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
}
}
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.