Adding an object to another using Java

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?

whats your class definition for Player and Creature? Do you have a stack trace? A shot in the dark: did you instantiate a collection (new ArrayList(), etc...) object in your Player object, or did you only define it? — Stefan
– Stefan, Commented Dec 9, 2012 at 13:31
Could you please post the code of Player? Is Creature added to a list inside player, or just into a member variable? If it is added to a list, the list may be null! — Mirco
– Mirco, Commented Dec 9, 2012 at 13:40

UltraInstinct · Accepted Answer · 2012-12-09 13:44:42Z

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

edited Dec 9, 2012 at 13:44

answered Dec 9, 2012 at 13:38

UltraInstinct

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Comments

SkyWalker · Accepted Answer · 2012-12-09 13:31:49Z

0

It is impossible to say without looking at the addCreature implementation. Look carefully the Stackstrace of the exception, it will show you the exact line number where the exception happened.

answered Dec 9, 2012 at 13:31

SkyWalker

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