Java "cannot cast to Comparable" when using TreeMap [duplicate]

Question

Possible Duplicate:
Java: SortedMap, TreeMap, Comparable? How to use?

I am using the Java JungI graph package and Netbeans 7. I am getting the following error from Java:

 Exception in thread "main" java.lang.ClassCastException: graphvisualization.MyVertex cannot be cast to java.lang.Comparable
    at java.util.TreeMap.put(TreeMap.java:542)

Here is the code associated with the error:

SortedMap<MyVertex, Double> vMap = new TreeMap<MyVertex, Double>();
       double curRank = 0;
       for(MyVertex v: g.getVertices())                 //g is a SparseGraph<MyVertex, MyEdge>
       {
           curRank = vertexRank.getVertexScore(v);
           vMap.put(v, curRank);                        //**Here is my Error**
       }

Class MyVertex is a class I made for graphs. The following is the code for MyVertex

public class MyVertex 
{
    int vID;                    //id for this vertex
    double centrality;          //centrality measure for this vertex
    int degree;                 //the degree of this vertex

    public MyVertex(int id)
    {
        this.vID = id;
        this.centrality=0;
        this.degree=0;
    }

    public double getCentrality()
    {
        return this.centrality;
    }

    public void setCentrality(double centrality)
    {
        this.centrality = centrality;
    }

    public int getDegree()
    {
        return this.degree;
    }

    public void setDegree(int deg)
    {
        this.degree = deg;
    }

    public void incrementDegree()
    {
        this.degree++;
    }

    public void decrementDegree()
    {
        this.degree--;
    }

    @Override
    public String toString()
    {
        return "v"+vID;
    }

    int compareTo(MyVertex v) 
    {
        return (this.degree < v.degree) ? 1 : 0;          //this will do descendingly
    }
}

1. How do I get MyVertex types to be casted to Comparables?
2. Why is this necessary? (I do not immediately see the reason)

Answer 1

How do I get MyVertex types to be casted to Comparables?

Implement Comparable interface.

public class MyVertex implements Comparable<MyVertex> {

  @Override
  public int compareTo(Object o) {
   // comparison logic goes here

  }
 }

or alternately, you can pass a comparator to the constructor of TreeMap.

 new TreeMap<MyVertex, Double>(new Comparator<MyVertex>()
        {
            public int compare(MyVertex o1, MyVertex o2)
            {
                //comparison logic goes here
            } 
    });

Why is this necessary?

Because you are storing in tree map which is a sorted map (sorted by keys). Map Keys need to be comparable to ensure a sort order in the map.

Answer 2

The behaviour is compliant with the javadoc of TreeMap:

Throws ClassCastException if the specified key cannot be compared with the keys currently in the map

There are essentially two ways to make it work:

• either make MyVertex implement Comparable<MyVertex>
• or pass a Comparator<MyVertex> to the constructor of your TreeMap

Note that before Java 7, the exception would only be thrown when adding a second item to the map whereas with Java 7 the exception is thrown when adding one item to the map.