在Java中创建多个数据类型的数组列表

Question

我正在使用的一些文件：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。我希望他们按顺序为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; 
    }
}

我怀疑它与我创建数组列表的方式有关。是否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，但它仍然给我带来了大量错误。

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();
    }
}

Answer 1

如果您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得一个元素时，元素的静态类型将是Object，你需要输入它所需的类型。

另请注意，您无法将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类型;即Integer和Double。

查看更多代码，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对象为List<Object>或Integer的{​​{1}}，则您无法像这样构建列表。

事实上，我看的越多，我就越认为你根本不需要一个清单。你应该做这样的事情：

Double

注意：根本没有 // 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])); !!

Answer 2

您可以在将字符串值传递到新的City对象之前解析它们。然后你可以将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);

Answer 3

通过查看已定义的City类，成员具有不同的原始数据类型。当您读取文件以创建City时，您需要使用构造函数中定义的数据类型传递构造函数参数。

修改您的City课程，如下所示：

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int，并按City类的要求加倍。

Answer 4

我相信不，但你可以这样做

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();
}
}

Answer 5

您可以这样做：

读者类

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();
    }
}

CITY CLASS

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; 
    }
}

Answer 6

  

有没有办法让它成为列表中的一些元素   不同类型的？

List可能包含不同类型的元素，但如果您使用String.split()填充它则不会 - 因为它会返回String[]。

您可以使用Java基元类型包装器上的String.split()方法将从[{1}}返回的字符串转换为所需类型，例如......

valueOf

Answer 7

就像Stephen建议的那样，您可以使用List<Object>，除此之外，您可以将String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，您已经发起了In两次。

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

将其修改为

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