Writing data back to binary file

Question

I am trying to write char, double and integer back to my binary file base on my String arraylist. However, after finishing writing and I read the binary file again it generates erros. Anyone could help with this I really appreciate it.

ArrayList<String>temp = new ArrayList<String>();
    for(int i = 0;i<temp.size();i++){
                String decimalPattern = "([0-9]*)\\.([0-9]*)";  
                boolean match = Pattern.matches(decimalPattern, temp.get(i));
                if(Character.isLetter(temp.get(i).charAt(0))){
                    os.writeChar(temp.get(i).charAt(0));
                }
                else if(match == true){
                    Double d = Double.parseDouble(temp.get(i));
                    os.writeDouble(d);
                }       
                else
                {
                 int in = Integer.parseInt(temp.get(i));
                 os.writeInt(in);
                }
            }
            os.close();
         }

Answer 1

This is a very simple example which will show you how to read your data sequentially:

public void readFile(String absoluteFilePath){
    ByteBuffer buf = ByteBuffer.allocate(2+4+8) // creating a buffer that is suited for data you are reading
    Path path = Paths.get(absoluteFilePath);

    try(FileChannel fileChannel = (FileChannel)Files.newByteChannel(path,Enum.setOf(READ))){
        while(true){
            int bytesRead = fileChannel.read(buf);
            if(bytesRead==-1){
                break;
            }
            buf.flip(); //get the buffer ready for reading.
            char c = buf.asCharBuffer().readChar(); // create a view buffer and read char
            buf.position(buf.position() + 2); //now, lets go to the int
            int i = buf.asIntBuffer().readInt(); //read the int
            buf.position(buf.position()+ 4); //now, lets go for the double.
            double d = buf.asDoubleBuffer().readDouble();
            System.out.println("Character: " + c + " Integer: " + i + " Double: " + d);
            buf.clear();
        }
    }catch(IOException e){
        e.printStackTrace();
    }// AutoClosable so no need to explicitly close
}  

---

Now, assuming that you always write data as (char,int,double) into your file and that there are no chances where your data would be out of order or incomplete as (char,int) / (char,double), you can simply read the data randomly by specifying the position in the file, measured in bytes, from where to fetch the data as:

channel.read(byteBuffer,position);  

In your case, the data is always 14 bytes in size as 2 + 4 + 8 so all your read positions will be multiples of 14.

First block at: 0 * 14 = 0  
Second block at: 1 * 14 = 14  
Third block at: 2 * 14 = 28

and so on..

Similar to reading, you can also write using

channel.write(byteBuffer,position)  

Again, positions will be multiples of 14.

This applies in case of FileChannel which is a super class of ReadableByteChannel which is a dedicated channel for reading, WritableByteChannel which is a dedicated channel for writing and SeekableByteChannel which can do both reading and writing but is a little bit more complex.

When using channels and ByteBuffer, take care as to how you read. There is nothing to prevent me from reading 14 bytes as a set of 7 characters. Although this looks scary, this gives you complete flexibility on how you read and write data