Re-assembling the "click and drag" xkcd comic

Assembler.java

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.RenderingHints;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;


public class Assembler {

	static int N = 48; 
	static int SIZE = 200;
	
	static int imagesUsed; 
	
	public static void main(String[] args) throws IOException {
		BufferedImage img = new BufferedImage( SIZE*N*2, SIZE*N*2, BufferedImage.TYPE_INT_ARGB ); 
		
		int center = SIZE*N; 
		place( img, "n", "e", center-SIZE, center, 1, -1 ); 
		place( img, "s", "e", center-SIZE, center-SIZE, 1, 1 ); 
		place( img, "n", "w", center, center, -1, -1 ); 
		place( img, "s", "w", center, center-SIZE, -1, 1 ); 
		
		System.out.println( "writing to disk ..." ); 
		ImageIO.write( img, "png", new File( "giant-" + System.currentTimeMillis() + ".png" ) );
		System.out.println( "used " + imagesUsed + " images" ); 
	}
	
	private static void place( BufferedImage dest, String ns, String ew, int startX, int startY, int xInc, int yInc ){
		Graphics2D g = dest.createGraphics();
		g.setRenderingHint( RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON );
		g.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR );
		
		for( int i = 1; i <= N; i++ ){
			for( int j = 1; j <= N; j++ ){
				String filename = "images/" + i + ns + j + ew + ".png";
				File src = new File( filename ); 
				if( src.exists() && src.length() > 10 ){
					System.out.println( "place " + filename );
					
					try {
						BufferedImage img = ImageIO.read( src );
						System.out.println( (startX+xInc*SIZE*j) + ": " + (startY+yInc*SIZE*i)); 
						g.drawImage( img, startX+xInc*SIZE*j,startY+yInc*SIZE*i, SIZE, SIZE, null );
						img.flush();
						imagesUsed ++; 
					}
					catch (IOException e) {
						e.printStackTrace();
					} 
				}
				else{
					g.setColor( ns == "n"? Color.white : Color.black ); 
					g.fillRect(startX+xInc*SIZE*j,startY+yInc*SIZE*i, SIZE, SIZE);
				}
			}
		}
	}
}

Downloader.java

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.URL;
import java.nio.channels.Channels;
import java.nio.channels.ReadableByteChannel;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;


public class Downloader {
	final static int N = 49; 
	static int downloadedImages = 0; 
	
	public static void main(String[] args) throws InterruptedException {
		ThreadPoolExecutor executor = new ThreadPoolExecutor( 10, 10, 10, TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>() );
		
		fetch( executor, "n", "e" ); 
		fetch( executor, "s", "e" ); 
		fetch( executor, "n", "w" ); 
		fetch( executor, "s", "w" ); 
		
		executor.shutdown();
		executor.awaitTermination( 1, TimeUnit.DAYS ); 
		System.out.println( "downloaded " + downloadedImages + " new images" ); 
	}
	
	
	private static void fetch( ThreadPoolExecutor executor, String ns, String ew ){
		for( int i = 1; i <= N; i++ ){
			for( int j = 1; j <= N; j++ ){
				String filename = i + ns + j + ew + ".png";
				executor.submit( new Fetcher( filename ) );
			}
		}
	}
	
	
	static class Fetcher implements Runnable{
		private String filename;
		
		public Fetcher( String filename ){
			this.filename = filename;
		}
		
		public void run(){
			File dest = new File( "images/" + filename ); 
			if( dest.exists() ){
				System.out.println( "skip " + filename ); 
			}
			else{
				System.out.println( "fetch " + filename ); 
				try( FileOutputStream fos = new FileOutputStream( dest ) ){
					URL website = new URL("http://imgs.xkcd.com/clickdrag/" + filename );
					ReadableByteChannel rbc = Channels.newChannel(website.openStream());
					fos.getChannel().transferFrom(rbc, 0, 1 << 24);
					downloadedImages ++; 
				}
				catch( IOException e ){
					System.err.println( "404: " + filename ); 
					try(PrintWriter out = new PrintWriter( dest ) ){
						out.close(); 
					}
					catch( FileNotFoundException e1){
						e1.printStackTrace();
					}
				}
			}
		}
	}
}