HashMap返回Null，无论如何

Question

I'm having a little trouble conceptualizing this question, so I apologize in advance if it's vague or been solved elsewhere.

Anyway, I've got a big class called Board . In board is ArrayList<Space> sList , a list of Space on the Board. Which each Space is bList , a HashMap that represents which Spaces border this particular Space. The init() method of Board creates all the Spaces and gives them their borders, for example

sList.add(new Location(7, true, "Police Station"));
sList.get(3).addToBList(sList.get(7)); //adds "Police Station" to the bList at sList index 3

However, if, while in Board , I try and look the bList of a certain Space, it always returns null, like I never added anything to bList. However, if I look within the Space itself, bList is always full of items. So, basically, I'm trying to invoke a variable from a class in another class and it only shows non-null values if I'm within that other class itself - any ideas (or clarifying questions)?

EDIT: I think I'm going to clarify my own question.

If Space has a getBList() getter that returns bList (which is a hashMap), and in Board I add certain values to that bList, why does bList return "{}" when I call getBList() in Board?

Also, this is all Arrow is (see bottom).

The code itself:

public class Board {

ArrayList<Space> sList;
    ArrayList<Player> pList;
    int moveCount = 0;
    ArrayList<Space> visited = new ArrayList<Space>();

    public Board(int p, int oldGod){ //num of players, Old God
        sList=new ArrayList<Space>();
        pList=new ArrayList<Player>();
        init();

        move(sList.get(0), sList.get(34));

}

public Space move(Space start, Space end){ 
        visited.add(start);
        Iterator it = start.bList.entrySet().iterator();//this is where the trouble starts. bList returns only {}
        if(start.equals(end)){
            System.out.println("Here! You've arrived at " + start);
        }
        else{System.out.println("Time to debug. You're at " + start);
        while(it.hasNext()){
            Map.Entry pair = (Map.Entry)it.next();
            for(Space s : visited){
                if(pair.getValue().equals(s)){
                }
                else{

                    return move((Space)pair.getValue(), end);
                }

            }
        }
        }
        return start;
    }

    private void init() {
        sList.add(new Location(1, true, "Ma's Boarding House"));
        sList.add(new Location(2, true, "South Church"));
        sList.get(1).addToBList(sList.get(2));
        //etc...
}
}


public abstract class Space {
    int district;
    String name;
    HashMap<Arrow, Space> bList = new HashMap<Arrow, Space>();
    ArrayList<Creep> cList = new ArrayList<Creep>();  
    ArrayList<Player> pList = new ArrayList<Player>();

    public String toString() {
        return name;
        }

    public void addToBList(Space s, Arrow a){
        bList.put(a, s);
    }

    public void borderList(){ //in here it always sees bList as full of variables
        Iterator it = bList.entrySet().iterator();
        while(it.hasNext()){
            Map.Entry pair = (Map.Entry)it.next();
            System.out.println(this + ": " + pair.getKey() + "-> " + pair.getValue()); //arrow towards
            it.remove();
        }
    }



}


public class Arrow {

    int color;

    public Arrow(int c){
        color=c;
    }

    public String toString(){
        if(color==0)
            return "Black";
        else if(color==1)
            return "White";
        else if(color==2)
            return "Both";
        else
            return "None";
    }

}

Answer 1

I suggest you read the Javadoc for Object.hashCode()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

and Object.equals()

Unless you override these, two object are not the same, unless they are the same object, regardless of what values you set in them.

For keys, you must implement both methods. For values of a map you might only implement equals(Object) or not if you don't use it.

BTW: I suggest you read the Javadoc for all the classes in java.lang and java.util

Answer 2

You need to implement methods equals() and hashcode() on your class Arrow . Looking at your code, I can't make out how/where are you adding the elements to your map. So I am just giving a simple example below if that helps. As another person mentioned, you must understand how hashmap works by reading more about it.

public class Arrow { private String elem1 = "init-value"; private int elem2 = 1001; { private String elem1 = "init-value"; private int elem2 = 1001;

public Arrow(String elem1, int elem2)
{
    this.elem1 = elem1;
    this.elem2 = elem2;
}

@Override
public boolean equals(Object obj)
{
    if( ! (obj instanceof Arrow) )
    {
        return false;
    }
    Arrow obj2 = (Arrow) obj;
    // put your actual logic for object equality here
    return this.elem1.equals(obj2.elem2) && this.elem2 == obj2.elem2;
}

@Override
public int hashCode()
{
    //put some logic to calculate hashcode for this object. This is used by
    // hashmap to find index/slot for your key
    return 17 * elem1.hashCode() * elem2;
}

}

Answer 3

the it.remove() kept removing my keys from the HashMap. It works now!