java.lang.ClassCastException：class java.util.TreeMap 无法转换为 class java.lang.Comparable

Question

I receive the following error when attempting to run my java program

Exception in thread "main" java.lang.ClassCastException: class java.util.TreeMap cannot be cast to class java.lang.Comparable (java.util.TreeMap and java.lang.Comparable are in module java.base of loader 'bootstrap')
    at java.base/java.util.TreeMap.compare(TreeMap.java:1569)
    at java.base/java.util.TreeMap.addEntryToEmptyMap(TreeMap.java:776)
    at java.base/java.util.TreeMap.put(TreeMap.java:785)
    at java.base/java.util.TreeMap.put(TreeMap.java:534)
    at exportsParser.exportsMap(exportsParser.java:53)
    at exportsParser.main(exportsParser.java:28)

The applicable code:

import edu.duke.*;
import java.io.*;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.regex.*;
import java.util.*;
import java.util.TreeMap;
import java.util.Iterator;

public class exportsParser{

    void println(Object obj){
        System.out.println(obj);
    }

    /* The rather involved pattern used to match CSV's consists of three
     * alternations: the first matches aquoted field, the second unquoted,
     * the third a null field.
     */
    private final static Pattern csv_pattern = Pattern.compile("\"([^\"]+?)\",?|([^,]+),?|,");

    public static void main(String[] argv) throws IOException {
        //println(csv_pattern);
        exportsParser parser = new exportsParser();
        BufferedReader reader = new BufferedReader(new FileReader("./exports_small.csv"));
        parser.exportsMap(reader);
    }
    public TreeMap<String, TreeMap<TreeMap<String,String> ,TreeMap<String, String>>> exportsMap(BufferedReader reader) throws IOException{

        if(reader.readLine() == null) return null;

        TreeMap<String, TreeMap<TreeMap<String,String>, TreeMap<String,String>>> exportsTable = new TreeMap<>();
        TreeMap<String, String> products = new TreeMap<>();
        TreeMap<String, String> value = new TreeMap<>();
        TreeMap<TreeMap<String,String>,TreeMap<String,String>> exportsData = new TreeMap<>();

        int countryIndex = 0;

        ArrayList<String> exportsList = new ArrayList<String>();
        String line;

        try{
            while((line = reader.readLine()) != null){
                exportsList = parse(line);

                String countryName = exportsList.get(0);
                products.put("items", exportsList.get(1));
                value.put("total", exportsList.get(2));
                println(products);
                println(value);
                exportsData.put(products, value);
                println(exportsData);

    //              exportsTable.put(countryName,exportsData);
                println(exportsTable);
            }
        }
        catch(IOException e)
        {
            e.printStackTrace();
        }
        reader.close();
        return exportsTable;
    }

    /* Parse one line.
     * @return List of Strings, minus their double quotes
     */
    public ArrayList<String> parse(String line) {
        ArrayList<String> list = new ArrayList<String>();
        Matcher mat = csv_pattern.matcher(line);
        // For each field
        while (mat.find()) {
            String match = mat.group();
            if (match == null)
                break;
            if (match.endsWith(",")) {  // trim trailing ,
                match = match.substring(0, match.length() - 1);
            }
            /*if (match.startsWith("\"")) { // assume also ends with
              match = match.substring(1, match.length() - 1);
              }*/
            if (match.length() == 0)
                match = null;
            list.add(match);
        }
        return list;
    }
}

To clarify, the issue arises when attempting to put the TreeMap data of products and value in exportsData. Same is applicable when attempting to add exportsData to the exportsTable correlating its key (Country) to the exportsData (Value). I understand what the errors means, I just have no idea as to how to fix it. Additionally libraries are not allowed (Purpose is to understand the flow of input data into "rows/columns" and experiment with Trees, HashMaps, etc)

Additionally, I cannot use a database for this as this is a requirement to manually do this. However what is not a requirement is using TreeMaps of course. We are allowed to experiment with the various Collection classes.

I have spent a while trying to get this to work but I have run out of thoughts and forum pages to read now. Eventually, this would be ideal to make it cater towards larger CSV files of unknown columns. However, for the practice run, we have been given the information before hand, hence the indexing in the code above.

CSV data:

Country,Exports,Value (dollars)
Germany,"motor vehicles, machinery, chemicals","$1,547,000,000,000"
Macedonia,"tobacco, textiles","$3,421,000,000"
Madagascar,"coffee, vanilla, shellfish","$864,800,000"
Malawi,"tea, sugar, cotton, coffee","$1,332,000,000"
Malaysia,"semiconductors, wood","$231,300,000,000"
Namibia,"diamonds, copper, gold, zinc, lead","$4,597,000,000"
Peru,"copper, gold, lead, zinc, tin,  coffee","$36,430,000,000"
Rwanda,"coffee, tea, hides, tin ore","$720,000,000"
South Africa,"gold, diamonds, platinum","$97,900,000,000"
United States,"corn, computers, automobiles, medicines","$1,610,000,000,000"

This is my first time using the above so it is prone to beginner errors.

Answer 1

Check javadoc . To cite - The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used. Natural ordering of keys means key must implement Comparable . Your keys are of type TreeMap , which does not have natural ordering - it does not implement Comparable . Naturally this leads to ClassCastException .

If you really must use TreeMap as key for your TreeMap, you must provide a Comparator to TreeMap constructor:

Comparator<TreeMap<String,String>> comparator = implement it;
TreeMap<TreeMap<String,String>,TreeMap<String,String>> exportsData = new TreeMap<>(comparator);

Seeing that your data is coming from a csv file, i would suggest to parse it to some custom class. It would be much more readable, and it will be easier to implement Comparable , or Comparator if needed.

Edit: You don't actually need 2 maps - for exports and value, it complicates thing more that needed. Those can be put in a single map. Keys are values from the first line in csv(or other keys, as in your case) and values are corresponding values from the parsed line. So you have:

Map<String, String> lineData;

Country may also be part of this map(if you need it). Normally it's this map, which will be rerpesented by your custom class, but it looks like your task is to work with collections, so i won't delve into that.

Since you want to map country names to data, now you need another map - keys will be string(country name) and values the map containing line data from above.

All of that can be stored in a list(you can store anything in a list). I'm leaving to you figuring out the exact way to implement it.