Java循环显示来自SQL国家/地区表的多种语言

Question

I am working on a Java assignment that requires pulling languages from a COUNTRY_LANGS_SQL table, then using a loop to display the Country, Population, Median Age, and Language(s) for each country. I'm able to display the 4 Countries, Population, Median Age, but for the life of me I cannot figure out what statement I need to declare in order for it to pull and loop the languages in. I've tried numerous variations with no success, so I'm hoping someone can take a look and point out what I'm doing wrong. Any help would be greatly appreciated.

UPDATE 3: So I'm very close to having this working now with it successfully executing and displaying language column, it's just not displaying the languages of the country (see below)

Name: Canada  Population: 34568211  Median Age: 41.5  ID      :   1  Language:   []
Name: Germany  Population: 81147265  Median Age: 45.7  ID      :   2  Language:   []
Name: South Africa  Population: 49601098  Median Age: 25.5  ID      :   3  Language:   []
Name: Japan  Population: 127253075  Median Age: 45.8  ID      :   4  Language:   []

I also only have one error now in the readLanguages method " The method getId() is undefined for the type List "

/**
 * @return list of countries read from the country database
 */
public List<Country> getCountries() {
    return countries;
}

private void readLanguages() {
    try (
        Connection connection = getConnection();
        PreparedStatement stmt = connection.prepareStatement(GET_COUNTRY_LANGS_SQL)
    ) {

        for (Country country : countries); {
            stmt.setInt(0, countries.getId());
            ResultSet rs = stmt.executeQuery();
        while (rs.next()) {
            int i = 0;
            countries.get(i).addLanguage(rs.getString("language")); 
            rs.close();
        }
    }}
    catch (SQLException e) {
        System.err.println("ERROR: " + e.getMessage());
        e.printStackTrace();
    }

It seems to not be pulling the Id's from the Country class, or it's not looping correctly. Any thoughts?

1)CountriesDB.java

        package edu.pcc.cis233j.countries;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

/**
 * Read data from the countries database
 * 
 * @author 
 */
public class CountriesDB {
    private static final String DB_NAME = "Countries";
    private static final String DB_URL = "jdbc:jtds:sqlserver://cisdbss.pcc.edu/" + DB_NAME;
    private static final String USERNAME = "";
    private static final String PASSWORD = "";
    private static final String GET_COUNTRIES_SQL = "SELECT * FROM COUNTRY";
    private static final String GET_COUNTRY_LANGS_SQL = "SELECT * FROM COUNTRY_LANGUAGE WHERE CountryId = ?";

    private List<Country> countries;

    /**
     * Create a CountriesDB object
     * Read from the Countries database and populate the countries list
     */
    public CountriesDB() {
        countries = readCountries();
    }

    /**
     * Create and return a connection to the database
     * @return connection to the countries database
     */
    private Connection getConnection() throws SQLException {
        Connection connection = DriverManager.getConnection(DB_URL, USERNAME, PASSWORD);
        return connection;
    }

    /**
     * Read country info from the Country table.
     * If an error occurs, a stack trace is printed to standard error and an empty list is returned.
     * @return the list of countries read
     */
    private List<Country> readCountries() {
        List<Country> countries = new ArrayList<>();
        try (
                Connection connection = getConnection();
                PreparedStatement stmt = connection.prepareStatement(GET_COUNTRIES_SQL);
                ResultSet rs = stmt.executeQuery()
                ) {
            while (rs.next()) {
                countries.add(new Country(rs.getInt("Id"),
                        rs.getString("Name"),
                        rs.getLong("Population"),
                        rs.getDouble("MedianAge")));
            }
        }
        catch (SQLException e) {
            System.err.println("ERROR: " + e.getMessage());
            e.printStackTrace();
        }
        return countries;
    }

    /**
     * @return list of countries read from the country database
     */
    public List<Country> getCountries() {
        return countries;
    }

    private void readLanguages() {
        try (
            Connection connection = getConnection();
            PreparedStatement stmt = connection.prepareStatement(GET_COUNTRY_LANGS_SQL)
        ) {

            for (Country country : countries); {
                stmt.setInt(0, countries.getId());
                ResultSet rs = stmt.executeQuery();
            while (rs.next()) {
                int i = 0;
                countries.get(i).addLanguage(rs.getString("language")); 
                rs.close();
            }
        }}
        catch (SQLException e) {
            System.err.println("ERROR: " + e.getMessage());
            e.printStackTrace();
        }
    }
}

2)Country.java

       package edu.pcc.cis233j.countries;

import java.util.ArrayList;
import java.util.List;

/**
 * A country in the world
 * @author Your Name & Cara Tang
 */
public class Country {
    private int id;
    private String name;
    private long population;
    private double medianAge;
    private List<String> language = new ArrayList<>();


    /**
     * Create a Country object with the given properties
     */
    public Country(int id, String name, long population, double medianAge) {
        this.id = id;
        this.name = name;
        this.population = population;
        this.medianAge = medianAge;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public long getPopulation() {
        return population;
    }

    public double getMedianAge() {
        return medianAge;
    }

    public List<String> getLanguage() {
        return language;
        }

    public void addLanguage(String string) {
         language.add(string);
        }

}

3) CountryMain.java

    package edu.pcc.cis233j.countries;

import java.util.List;

/**
 * Read from the Country database and print data on the countries
 * @author TJ
 */
public class CountryMain {
    public static void main(String[] args) {
        CountriesDB cdb = new CountriesDB();
        List<Country> countries = cdb.getCountries();

        for (Country country : countries) {
              System.out.println("Name: " + country.getName()
                                   + "  Population: " + country.getPopulation()
                                   + "  Median Age: " + country.getMedianAge()
                                   + "  ID      :   " + country.getId()
                                   + "  Language:   " + country.getLanguage()
                      );
        }
    }
}

Answer 1

First, the lines:

countries.add(new Country(rs.getString("Language"),
                    rs.getString("Language")));

refer to rs before it is declared or populated. Also, they appear to be trying to build a Country from two strings (both the same).

Second,

countries.add(new Language ());

Is crazy because countries has Country objects in it, so you can't just add a Language object, and a new one at that (not from the database).

It's almost as if you didn't write the code yourself and consequently don't understand it...

I think what you need is closer to this:

private void readLanguages() {
    try (
        Connection connection = getConnection();
        PreparedStatement stmt = connection.prepareStatement(GET_COUNTRY_LANGS_SQL)
    ) {
        ResultSet rs = null;
        for (Country country : countries); {
            stmt.setInt(1, country.getId());
            rs = stmt.executeQuery();
            while (rs.next()) {
              country.setLanguage(rs.getString("Language")); //assumes 1 per country - others are discarded
            }
            rs.close();
        }
    }
}

not perfect java - but should give you an idea where you are going wrong .

Other notes:

• The easy way to do this (assuming one language per country) would be to join the tables in the SELECT.
• You should also close your database connection when finished with it.
• I'm not sure that your try (stuff) {statements} syntax is correct.
• If there are multiple languages per country, you need that supported in your data structures/classes.
• I'm assuming your Country class has getters and setters for the fields.

Note: the question has been updated but I think this is still mostly relevant.

Answer 2

As by design, one Country can have multiple Language s, so your model should look like:

class Country {
    private int id;
    private String name;
    private List<String> language;
    //....
}

There are different ways you can construct it, most straight-forward one is something like (in pseudo-code):

List<Country> resultCountries=....;
countryRows = executeGetCountrySql
foreach countryRow in countryRows {
    id = row.get("ID");
    name = row.get("NAME");
    List<String> langs = new ArrayList<();
    langRows = executeGetLanguageByCountryIdSql(id);
    foreach langRow in langRows {
        langs.add(langRow.get("LANGUAGE"));
    }
    resultCountries.add(new Country(id, name, langs));
}

Some DB iirc does not allow concurrent resultsets. One way you can do is to read all countries first, and loop through each country and get its corresponding languages, and set it back to country.

Another way is to join the country table with country_language, and construct new country / add new language to existing country for each result row.