read-cifar10.java

import java.awt.Color;
import java.awt.image.BufferedImage;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

import javax.imageio.ImageIO;

public class Main {

    public static void main(String[] args) throws IOException {
        FileInputStream inputStream = new FileInputStream("./data_batch_1.bin");
        byte[] b = new byte[3073];
        inputStream.read(b);

        BufferedImage image = new BufferedImage(32, 32, BufferedImage.TYPE_INT_RGB);
        for (int row = 0; row < 32; row++) {
            for (int col = 0; col < 32; col++) {
                Color color = new Color(
                        b[1 + 1024 * 0 + row * 32 + col] & 0xFF,
                        b[1 + 1024 * 1 + row * 32 + col] & 0xFF,
                        b[1 + 1024 * 2 + row * 32 + col] & 0xFF);
                image.setRGB(col, row, color.getRGB());
            }
        }

        boolean result = ImageIO.write(image, "jpeg", new FileOutputStream("./out.jpg"));
        if (!result) {
            System.err.println("failed");
        }
    }
}

write-cifar10.java

import java.awt.Color;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;

import javax.imageio.ImageIO;

public class Main {

    public static void main(String[] args) throws IOException {
        BufferedImage bufferedImage = ImageIO.read(new File("./image.jpg"));
        assert bufferedImage.getHeight() == 32;
        assert bufferedImage.getWidth() == 32;
        assert bufferedImage.getColorModel().getNumComponents() == 3;

        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        outputStream.write((byte) 0);
        for (int i = 0; i < 3; i++) {
            for (int row = 0; row < bufferedImage.getHeight(); row++) {
                for (int col = 0; col < bufferedImage.getWidth(); col++) {
                    Color color = new Color(bufferedImage.getRGB(col, row));
                    switch (i) {
                    case 0:
                        outputStream.write(color.getRed());
                        break;
                    case 1:
                        outputStream.write(color.getGreen());
                        break;
                    case 2:
                        outputStream.write(color.getBlue());
                        break;
                    default:
                        break;
                    }
                }
            }
        }
        assert outputStream.size() == 3073;
        outputStream.writeTo(new FileOutputStream("./out.bin"));
    }
}