Question

I have a question related to Key Type initialization in HashMap. For example, I have defined the below Node class with over-ridden equals() and hashCode() as follows:

public class Node implements Comparable<Node> {
    private int Id;
    public Node(int i) {
    ...
    }
    void setId(int i) {
        Id = i;
    }
    int getId() {
        return Id;
    }
    @Override
    public boolean equals(Object o) {
        if (o == null) {
            throw new NullPointerException();
        }
        if (o instanceof Node && this != o) {
            if (((Node) o).getId() == this.getId())
                return true;
        }
        return false;
    }
    public int hashCode() {
        return Id;
    }
}

Now I am building a HashMap with key as type Node as follows:

public class AdjList {

    public HashMap<Node,Double> adj;
    public AdjList() {
        adj = new HashMap<Node,Double>(maxSize);
    }
    ...
}

As you can possibly figure out, I am trying to generate a graph adjacency list with the node class as HashMap.

Now, my question is when I call AdjList() constructor where I create a new HashMap with some maxSize, will it initialize the Node() class as key type? Or I need to separately initialize Node() clas for the key? If I need to initialize Node() in AdjList constructor, then how it can be possible?

Any suggestion will be valuable and useful suggestions will be rewarded.

Thanks,
Somnath

Answer 1

when I call AdjList() constructor where I create a new HashMap with some maxSize, will it initialize the Node() class as key type?

No! You are instantiate the Map with initialCapacity which is the loadFactor value not maxSize (See the documentation).

You may define a method in AddList that adds an entry.

public void add(int i,Double d)
{
  adj.put(new Node(i),d);
} 

Second, you’ve implemented Comparable so you must have to define the compareTo method.