Solution: The Image Processor (PECS)

import java.util.ArrayList;
import java.util.List;
public class ImageProcessorSolution {
    static class Image {
        String filename;
        public Image(String name) { this.filename = name; }
        public String toString() { return "Image: " + filename; }
    }
    static class Jpeg extends Image {
        public Jpeg(String name) { super(name); }
        public String toString() { return "JPEG: " + filename; }
    }
    static class Png extends Image {
        public Png(String name) { super(name); }
        public String toString() { return "PNG: " + filename; }
    }
    // Generic method using PECS (Producer Extends, Consumer Super)
    public static <T> void transferImages(List<? extends T> source, List<? super T> destination) {
        for (T img : source) {
            destination.add(img);
        }
    }
    public static void main(String[] args) {
        List<Jpeg> cameraRoll = new ArrayList<>();
        cameraRoll.add(new Jpeg("photo.jpg"));
        List<Png> scannerTray = new ArrayList<>();
        scannerTray.add(new Png("scan.png"));
        List<Image> processQueue = new ArrayList<>();
        // T is inferred as 'Image'
        // Works for Jpeg (Producer Extends Image)
        transferImages(cameraRoll, processQueue);
        // Works for Png (Producer Extends Image)
        transferImages(scannerTray, processQueue);
        System.out.println("Queue contains: " + processQueue);
    }
}