Objects as keys in a HashMap - how to reference them (java)

Question

I am currently working on a small project simulating a game. Each Player object has an inventory which is a HashMap<Item, Integer> where the Integer is the quantity of that Item.

I am currently trying to write a method in the Player class, that allows the player to buy an Item from a Shop, if the Player have enough Coins in their inventory. Each Player starts with 50 Coins in their inventory.

Although, when I am trying to acces the coins in the players inventory (using inventory.get(coins)), I get an "coins cannot be resolved to a variable error".

CONSTRUCTOR

private String name; private HashMap<Item, Integer> inventory; private String location; public Player (String name){ this.name = name; location = "Home"; inventory = new HashMap<>(); Resource coins = new Resource("Coins", 1, 1); Tool pickaxe = new Tool("Pickaxe", 100, 100); Tool axe = new Tool("Axe", 100, 100); Tool shovel = new Tool("Shovel", 100, 100); Crop turnip = new Crop("Turnip", 20, "Spring"); this.inventory.put(coins, 50); this.inventory.put(pickaxe, 1); this.inventory.put(axe, 1); this.inventory.put(shovel, 1); this.inventory.put(turnip, 10); }

METHOD THAT FAILS

public void buyItemPierre(Shop pierres, Item item){ if (location.equals("Pierres")){ if (pierres.getForSale().containsKey(item)){ if (inventory.get(**coins**) >= pierres.getForSale().get(item)){ // ERROR HERE } } else{ System.out.println("Sorry, that item is not for sale here"); } } else{ System.out.println("You have to visit Pierres before you can buy anything from there!"); } }

I've tried instantiating the objects in the main method, although I get the same error. There must be something that I don't get regarding how to reference objects as keys in a HashMap... How else can I check if the player has enough coins? Thank you in advance!

This could be fixed by proper usable of variables, but honestly: don't. Don't use Objects like them as Map keys. They don't bring you any advantage. Use something static like a String "coins" or a own Enum. Much more flexible. — Tom
– Tom, Commented Dec 30, 2022 at 16:51

egeorge · Accepted Answer · 2022-12-30 16:53:32Z

1

You have declared coins as a local variable in the constructor.

 Resource coins = new Resource("Coins", 1, 1);

So as soon as the constructor is complete, that variable goes away. You still have the object as a key in the map, but if you want to use the coins variable to look it up in the map, you have to declare it as a member (outside the constructor).

private Resource coins= new Resource("Coins", 1, 1);

answered Dec 30, 2022 at 16:53

egeorge

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5 Comments

Sofie Buur Over a year ago

Ah okay that does make sense! But how can I then add a new object to the inventory and access it later? Should I then declare that as a member too?

markspace Over a year ago

Just like you have a buyItemPierre method, you need a addItem method so you can add items to the map.

markspace Over a year ago

Aside: the type of map you are using is also called a "multiset" or a "bag":en.wikipedia.org/wiki/Multiset

egeorge Over a year ago

I would create a wrapper class around Item to hold the count. public class InventoryItem { Item item; Integer count; ...constructors and setter} And then declare your map to hold these, indexed by a string. private Map<String, InventoryItem> inventory=new HashMap<>(); Then, in your constructor, use inventory.put("Coins", new InventoryItem(new Resource("Coins",1,1), 50);

Sofie Buur Over a year ago

Thank you so much for all your responses! Really appreciated

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