Question

I have recently been giving Java spending a good few months on C#, just got caught up on something I’m unsure how to format.

A little example I’ve thrown together to explain my problem.

I am trying to add a an object of type ‘Creature’ to my object of type ‘Player’

Currently like this:

                                                        //Null items are objects
Player newPlayer = new Player("This", "Is", "Here", 1 , 1, 1, null, null, null);        

Creature c = null;                                  //Null items are objects
c = new Creature("Name", "Species",  100, 5.5, 10.5, 1, 100, null, null, null);

newPlayer.addCreature(c);

However the problem I’m getting is java.lang.NullPointException.

The player class can be seen here:

public Player(String Username, String Password, String Email, int Tokens, int Level, int Experience, Vector<Creature> Creature, Vector<Food> Food, Vector<Item> Item) {
    m_username = Username;
    m_password = Password;
    m_email = Email;
    m_tokens = Tokens;
    m_level = Level;
    m_experience = Experience;
    m_creature = Creature;
    m_food = Food;
    m_item = Item;

}   

public void addCreature(Creature c)
{
    m_creature.add(c);      
}

And the creature:

public Creature(String Name, String Species, int Health, double Damage, double Regen, int Level, int Exp, Vector<Effect> ActiveEffect, Vector<Attack> Attack, Vector<Specialisation> Specialisation )
{
    m_name = Name;
    m_species = Species;
    m_health = Health;
    m_damageMultiplier = Damage;
    m_regenRate = Regen;
    m_level = Level;
    m_experience = Exp;
    m_activeEffect = ActiveEffect;      
    m_attack = Attack;
    m_specialisation = Specialisation;      

}

How do I create the instance using this?

Answer 1

That is because the reference to the vector that you are storing is null. You are passing nulls for the constructor.

When you pass new vector<Creature>() you are actually passing a reference to a newly constructed vector. It doesn’t contain any creature objects as yet. Earlier it was failing because you were trying to call add(..) function on a reference that was set to null.

Try this:

Player newPlayer = new Player("This", "Is", "Here", 1 , 1, 1, new Vector<Creature>(), new Vector<Food>(), new Vector<Item>());
                                                              ^ new empty vector      ^ new empty vector  ^ new empty vector