Gilded rose solution

Adjuster.java

public interface Adjuster {
	public abstract void adjust(Item item);
}

AdjustQualityBy.java

public class AdjustQualityBy implements Adjuster {
	private int adjustment;

	public AdjustQualityBy(int adjustment) {
		this.adjustment = adjustment;
	}

	public void adjust(Item item) {
		item.setQuality(item.getQuality() + adjustment);
		if (item.getSellIn() < 0)
			item.setQuality(item.getQuality() + adjustment);
	}
}

BackstagePassAdjuster.java

public class BackstagePassAdjuster implements Adjuster {

	public void adjust(Item item) {
		item.setQuality(item.getQuality() + 1);
		if (item.getSellIn() < 10)
			item.setQuality(item.getQuality() + 1);
		if (item.getSellIn() < 5)
			item.setQuality(item.getQuality() + 1);
		if (item.getSellIn() < 0)
			item.setQuality(0);
	}
}

GildedRose.java

import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class GildedRose {
	
	private static final Map<String, Product> products;
	static {
		products = new HashMap<String, Product>();
		products.put("NORMAL ITEM", new Product(new AdjustQualityBy(-1)));
		products.put("Aged Brie", new Product(new AdjustQualityBy(+1)));
		products.put("Backstage passes to a TAFKAL80ETC concert", new Product(new BackstagePassAdjuster()));
		products.put("Conjured Mana Cake", new Product(new AdjustQualityBy(-2)));
	}

	public static void updateQuality(List<Item> items) {
		for (int i = 0; i < items.size(); i++)
			update(items.get(i));
	}

	private static void update(Item item) {
		Product p = products.get(item.getName());
		if (p != null)
			p.update(item);
	}

}

Product.java

public class Product {
	private Adjuster qualityAdjuster;

	public Product(Adjuster qualityAdjuster) {
		this.qualityAdjuster = qualityAdjuster;
	}

	public void update(Item item) {
		item.setSellIn(item.getSellIn() - 1);
		
		qualityAdjuster.adjust(item);
		
		if (item.getQuality() < 0)
			item.setQuality(0);
		if (item.getQuality() > 50)
			item.setQuality(50);
	}

}