如何在Java中使用TreeMap在.get（）和.put（）中使用字符串

Question

I am currently working on creating a Prim's Minimum Spanning Tree for an undirected, weighted graph that uses String values for the vertices. To create the graph, my teacher said we can use the edge and graph classes from our textbook. However, the book uses Integers for vertices rather than strings. I tried replacing all Integers with Strings, but I received a compiler error for every line that used .get() from the generic TreeMap because it could not find the symbol method get(java.lang.String). After a little work, I discovered that initializing the TreeMap and using .add() works with Strings, but not the .get() or .put() methods. Here is the code exactly the way it is in the book except Integer is replaced with String.

How can I make the .get() and .put() methods to work with Strings?

import java.util.*;

class Graph {
    private int numVertices;    //number of vertices in the graph
    private int numEdges;        //number of edges in the graph

    private Vector<TreeMap<String, String>> adjList;

    //constructor
    public Graph(int n) {
        numVertices=n;
        numEdges=0;
        adjList=new Vector<TreeMap<String, String>>();
        for(int i=0;i<numVertices;i++) {
            adjList.add(new TreeMap<String, String>());
        }
    }

    //Determines the number of vertices in the graph
    public int getNumVertices() {
        return numVertices;
    }

    //Determines the number of edges in the graph
    public int getNumEdges() {
        return numEdges;
    }

    //Determines the weight of the edge between vertices v and w
    public String getEdgeWeight(String v, String w) {
        return adjList.get(v).get(w);
    }

    //Add the edge to both v's and w's adjacency list
    public void addEdge(String v, String w, int wgt) {
        adjList.get(v).put(w,wgt);
        adjList.get(w).put(v,wgt);
        numEdges++;
    }

    //Adds an edge to the graph
    public void addEdge(Edge e) {
        //Extract the vertices and weight from the edge e
        String v=e.getV();
        String w=e.getW();
        int weight=e.getWeight();
        addEdge(v, w, weight);
    }

    //Finds the edge connecting v and w
    public Edge findEdge(String v,String w) {
        int wgt=adjList.get(v).get(w);
        return new Edge(v, w, wgt);
    }

    //package access
    //Returns the adjacency list for given vertex
    TreeMap<String, String> getAdjList(String v) {
        return adjList.get(v);
    }
}

Answer 1

Your problem is not with the TreeMap . TreeMap is key based collection. You are facing issues with your Vector ' adjList '. Vector is an Indexed based collection you can get an item with its index only.

Try changing your method as below

public String getEdgeWeight(int v, String w) {
    return adjList.get(v).get(w);
}