Example IO

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {
    static String filename = "myfile.txt";
    static int PAGE_SIZE = 512;
    static RandomAccessFile file;
    static byte ONE_BYTE = 1;

    public static void main(String[] args) throws IOException {
        file = new RandomAccessFile(new File(filename), "rw");

        // Table: isFull: 1, primarykey: studentid, tablename: student,
        // numberoffields: 3, fieldnames: studentid, name, surname, fieldsizes:
        // 10, 15, 15
        List<String> fields = new ArrayList<String>();
        fields.add("studentid");
        fields.add("name");
        fields.add("surname");

        List<Integer> fieldSizes = new ArrayList<Integer>();
        fieldSizes.add(10);
        fieldSizes.add(15);
        fieldSizes.add(15);

        Scanner scanner = new Scanner(System.in);
        String tablename = readAnything(scanner, 30, "Table name ");

        try {
            createTable(tablename, fields, fieldSizes, "studentid");
        } catch (MyDBException e) {
            System.err.println(e.getMessage());
        }

        // Get Table Data
        ByteBuffer readBuffer = readPage(0);
        byte first = readBuffer.get();
        if (first == ONE_BYTE) {
            String primaryKey = readString(readBuffer, 25);
            System.out.println(String.format("Primary key is %s", primaryKey));

            String tableName = readString(readBuffer, 30);
            System.out.println(String.format("Table name is %s", tableName));

            int numberOfFields = readBuffer.getInt();
            System.out.println(String.format("Number of fields: %d", numberOfFields));

            for (int i = 0; i < numberOfFields; i++) {
                String fieldName = readString(readBuffer, 25);
                System.out.println(String.format("%d. field name: %s", i, fieldName));
            }

            for (int i = 0; i < numberOfFields; i++) {
                int fieldSize = readBuffer.getInt();
                System.out.println(String.format("%d. field size: %d", i, fieldSize));
            }
        }

        file.close();
    }

    static ByteBuffer readPage(int pageId) throws IOException {
        file.seek(PAGE_SIZE * pageId);

        byte[] arr = new byte[PAGE_SIZE];
        file.read(arr);

        ByteBuffer buffer = ByteBuffer.wrap(arr);
        return buffer;
    }

    static byte[] stringToByteArray(int length, String s) {
        ByteBuffer buffer = ByteBuffer.allocate(length);
        buffer.put(s.getBytes());
        return buffer.array();
    }

    static String byteArrayToString(byte[] bytes) {
        int length = 0;
        for (int i = 0; i < bytes.length; i++) {
            if (bytes[i] == 0) {
                break;
            }
            length++;
        }
        return new String(bytes, 0, length);
    }

    static String readString(ByteBuffer buffer, int length) {
        byte[] bytes = new byte[length];
        buffer.get(bytes);
        return byteArrayToString(bytes);
    }

    static void writePage(int pageId, ByteBuffer buffer) throws IOException {
        file.seek(PAGE_SIZE * pageId);
        file.write(buffer.array());
    }

    static void createTable(String tablename, List<String> fields, List<Integer> fieldSizes, String primaryKey) throws IOException, MyDBException {
        if (tablename.getBytes().length > 30) {
            throw new MyDBException("Table name cannot be larger than 30 bytes!");
        }

        ByteBuffer buffer = ByteBuffer.allocate(PAGE_SIZE);
        buffer.put(ONE_BYTE);
        buffer.put(stringToByteArray(25, primaryKey));
        buffer.put(stringToByteArray(30, tablename));
        buffer.putInt(fields.size());
        for (String fieldName : fields) {
            buffer.put(stringToByteArray(25, fieldName));
        }

        for (Integer fieldSize : fieldSizes) {
            buffer.putInt(fieldSize);
        }

        writePage(0, buffer);
    }

    static String readAnything(Scanner scanner, int length, String type) {
        while (true) {
            String str = scanner.next();
            if (str.getBytes().length > length) {
                System.err.println(type + " cannot be longer than " + length + ", please enter again: ");
            } else {
                return str;
            }
        }
    }

    public static class MyDBException extends Exception {
        private static final long serialVersionUID = -5228951309945306582L;

        public MyDBException(String message) {
            super(message);
        }
    }
}