A map of maps will put only the last entry provided to the inner map

I am making Map<String, Map<String, Item>> maps = new HashMap<>(); this is my code :

  private static void addItem(String[] commandParsed, Set<Item> inventory, Map<String, Map<String, Item>> maps){
            Item item = new Item(commandParsed[1], Double.parseDouble(commandParsed[2]), commandParsed[3]);
            String mapName = commandParsed[3];
            Map<String, Item> map = new HashMap<>();
            if (inventory.contains(item)){
                System.out.printf("Error: Item %s already exists%n", item.name);
            }
            else {
                inventory.add(item);
                maps.put(mapName, map);
                map.put(item.name, item);
                System.out.printf("Ok: Item %s added successfully%n", item.name);
            }
        }

Sample input:

add CowMilk 1.90 dairy
add BulgarianYogurt 1.90 dairy
add SmartWatch 1111.90 technology

inventory is a separate set, to which I also add the items, but it adds them there correctly. My method creates the map dairy and map technology as expected. However each map contains only one entry, for example CowMilk is overwritten by BulgarianYogurt

You overwrite map each time in this line: maps.put(mapName, map);;
Put an empty map if it's the first entry only: maps.computeIfAbsent(mapName, key -> new HashMap<>());.
Then add the entry: maps.get(mapName).put(item.name, item).

All you need to do is the following:

• This will put add the hashMap for the key if it doesn't exist.
• In either case, it returns the new Map or the old one allowing you to
  enter the key/value pair.

maps.computeIfAbsent(mapName, k->new HashMap<>()).put(item.name, item);

Also, it does not seem to be necessary to maintain a list of already added items. Why not just check the current map. Putting it altogether you could do.

private static void addItem(String[] commandParsed, Map<String, Map<String, Item>> maps){
    Item item = new Item(commandParsed[1],
                         Double.parseDouble(commandParsed[2]),
                         commandParsed[3]);
    String mapName = commandParsed[3];
    if (maps.containsKey(mapName) && maps.get(mapName).containsKey(item.name)) {
  	    System.out.printf("Error: Item %s already exists%n", item.name);
    }  else {
  	      maps.computeIfAbsent(mapName, k->new HashMap<>()).put(item.name,item);
    }	    
}

Please let me know if I misunderstood your intent.