What is the best way to create byte data structure not fixed size in JAVA

You can implement this with an ArrayList under the hood. ArrayList is an array that expands when more data is added than capacity permits.

Your ByteCache might look like

public class ByteCache {

    ArrayList<Byte> backing = new ArrayList<Byte>();

    public ByteCache(){
    }

    public ByteCache(Byte[] bytes){
        add(bytes);
    }

    public void add(Byte[] bytes){
        for(Byte b : bytes){
            backing.add(b);
        }
    }

    public int length(){
        return backing.size();
    }

    public Byte[] get(int offset, int length){
        if(offset < 0 || length < 1){
            return null;
        }

        Byte[] toRet = new Byte[length];

        for(int i = offset; i < offset + length; i++){
            if(i == backing.size()){
                break;
            }
            toRet[i - offset] = backing.get(i);
        }
        return toRet;
    }
}

You would need to implement your own get() and add() methods, but for length() a call to ArrayList's correct method should be enough.

P.S. ArrayList doesn't exactly expand - a new array is created that is double the size and all items are copied over.