Question

I am doing a piece of coursework in Java using BlueJ. We need to add new features to the text based game, Zuul. I have decided to start working on an inventory and item system. I am having trouble working out the best way to do this so I just winged it. Here is my code. Sorry I haven’t gotten round to commenting everything yet. The game compiles but I get an exception in the console when I run the game.

Error:

java.lang.NullPointerException
    at Game.createPlayer(Game.java:15)
    at Game.<init>(Game.java:7)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:525)
    at bluej.runtime.ExecServer$3.run(ExecServer.java:740)

Game Class (This is equivalent of the Main class in Java, this is where the game is run from):

import java.util.*;

public class Game
{
    public Game()
    {
        createPlayer();
        createItems();
    }

    private Entity localPlayer;

    public void createPlayer(){
        Player localPlayer = new Player("Player Name", 0, 0, 0, 0, 0);
        localPlayer.equipArmour("Helm", armourDB.get("Helm")); // This is where I think I have gone wrong
    }

    // Create global hashmap variables
    private HashMap<String, Weapon> weaponsDB;
    private HashMap<String, Armour> armourDB;
    private HashMap<String, Supplement> supplementDB;

    public void createItems(){
        // Create weapons
        weaponsDB = new HashMap<String, Weapon>();

        Weapon weaponFists = new Weapon("Fists", "Weapon", 0, 0, 0, 0, 0, "Melee");
        Weapon weaponSword = new Weapon("Sword", "Weapon", 0, 0, 0, 0, 0, "Melee");
        Weapon weaponBow = new Weapon("Bow", "Weapon", 0, 0, 0, 0, 0, "Ranged");
        Weapon weaponDagger = new Weapon("Dagger", "Weapon", 0, 0, 0, 0, 0, "Melee");

        weaponsDB.put("Fists", weaponFists);
        weaponsDB.put("Sword", weaponSword);
        weaponsDB.put("Bow", weaponBow);
        weaponsDB.put("Dagger", weaponDagger);

        // Create armour
        armourDB = new HashMap<String, Armour>();

        Armour armourBreastplate = new Armour("Breatplate", "Chest", 0, 0, 0, 0, 0);
        Armour armourHelm = new Armour("Helm", "Head", 0, 0, 0, 0, 0);

        armourDB.put("Breastplate", armourBreastplate);
        armourDB.put("Helm", armourHelm);

        // Create supplements
        supplementDB = new HashMap<String, Supplement>();

        Supplement supplementHealthPotion = new Supplement("Health Potion", 0, 0);
        Supplement supplementPowerPotion = new Supplement("Power Potion", 0, 0);

        supplementDB.put("Health Potion", supplementHealthPotion);
        supplementDB.put("Power Potion", supplementPowerPotion);
    }
}

Entity Class (Construction for the player class and enemy class):

import java.util.*;

public class Entity
{
    private boolean entityStatus;

    private String entityName;
    private int entityHealth;
    private int entityPower;
    private int entityHealthRegen;
    private int entityPowerRegen;
    private int entityAttackPower;

    private HashMap<String, Armour> entityEquipment;
    private ArrayList<Item> entityInventory;    

    public Entity(
        String paramEntityName,
        int paramEntityHealth,
        int paramEntityPower,
        int paramEntityHealthRegen,
        int paramEntityPowerRegen,
        int paramEntityAttackPower)
    {
        entityStatus = true;

        entityName = paramEntityName;
        entityHealth = paramEntityHealth;
        entityPower = paramEntityPower;
        entityHealthRegen = paramEntityHealthRegen;
        entityPowerRegen = paramEntityPowerRegen;
        entityAttackPower = paramEntityAttackPower;

        entityEquipment = new HashMap<String, Armour>(); // Set all possible equipment slots to null on initial run
        entityEquipment.put("Head", null);
        entityEquipment.put("Shoulders", null);
        entityEquipment.put("Chest", null);
        entityEquipment.put("Hands", null);
        entityEquipment.put("Legs", null);
        entityEquipment.put("Feet", null);
        entityEquipment.put("Weapon", null);

        entityInventory = new ArrayList<Item>();
    }

    public boolean getEntityStatus(){
        return entityStatus;
    }

    public String getEntityName(){
        return entityName;
    }

    public int getEntityHealth(){
        return entityHealth;
    }

    public int getEntityPower(){
        return entityPower;
    }

    public int getEntityHealthRegen(){
        return entityHealthRegen;
    }

    public int getEntityPowerRegen(){
        return entityPowerRegen;
    }

    public int getEntityAttackPower(){
        return entityAttackPower;
    }

    // Equips the player with an item into the equipment slot
    public void equipArmour(String paramEquipmentSlot, Armour paramArmourName){
        entityEquipment.put(paramEquipmentSlot, paramArmourName);
    }

    public void printInventory(){
        System.out.println("Something");
    }
}

I think the main problem is that I cannot wrap my head around the use of hashtags, I need to see a live example to see how it works. Can anyone help? If you need anything else from me, let me know.

Answer 1

Well this is the problem:

armourDB.get("Helm")

You haven’t initialized armourDB at that point. If you call createItems() before createPlayer() it should be okay for that particular line. But you still won’t be initializing the isntance variable called localPlayer. You’ll only be assigning a value to the local variable declared in createPlayer.

It’s not really clear what you’re trying to achieve to be honest, but those are the first two problems…