在 Java 中创建多个数据类型的数组列表答案

【问题标题】：Creating an array list of multiple data types in Java在 Java 中创建多个数据类型的数组列表
【发布时间】：2016-11-13 03:33:13
【问题描述】：

我正在处理的一些文件：http://pastebin.com/WriQcuPs

目前我必须制作人口、纬度和经度字符串，否则我将无法获得所需的输出。我希望它们按顺序是 int、double 和 double。

public class City {
    String countrycode;
    String city;
    String region;
    String population;
    String latitude;
    String longitude;

    public City (String countrycode, String city, String region, String population, String latitude, String longitude) {
        this.countrycode = countrycode;
        this.city = city; 
        this.region = region;
        this.population = population;
        this.latitude = latitude;
        this.longitude = longitude;
    }

    public String toString() {
        return this.city + "," + this.population 
                + "," + this.latitude + ","
                + this.longitude; 
    }
}

我怀疑这与我创建数组列表的方式有关。有没有办法使列表中的某些元素具有不同的类型？我尝试将其更改为 ArrayList<City> 并更改 City 类中的数据类型，但它仍然给了我很多错误。

public class Reader {

    In input = new In("file:world_cities.txt");
    private static City cityInfo;

    public static void main(String[] args) {
        // open file
        In input = new In("world_cities.txt");
        input = new In("world_cities.txt");
        try {
        // write output to file
        FileWriter fw = new FileWriter("cities_out.txt");
        PrintWriter pw = new PrintWriter(fw);

        int line = 0;

        // iterate through all lines in the file
        while (line < 47913) {

            // read line
            String cityLine = input.readLine();

            // create array list
            ArrayList<String> cityList = new ArrayList<String>(Arrays.asList(cityLine.split(",")));

            // add line to array list
            cityList.add(cityLine);

            // increase counter
            line += 1;      

            // create instance
            cityInfo = new City(cityList.get(0), cityList.get(1), cityList.get(2), cityList.get(3), cityList.get(4), cityList.get(5));
            System.out.println(cityInfo);

            // print output to file 
            pw.println(cityInfo); 
        }

        // close file
        pw.close();
        }

        // what is printed when there is an error when saving to file
        catch (Exception e) {
            System.out.println("ERROR!");
        }

        // close the file
        input.close();
    }
}

【问题讨论】：

标签： java arraylist

【解决方案1】：

如果按如下方式声明列表，则可以将任何引用类型的实例放入其中：

    List<Object> list = new ArrayList<>();

但缺点是当你从列表中获取一个元素时，该元素的静态类型将是Object，你需要将它类型转换为你需要的类型。

另请注意，您不能将int 或double 放入List。原始类型不是引用类型，List API 要求元素是引用类型的实例。您需要使用相应的包装器类型；即Integer 和Double。

查看您的更多代码，我发现了这一点：

ArrayList<String> cityList = 
    new ArrayList<String>(Arrays.asList(cityLine.split(",")));

如果您将列表更改为List<Object>，其中对象为Integer 或Double，您将无法这样构建您的列表。

事实上，我越看这个，我越觉得你根本不需要列表。你应该这样做：

    // read line
    String cityLine = input.readLine();

    String[] parts = cityLine.split(",");
    // increase counter
    line += 1;      

    // create instance
    cityInfo = new City(parts[0], parts[1], parts[2], 
                        Integer.parseInt(parts[3]),
                        Double.parseDouble(parts[4]),
                        Double.parseDouble(parts[5]));

注意：那里根本没有List！！

【讨论】：

太棒了！效果很好。非常感谢！

【解决方案2】：

您可以在将字符串值传递到新的City 对象之前对其进行解析。然后，您可以将 City 中的构造函数和变量更改为 int、double 和 double。

int pop = Integer.parseInt(cityList.get(3));
double latitude = Double.parseDouble(cityList.get(4));
double longitude = Double.parseDouble(cityList.get(5));
cityInfo = new City(cityList.get(0), cityList.get(1), cityList.get(2), pop, latitude, longitude);

【讨论】：

【解决方案3】：

通过查看City 类，您已经定义了成员属于不同的原始数据类型。当您读取文件以创建城市时，您需要使用在构造函数中定义的数据类型传递构造函数参数。

修改你的City类如下：

public class City {
    String countrycode;
    String city;
    String region;
    int population;
    double latitude;
    double longitude;
...

尝试以下方法：

cityInfo = new City(cityList.get(0), cityList.get(1), cityList.get(2), Integer.parseInt(cityList.get(3)), Double.parseDouble(cityList.get(4)), Double.parseDouble(cityList.get(5)));

这会将字符串转换为 City 类所需的 int 和 double。

【讨论】：

【解决方案4】：

我相信不，但你可以做到这一点

public class City {
String countrycode;
String city;
String region;
String population;
double latitude;
double longitude;

public City (String countrycode, String city, String region, String population, double latitude, double longitude) {
    this.countrycode = countrycode;
    this.city = city; 
    this.region = region;
    this.population = population;
    this.latitude = latitude;
    this.longitude = longitude;
}

public String toString() {
    return this.city + "," + this.population 
            + "," + this.latitude + ","
            + this.longitude; 
}
}

public class Reader {

In input = new In("file:world_cities.txt");
private static City cityInfo;

public static void main(String[] args) {
    // open file
    In input = new In("world_cities.txt");
    input = new In("world_cities.txt");
    try {
    // write output to file
    FileWriter fw = new FileWriter("cities_out.txt");
    PrintWriter pw = new PrintWriter(fw);

    int line = 0;

    // iterate through all lines in the file
    while (line < 47913) {

        // read line
        String cityLine = input.readLine();

        // create array list
        ArrayList<String> cityList = new ArrayList<String>(Arrays.asList(cityLine.split(",")));

        // add line to array list
        cityList.add(cityLine);

        // increase counter
        line += 1;      

        // create instance
        cityInfo = new City(cityList.get(0), cityList.get(1), cityList.get(2), cityList.get(3), Doule.parseDouble(cityList.get(4)), Doule.parseDouble(cityList.get(5)));
        System.out.println(cityInfo);

        // print output to file 
        pw.println(cityInfo); 
    }

    // close file
    pw.close();
    }

    // what is printed when there is an error when saving to file
    catch (Exception e) {
        System.out.println("ERROR!");
    }

    // close the file
    input.close();
}
}

【讨论】：

【解决方案5】：

你可以这样做：

阅读器类

import java.io.FileWriter;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;

public class Reader {

    In input = new In("file:world_cities.txt");
    private static City cityInfo;

    public static void main(String[] args) {
        // open file
//        In input = new In("world_cities.txt");
//        input = new In("world_cities.txt");
        try {
        // write output to file
        FileWriter fw = new FileWriter("cities_out.txt");
        PrintWriter pw = new PrintWriter(fw);

        int line = 0;

        // iterate through all lines in the file
        while (line < 47913) {

            // read line
            String cityLine = input.readLine();

            // create array list
            ArrayList<String> cityList = new ArrayList<String>(Arrays.asList(cityLine.split(",")));

            // add line to array list
            cityList.add(cityLine);

            // increase counter
            line += 1;      

            // create instance
            cityInfo = new City(cityList.get(0), cityList.get(1), cityList.get(2), Integer.parseInt(cityList.get(3)), Double.parseDouble(cityList.get(4)), Double.parseDouble(cityList.get(5)));
            System.out.println(cityInfo);

            // print output to file 
            pw.println(cityInfo); 
        }

        // close file
        pw.close();
        }

        // what is printed when there is an error when saving to file
        catch (Exception e) {
            System.out.println("ERROR!");
        }

        // close the file
        input.close();
    }
}

城市等级

public class City {
    String countrycode;
    String city;
    String region;
    int population;
    double latitude;
    double longitude;

    public City (String countrycode, String city, String region, int population, double latitude, double longitude) {
        this.countrycode = countrycode;
        this.city = city; 
        this.region = region;
        this.population = population;
        this.latitude = latitude;
        this.longitude = longitude;
    }

    public String toString() {
        return this.city + "," + this.population 
                + "," + this.latitude + ","
                + this.longitude; 
    }
}

【讨论】：

【解决方案6】：

有没有办法让列表中的一些元素不同的类型？

List 可以包含不同类型的元素，但如果您使用 String.split() 填充它则不行——因为这会返回 String[]。

例如，您可以使用 Java 原始类型包装器上的 valueOf 方法将从 String.split() 返回的字符串转换为所需的类型...

Integer population = Integer.valueOf("1234567");  // Returns 1234567 as an Integer, which can autobox to an int if you prefer

【讨论】：

【解决方案7】：

就像斯蒂芬建议的那样，您可以使用List<Object>，除此之外，您可以将字符串传递给 City，但让 City 自己处理数据类型。

public class City {
    String countrycode;
    String city;
    String region;
    int population;
    double latitude;
    double longitude;

    public City (String countrycode, String city, String region, String population, String latitude, String longitude) {
        this.countrycode = countrycode;
        this.city = city; 
        this.region = region;
        try{
            this.population = Integer.parseInt(population);
            this.latitude = Double.parseDouble(latitude);
            this.longitude = Double.parseDouble(longitude);
        }catch(Exception e){
            e.printStacktrace();
        }

    }

    public String toString() {
        return this.city + "," + this.population 
                + "," + this.latitude + ","
                + this.longitude; 
    }
}

顺便说一句，你已经发起了两次In。

In input = new In("world_cities.txt");
input = new In("world_cities.txt");

修改为

In input = new In("world_cities.txt");

【讨论】：

哎呀！好收获！