在 Java 中创建多种数据类型的数组列表 [英] Creating an array list of multiple data types in Java
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.
公共类城市{字符串国家/地区代码；弦城；字符串区域；字符串人口；字符串纬度；字符串经度；公共城市(字符串国家代码，字符串城市，字符串地区，字符串人口，字符串纬度，字符串经度){this.countrycode = 国家/地区代码；this.city = 城市；this.region = 区域；this.population = 人口；this.latitude = 纬度；this.longitude = 经度；}公共字符串 toString() {返回 this.city + "," + this.population+ "," + this.latitude + ","+ this.longitude;}}<小时>
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 并更改 City 类中的数据类型，但它仍然给了我很多错误.
公共类阅读器{In input = new In("file:world_cities.txt");私有静态城市城市信息；公共静态无效主(字符串 [] args){//打开文件In input = new In("world_cities.txt");input = new In("world_cities.txt");尝试 {//将输出写入文件FileWriter fw = new FileWriter("cities_out.txt");PrintWriter pw = new PrintWriter(fw);整数行 = 0;//遍历文件中的所有行而(行 < 47913){//读取行String cityLine = input.readLine();//创建数组列表ArrayListcityList = new ArrayList(Arrays.asList(cityLine.split(",")));//添加行到数组列表cityList.add(cityLine);//增加计数器线 += 1;//创建实例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);//打印输出到文件pw.println(cityInfo);}//关闭文件pw.close();}//保存到文件时出错时打印的内容捕获(异常 e){System.out.println("错误！");}//关闭文件input.close();}} 如果声明列表如下，则可以将任何引用类型的实例放入其中:
列表<对象>list = new ArrayList<>();但缺点是当您从列表中获取一个元素时，该元素的静态类型将是 Object，您需要将其类型转换为您需要的类型.
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城市列表 =new ArrayList(Arrays.asList(cityLine.split(","))); 如果您将列表更改为 List，其中对象是 Integer 或 Double，您将无法像这样建立你的清单.
事实上，我越看这个，我就越觉得你根本不需要清单.你应该做这样的事情:
//读取行String cityLine = input.readLine();String[] 部分 = cityLine.split(",");//增加计数器线 += 1;//创建实例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！！
Some of the file I'm working with: http://pastebin.com/WriQcuPs
Currently I had to make the population, latitude, and longitude strings or else I wouldn't get the desired output. I want for them to be int, double, and double in that order.
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;
}
}
I suspect it has something to do with how I created the array list. Is there a way to make it so that some of the elements of the list are of a different type? I tried changing it to ArrayList<City> and changing the data types in the City class but it still gave me a ton of errors.
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();
}
}
If you declare the list as follows, you can put instances of any reference type into it:
List<Object> list = new ArrayList<>();
But the downside is that when you get an element from the list, the static type of the element will be Object, and you will need to type cast it to the type that you need.
Also note, that you can't put an int or a double into a List. Primitive types are not reference types, and the List API requires the elements to be instances of reference types. You need to use the corresponding wrapper types; i.e. Integer and Double.
Looking at more of your code, I spotted this:
ArrayList<String> cityList =
new ArrayList<String>(Arrays.asList(cityLine.split(",")));
If you change the list to List<Object> where the objects are either Integer or Double, you won't be able to build your list like that.
In fact, the more I look this, the more I think that you don't need a list at all. You should be doing something like this:
// 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]));
Notice: there is no List there at all!!
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