Compressor.java

import java.util.Collection;
import java.util.HashMap;
import java.util.Map;


public class Compressor {
	
	private Compressor() {};
	
	public static <V extends ICompressible<K, V>, K> Collection<V> compress(Collection<V> original) {
		Map<K, V> map = new HashMap<K, V>(original.size());
		
		for (V o : original) {
			if (map.containsKey(o.compressKey())) {
				V n = map.get(o.compressKey());
				V c = n.compress(o);
				map.put(o.compressKey(), c);
			} else {
				map.put(o.compressKey(), o);
			}
		}
		
		return map.values();
	}
	
}

ICompressible.java

public interface ICompressible<K, V> {
	
	K compressKey();

	V compress(V o);
}

Item.java

import java.util.ArrayList;
import java.util.Collection;

public class Item implements ICompressible<String, Item> {

	private String id;
	private int quantity;

	public Item(String id, int quantity) {
		this.id = id;
		this.quantity = quantity;
	}

	@Override
	public String compressKey() {
		return id;
	}

	@Override
	public Item compress(Item o) {
		return new Item(id, o.quantity + quantity);
	}

	public static void main(String[] args) {
		Item i1 = new Item("Square", 5);
		Item i2 = new Item("Triangle", 10);
		Item i3 = new Item("Square", 3);
		
		Collection<Item> items = new ArrayList<Item>();
		items.add(i1);
		items.add(i2);
		items.add(i3);
		
		Collection<Item> compressedItems = Compressor.compress(items);
		
		System.out.println(items);
		// => [Square:5, Triangle:10, Square:3]

		System.out.println(compressedItems);
		// => [Square:8, Triangle:10]
	}

}