Przykład zastosowania RandomAccessFile

package com.company;

import java.io.IOException;
import java.io.RandomAccessFile;

public class Main {

    public static void write_raf(String path) throws IOException {
        RandomAccessFile file = null;
        try {
            // "r", "rw", "rws", "rwd"
            file = new RandomAccessFile(path, "rw");

            String imie = "Adam";
            String nazwisko = "Kowalski";
            int wiek = 50;
            char plec = 'M';
            double pensja = 2000;

            file.writeUTF(imie);
            file.writeUTF(nazwisko);
            file.writeInt(wiek);
            file.writeChar(plec);
            file.writeDouble(pensja);

            imie = "Julia";
            nazwisko = "Nowak";
            wiek = 30;
            plec = 'K';
            pensja = 2000;

            file.writeUTF(imie);
            file.writeUTF(nazwisko);
            file.writeInt(wiek);
            file.writeChar(plec);
            file.writeDouble(pensja);

        } finally {
            if (file != null) {
                file.close();
            }
        }
    }

    public static void read_raf(String path) throws IOException {
        RandomAccessFile file = null;
        try {
            // "r", "rw", "rws", "rwd"
            file = new RandomAccessFile(path, "rw");

            while (file.getFilePointer() < file.length()) {
                String imie = file.readUTF();
                String nazwisko = file.readUTF();
                int wiek = file.readInt();
                char plec = file.readChar();
                double pensja = file.readDouble();

                System.out.printf("Imię: %s\nNazwisko: %s\nWiek: %d\nPensja: %.2f\n", imie, nazwisko, wiek, pensja);
            }

        } finally {
            if (file != null) {
                file.close();
            }
        }
    }

    public static void modify_raf(String path) throws IOException {
        RandomAccessFile file = null;
        try {
            // "r", "rw", "rws", "rwd"
            file = new RandomAccessFile(path, "rw");

            while (file.getFilePointer() < file.length()) {
                String imie = file.readUTF();
                String nazwisko = file.readUTF();
                int wiek = file.readInt();
                char plec = file.readChar();
                long poz = file.getFilePointer();
                double pensja = file.readDouble();

                if (plec == 'K') {
                    pensja = pensja * 2.0;

                    file.seek(poz);
                    file.writeDouble(pensja);
                }
            }

        } finally {
            if (file != null) {
                file.close();
            }
        }
    }

    public static void complex_mult(double [] u, double [] v, double [] res) {
        double a = u[0], b = u[1], c = v[0], d = v[1];
        res[0] = a * c - b * d;
        res[1] = a * d + b * c;
    }

    public static void complex_add(double [] u, double [] v, double [] res) {
        double a = u[0], b = u[1], c = v[0], d = v[1];
        res[0] = a + c;
        res[1] = b + d;
    }

    public static double complex_abs(double [] u) {
        return Math.sqrt(u[0] * u[0] + u[1] * u[1]);
    }

    public static void write_julia(String path) throws IOException {
        RandomAccessFile plik = null;
        try {
            plik = new RandomAccessFile(path, "rw");

            int w = 800;
            int h = 800;

            double re_min = -2.0;
            double re_max = 2.0;
            double im_min = -2.0;
            double im_max = 2.0;

            double [] c = new double [] { 0.0, 0.67 };

            plik.writeBytes("P5\n# Julia image\n");
            plik.writeBytes(w + " " + h + " 255\n");

            for (int i = 0; i < h; i++) {
                double im = (im_max - im_min) / h * i + im_min;
                for (int j = 0; j < w; j++) {
                    double re = (re_max - re_min) / w * j + re_min;
                    double [] z = new double [] { re, im };
                    int n = 255;
                    while ( complex_abs(z) < 10 && n >= 5 ) {
                        complex_mult(z, z, z);
                        complex_add(z, c, z);
                        n -= 5;
                    }
                    plik.writeByte(n);
                }
            }

        } finally {
            if (plik != null) {
                plik.close();
            }
        }
    }


    public static void write_circle(String path) throws IOException {
        RandomAccessFile plik = null;
        try {
            plik = new RandomAccessFile(path, "rw");
            plik.writeBytes("P5\n");
            int promien = 200;
            plik.writeBytes((promien*2) + " " + (promien*2) + " 255\n");

            int k = 0;
            for (int i = 0; i < 2 * promien; i++) {
                for (int j = 0; j < 2 * promien; j++) {
                    int x = i - promien;
                    int y = j - promien;
                    double odl = Math.sqrt(x * x + y * y);
                    //double v = odl < promien ? 1 : 0 ;
                    double v = (promien - odl) / promien;
                    byte kolor = (byte)Math.max(0, Math.min(255, v * 255));
                    plik.writeByte(kolor);
                    ++k;
                }
            }
        } finally {
            if (plik != null) {
                plik.close();
            }
        }
    }

    public static void main(String[] args) throws IOException {
        write_raf("dane.dat");

        modify_raf("dane.dat");

        System.out.println("Po modyfikacji: ");
        read_raf("dane.dat");

        write_circle("circle.pgm");
        write_julia("julia.pgm");
    }
}