Java“不能转换为可比较的”当使用TreeMap时 [英] Java "cannot cast to Comparable" when using TreeMap

问题描述

可能重复：

Java：SortedMap，TreeMap，Comparable？如何使用？

我使用的是Java JungI 图形包和Netbeans 7.我从Java获得以下错误：

 线程main中的异常java.lang.ClassCastException：graphvisualization.MyVertex不能转换为java.lang.Comparable 
 at java.util。 TreeMap.put（TreeMap.java:542）
  

这里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是SparseGraph< MyVertex，MyEdge> 
 {
 curRank = vertexRank.getVertexScore（v）; 
 vMap.put（v，curRank）; // **这是我的错误** 
} 
  

类MyVertex是一个类为图表做。下面是MyVertex的代码

  public class MyVertex 
 {
 int vID; //此顶点的id 
 double centrality; //这个顶点的中心度度量
 int degree; //这个顶点的度数
 
 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（）
 {
 returnv+ vID; 
} 
 
 int compareTo（MyVertex v）
 {
 return（this.degree< v.degree）？ 1：0; // this will do descendingly 
} 
} 
  

1. 如何获取MyVertex类型转换为Comparables？


2. 为什么需要这样做？ （我不立即看到原因）

解决方案

是否可以将MyVertex类型转换为Comparables？

实现可比较的接口。

  public class MyVertex implements Comparable< MyVertex> {
 
 @Override 
 public int compareTo（Object o）{
 //比较逻辑在这里
 
} 
} 
  

或者，您可以将比较器传递给构造函数 TreeMap 。

 新的TreeMap< MyVertex，Double>（new Comparator< ; MyVertex>（）
 {
 public int compare（MyVertex o1，MyVertex o2）
 {
 //比较逻辑在这里
} 
} ; 
 





为什么需要？

因为你存储在树映射中，这是一个排序的映射（按键排序）。地图键需要具有可比性才能确保地图中的排序顺序。

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)

解决方案

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.

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