JAVAClassCastException:类java。util。树映射不能转换为java类。可比的

Question

当试图运行我的java程序时，我收到以下错误

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)

适用代码：

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;
    }
}

澄清一下，当试图将产品和价值的TreeMap数据放入exportsData时，会出现问题。当试图将exportsData添加到将其键（国家）与exportsData（价值）相关联的exportsTable时，同样适用。我明白错误的含义，只是不知道如何修复它。此外，不允许库（目的是了解输入数据流向“行/列”，并尝试使用Trees、HashMaps等）

此外，我不能使用数据库来进行此操作，因为这是手动执行此操作的要求。然而，不需要的是使用树形图。我们可以尝试各种收集课程。

我花了一段时间试图让这个工作，但我已经用完的想法和论坛页面阅读了。最终，这将是理想的，使其能够满足未知列的更大CSV文件。然而，对于实践运行，我们已经得到了手头的信息，因此在上面的代码中建立了索引。

CSV数据：

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"

这是我第一次使用上述方法，所以初学者很容易出错。

Answer 1

查看javadoc。举个例子-映射是根据其键的自然顺序进行排序的，或者由映射创建时提供的比较器进行排序的，具体取决于使用的构造函数 键的自然顺序意味着键必须实现可比性。您的密钥属于TreeMap类型，它没有自然排序-它不实现可比。这自然会导致ClassCastException。

如果您真的必须使用TreeMap作为TreeMap的键，则必须提供TreeMap构造函数的比较器：

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

鉴于您的数据来自csv文件，我建议将其解析为一些自定义类。它的可读性要高得多，而且如果需要，可以更容易地实现Comparable，或者Comparator。