storing an integer in a specific number of bytes in an array of bytes - JAVA

Question

I'm trying to implement a database in java using slotted pages , so basically what I want to do is to store my data in a specific number of bytes . so this is the page where I have to store it .

    protected byte[] myData = new byte[PAGE_SIZE*1024]; //array for storing my data

now I want to store an Integer in the first 4 bytes of myData , when I do that automatically is stored in just one byte if the Integer doesn't exceed 255 , but what I want to do is use 4 bytes for my Integer it doesn't matter if it's 1 or one billion .

my question is , is it possible to do that in java ? to control how many bytes my data must allocate , like I assign 3 to the first 4 bytes of my byte array ?.

        if (recordFitsIntoPage(record)) {

            byte [] fix_rec = new byte [record.getFixedLength()];
            byte [] var_rec= new byte [record.getVariableLength()];

            var_rec = var_rec(record);
            fix_rec = fix_rec(record);

            byte  [] box = { (byte) record.getVariableLength() ,(byte) offsetEnd };
            System.arraycopy(fix_rec, 0,data,offset,record.getFixedLength());
            System.arraycopy(var_rec, 0,data,offsetEnd,record.getVariableLength());
            read_bytes(data);
            this.numRecords++;
            }else {

                throw new  Exception("no more space left");

            }

I have a fixed-sized variables that I need to store them in my case for example in 12 bytes , I have been using System.arraycopy() but it's not relevant in my case , after I execute the code I get out of bound exception "last source index 12 out of bounds for byte[9]" because it uses just 9 bytes to store my Data not 12 .

Details matter. In java, there is *i*nt and **I**nteger. Which one is it here? Ther is no **i**nteger. — GhostCat, Nov 11 '19 at 14:08
And have you done some prior research, and looked at questions like https://stackoverflow.com/questions/1936857/convert-integer-into-byte-array-java ? — GhostCat, Nov 11 '19 at 14:08
Rather than describe your work, show it. It's not clear how you are converting those integers, if you are using anything like `ByteBuffer`, for example. If you wrote something that stores only one byte, you should show us what it is. — RealSkeptic, Nov 11 '19 at 14:23
Do I understand correctly that you want to be able to write an int value as a variable number of bytes, depending on its value? How will you know how many bytes each value uses when you read the data later? For instance, if you have an array with two `0xff` bytes, does that represent a single int with the value `0xffff`, or does it represent two ints which each have the value `0xff`? — VGR, Nov 11 '19 at 16:12
it doesn't depend really on its value for me it's fixed with 4 bytes , so the problem was how can I write an int in 4 bytes in an array of bytes , @V — ishak lahlouh, Nov 11 '19 at 16:30

dguy · Answer 1 · 2019-11-11T15:50:45.633

This method creates an array of 32 bytes of any integer given - be it 1 or one billion:

private static byte[] bigIntegerToBytes(BigInteger b, int numBytes) {
        byte[] src = b.toByteArray();
        byte[] dest = new byte[numBytes];
        boolean isFirstByteOnlyForSign = src[0] == 0;
        int length = isFirstByteOnlyForSign ? src.length - 1 : src.length;
        int srcPos = isFirstByteOnlyForSign ? 1 : 0;
        int destPos = numBytes - length;
        System.arraycopy(src, srcPos, dest, destPos, length);
        return dest;
    }

You have an array of byte ready to store:

    byte[] myData = new byte[PAGE_SIZE*1024];

You have a hand-picked integer as well:

    BigInteger myInteger = new BigInteger("50000000000");

Then we change our integer to 32-length byte[]

    byte[] bytesOfInteger = bigIntegerToBytes(myInteger,32);

Finally, you copy first 4 bytes of integer to your byte[] myData

    System.arraycopy(bytesOfInteger, 0, myData, 0, 3);

So this shows that you can allocate any decent big integer into a fixed 32 byte[].

storing an integer in a specific number of bytes in an array of bytes - JAVA

1 Answers1