different.
I searched in stackoverflow about this but it doesn't match my case. The point is my data is .png picture so the string converted is unreadable. Thanks in advance.

Solved
Using something like this.
byte[] mByteEntity = EntityUtils.toByteArray(entity);
byte[] mByteDecrypted = clip_xor(mByteEntity,"your_key".getBytes());
baos.write(mByteDecrypted);
InputStream in = new ByteArrayInputStream(baos.toByteArray());
and this is function clip_xor
protected byte[] clip_xor(byte[] data, byte[] key) {
int num_key = key.length;
int num_data = data.length;
try {
if (num_key > 0) {
for (int i = 0, j = 0; i < num_data; i++, j = (j + 1)
% num_key) {
data[i] ^= key[j];
}
}
} catch (Exception ex) {
Log.i("error", ex.toString());
}
return data;
}
Hope this will useful for someone face same problem. Thanks you your all for helping me solve this.
Special thanks for P'krit_s

primitive arrays are actually Objects (that's why they have .equals method) but they do not implement the contract of equality (hashCode and equals) needed for comparison. You cannot also use == since according to docs, .getBytes will return a new instance byte[]. You should use Arrays.equals(byteEntity, byteTest) to test equality.

Have a look to the answer here.
In that case my target was transform a png image in a bytestream to display it in embedded browser (it was a particular case where browser did not show directly the png).
You may use the logic of that solution to convert png to byte and then to String.
Then reverse the order of operations to get back to the original file.