Como usar RandomAccesFile ? Preciso de Metadata?

public void insere(Usuario umUsuario){
		RandomAccessFile file;
		try {
			file = new RandomAccessFile(nomeDoArquivo,"rw");
			FileChannel fc = file.getChannel( );
			fc.position(fc.size());
			
			ByteBuffer bb = ByteBuffer.allocateDirect (20);
			byte[] userName = umUsuario.getUserName().getBytes(); 
			bb.put(userName);
			fc.write(bb);

			bb = ByteBuffer.allocate(10);
			byte[] password = umUsuario.getPassword().getBytes(); 
			bb.put(userName);
			fc.write(bb);
			
			fc.close();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		
		

	}
	
	public Usuario busca(long pos){
		RandomAccessFile file;
		pos--;
		Usuario result = new Usuario();
		try {
			file = new RandomAccessFile(nomeDoArquivo,"rw");
			FileChannel fc = file.getChannel( );
			fc.position(1);
			
			ByteBuffer bb = ByteBuffer.allocateDirect (20);
			fc.read(bb);
			bb.rewind();
			byte[] userName = new byte[20];
			bb.put(userName);
			result.setUserName(new String(userName));
			
			bb = ByteBuffer.allocateDirect (10);
			fc.read(bb);
			bb.rewind();
			byte[] password = new byte[10];
			bb.get(password);
			result.setUserName(new String(password));
			
			fc.close();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} finally{
			return result;
		}