使用TreeMap时Java“无法转换为可比”

Question

我正在使用Java
JungI图形包和Netbeans7。我从Java中收到以下错误：

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

这是与错误相关的代码：

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**
       }

MyVertex类是我为图形设计的类。以下是MyVertex的代码

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. 如何将MyVertex类型转换为可比对象？
2. 为什么这是必要的？（我没有立即看到原因）

Answer 1

如何将MyVertex类型转换为可比对象？

实现可比接口。

public class MyVertex implements Comparable<MyVertex> {

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

  }
 }

或者，您可以将传递comparator给的构造函数TreeMap。

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

为什么这是必要的？

因为您存储在树状地图中，该树状地图是排序的地图（按键排序）。映射键必须具有可比性，以确保映射中的排序顺序。