Java – ArrayList of arrays, how to make the size of the last array different from other arrays in the list

Explain what I want to achieve:

I get a file. I have to read data from the file and create a block with a size of 1 KB For example, if the file size is 5.8 KB, I will have 5 blocks, 1 KB each, and the last block is 0.8 KB After having these in the block, I must perform Sha 256 encoding from the last block and attach it to the penultimate block, then I must apply the encoding to the penultimate block and attach it to the third block, and so on

problem

If I give more than 1024 bytes of file size, my code works well But if my last block size is not 1024, the code will not work as expected

The way I do this now is:

BufferedInputStream bis = new BufferedInputStream(new FileInputStream(f));
int sizeOfBlock = 1024;
int sizeOfHash = 256;

MessageDigest md;
md = MessageDigest.getInstance("SHA-256");

byte[] block = new byte[sizeOfBlock];

List <byte []> blockList = new ArrayList <byte []>();

int tmp = 0;
while ((tmp = bis.read(block)) > 0) {
    System.out.println(tmp);
    blockList.add(block);           
}       

for (int j = blockList.size()-1; j > 0;){
    System.out.println(blockList.get(j).length); // for the first iteration it shouldnt be 1024 if the file size is not a multiple of 1024
    md.update(blockList.get(j--));
    byte[] hash = md.digest();
    byte[] appendBlock = new byte[blockList.get(j).length + hash.length];

    System.arraycopy(blockList.get(j),appendBlock,blockList.get(j).length);
    System.arraycopy(md.digest(),blockList.get(j).length,hash.length);
    blockList.set(j,appendBlock);      
}

System.out.println(blockList.get(0).length);
md.update(blockList.get(0));
byte[] hash = md.digest();

String result = bytesToHex(hash); // converting function from byte to hex
System.out.println(result);

Solution

You seem to add the same 1024 byte array to the array list again and again

What you should do is as follows:

while ((tmp = bis.read(block)) > 0) {
    byte[] currentBlock = new byte[tmp];
    System.arraycopy (block,currentBlock,tmp);
    System.out.println(tmp);
    blockList.add(currentBlock);           
}