Java 方法讀取文本文件並返回 ArrayList 類型 object

Question

    public static void main(String[] args)
    {
        ArrayList <Locations>       LocationsList       = readFile("Locations.csv", "Locations");
        //ArrayList <Movies>          MoviesList          = readFile("Movies.csv", "Movies");
        //ArrayList <Operators>       OperatorsList       = readFile("Operators.csv", "Operators");
        //ArrayList <PersonCategory>  PersonCategoryList  = readFile("PersonCategory.csv", "PersonCategory");
    }
    
    public static ArrayList readFile(String fileName, String whichFile)
    {
        ArrayList list = new ArrayList();

        try
        {
            BufferedReader br = new BufferedReader(new FileReader(fileName));
            
            String indata;
            
            int line = 0;
            while((indata=br.readLine())!=null)
            {
                StringTokenizer st = new StringTokenizer(indata,",");
                
                if(line != 0)
                {
                    if(whichFile.equals("Locations"))
                    {
                        int id = Integer.parseInt(st.nextToken());
                        String city = st.nextToken();
                        if(city.charAt(0) == '"')
                        {
                            String c = st.nextToken();
                            city = city.substring(1,city.length()) +"," +c.substring(0,c.length()-1);
                        }
                        int stateId = Integer.parseInt(st.nextToken());
                        
                        Locations x = new Locations(id, city, stateId);
                        list.add(x);
                    }
                    
                    else if(whichFile.equals("Movies"))
                    {
                        int id = Integer.parseInt(st.nextToken());
                        String name = st.nextToken();
                        int ratingId = Integer.parseInt(st.nextToken());
                        
                        Movies x = new Movies(id, name, ratingId);
                        list.add(x);
                    }                              
                }
                
                line++;
            }
            
            br.close();
        }
        catch (FileNotFoundException fnfe){System.out.println(fnfe.getMessage());}
        catch (IOException io){System.out.println(io.getMessage());}
        catch (Exception e){System.out.println(e.getMessage());}
        
        return list;
    }

我正在嘗試創建一種方法，該方法將讀取文本文件並可以返回 ArrayList 類型 object 以使用多個 Class。 使用我上面的代碼，它可以成功運行。

但是，有如下警告行：“ ArrayList類型的表達式需要未經檢查的轉換以符合ArrayList<Locations> ”

我該如何解決？

Answer 1

嘗試這個。

public static <T> ArrayList<T> readFile(String fileName, Function<String[], T> converter) throws IOException {
    ArrayList<T> result = new ArrayList<>();
    try (BufferedReader reader = Files.newBufferedReader(Paths.get(fileName))) {
        String line = reader.readLine();
        String[] fields = line.split(",");
        T object = converter.apply(fields);
        result.add(object);
    }
    return result;
}

並定義將 CSV 線轉換為 object 的轉換器。

static Locations convertLocations(String[] fields) {
    int id = Integer.parseInt(fields[0]);
    String city = fields[1];
    if (city.charAt(0) == '"') {
        String c = fields[2];
        city = city.substring(1, city.length()) + "," + c.substring(0, c.length() - 1);
    }
    int stateId = Integer.parseInt(fields[3]);
    Locations x = new Locations(id, city, stateId);
    return x;
}

static Movies convertMovies(String[] fields) {
    /* Make Movies object from fields */
}

並將它們結合起來。

ArrayList<Locations> LocationsList = readFile("Locations.csv", fields -> convertLocations(fields));
ArrayList<Movies> MoviesList = readFile("Movies.csv", fields -> convertMovies(fields));

Answer 2

您需要使用例如創建正確的基於泛型的ArrayList ： new ArrayList<Location>()

您可以通過將 class 傳遞給 readFile 來解決此問題，如下所示：

public static <T> ArrayList<T> readFile(....., Class<T> clazz)
{
   ArrayList<T> list = new ArrayList<T>();
   ...
}

Answer 3

本質上，您需要為通用 class ArrayList指定類型參數。

由於您將從不同類創建的對象添加到同一個列表中，因此您可以創建一個接口，例如MyInterface

public interface MyInterface {
    ....
}

從readFile返回的所有類都必須實現此接口。 例如。

public class Movies implements MyInterface {
    ....
}

現在，您可以在適當的位置添加類型參數MyInterface ：

public static void main(String[] args) {
        ArrayList<MyInterface> LocationsList = readFile("Locations.csv", "Locations");
        ....
    }

public static ArrayList<MyInterface> readFile(String fileName, String whichFile) {
 
  ArrayList<MyInterface> list = new ArrayList<>();
            ....
        }

根據回復添加以下信息

實際上，您可能選擇將接口留空，但是您必須將對象顯式轉換為具體類才能做任何有用的事情。

1. 您可以在需要時投射每個 object

        MyInterface myInterfaceObject = locationsList.get(0)
        Locations locations = Locations.class.cast(myInterfaceObject);

或者

        MyInterface myInterfaceObject = locationsList.get(0)
        Locations locations = (Locations) myInterfaceObject;

2. 或者您可以為每種具體類型編寫一個列表轉換器 function

public class ListConverter {
  public ArrayList<Locations> toLocationsArraylist(ArrayList<MyInterface> inList) {
      ArrayList<Locations> outList = new ArrayList<>();
      for (MyInterface listItem : inList) {
          outList.add((Locations) listItem);
      }
      return outList;
  }
}

接着

public static void main(String[] args) {
        ArrayList<MyInterface> myInterfaceList = readFile("Locations.csv", "Locations");
        ArrayList<Locations> locationList = ListConverter.toLocationsArraylist(myInterfaceList);
       
    }

如果您確實考慮使用此解決方案，請考慮更適當地重命名 MyInterface ，例如，重命名為CsvRecord或任何特定於域的名稱。

Answer 4

這是我從@saka1029 獲取的最終代碼並進行了一些調整，以便它可以讀取文件中除第一行之外的每一行。

    public static <T> ArrayList<T> readFile(String fileName, Function<String[], T> converter)
    {
        ArrayList <T> list = new ArrayList<>();

        try
        {
            BufferedReader br = new BufferedReader(new FileReader(fileName));
            br.readLine();
            
            String inData;
            
            while((inData=br.readLine()) != null)
            {
                String[] fields = inData.split(",");
                T object = converter.apply(fields);
                list.add(object);
            }
            
            br.close();
        }
        catch (FileNotFoundException fnfe){System.out.println(fnfe.getMessage());}
        catch (IOException io){System.out.println(io.getMessage());}
        catch (Exception e){System.out.println(e.getMessage());}
        
        return list;
    }

這是我對 @saka1029 答案中convertLocations方法的更正版本。

    static Locations convertLocations(String[] fields)
    {
        int id = Integer.parseInt(fields[0]);
        String city = fields[1];
        int stateId;
        if (city.charAt(0) == '"')
        {
            String c = fields[2];
            city = city.substring(1, city.length()) + "," + c.substring(0, c.length() - 1);
            stateId = Integer.parseInt(fields[3]);
        }
        else
            stateId = Integer.parseInt(fields[2]);
        
        Locations x = new Locations(id, city, stateId);
        
        return x;
    }