使用TreeMap时,Java"无法转换为Comparable"

Question

可能重复:
Java:SortedMap,TreeMap,Comparable？如何使用？

我正在使用Java JungI图形包和Netbeans 7.我从Java得到以下错误:

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

以下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类是我为图表制作的一个类.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类型转换为可比较的？
2. 为什么这有必要？(我没有立即看到原因)

Answer 1

如何将MyVertex类型转换为可比较的？

实现Comparable接口.

public class MyVertex implements Comparable<MyVertex> {

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

  }
 }

或者,您可以将a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
            } 
    });

为什么这有必要？

因为您存储在树形图中,它是一个有序地图(按键排序).地图键需要具有可比性,以确保地图中的排序顺序.

Answer 2

该行为符合TreeMap的javadoc:

如果无法将指定的键与当前映射中的键进行比较,则抛出ClassCastException

基本上有两种方法可以使它工作:

• 或者制造MyVertex工具Comparable<MyVertex>
• 或者传递Comparator<MyVertex>给TreeMap的构造函数

请注意,在Java 7之前,只有在向地图添加第二个项目时才会抛出异常,而使用Java 7时,在向地图添加一个项目时会抛出异常.

Answer 3

MyVertexclass应该实现Comparable为树映射使用compareTo方法根据键对映射进行排序.

public class MyVertex implements Comparable<MyVertex> {

  @Override
  public int compareTo(MyVertex o) {
   // do the comparison logic

  }
 }

其他选项是将比较器对象传递给TreeMap http://docs.oracle.com/javase/1.5.0/docs/api/java/util/TreeMap.html#TreeMap(java.util.Comparator)