将 ArrayList 从对象更改为更具体的类型

Question

是否可以更改数组列表的对象类型，即从对象 ArrayList 更改为特定对象 ArrayList。我试过为每个人使用一个。或者，有没有一种方法可以更改文件处理方法，使其可以 return 特定类型，具体取决于它从哪个文件读取而无需重复代码？

我的尝试：

ArrayList<Object> librarianList = FileHandling.getLibrarianRecords(fileName);
ArrayList<Librarian> libList = new ArrayList<>();
for (Object addType: librarianList) {
libList.add(addType);
}

getLibrarianRecords 代码

public static ArrayList<Object> getLibrarianRecords(String filename){
                ArrayList<Object> fromFile = new ArrayList<>(); //Array of
                // existing librarians

                try{
                        FileInputStream fIS =
                                new FileInputStream(SYSTEM_PATH + filename);
                        ObjectInputStream oIS = new ObjectInputStream(fIS);
                        fromFile = (ArrayList<Object>)oIS.readObject();
                } catch (IOException ex){
                        System.out.println("Failed to read from file " + ex.getMessage());
                        ex.printStackTrace(); //Catches an IO exception.
                } catch (ClassNotFoundException ex){
                        System.out.println("Error class not found" + ex.getMessage());
                        ex.printStackTrace(); //Catches a class not found
                        // exception.
                }

                return fromFile; //Returns the array list.
        }

Answer 1

感谢@user3170251 推荐演员

ArrayList<Object> librarianList = FileHandling.getLibrarianRecords(fileName);
ArrayList<Librarian> libList = new ArrayList<>();
for (Object addType: librarianList) {
libList.add((Librarian) addType);
}

对于更改类型，这确实有效。

Answer 2

您可以使用这个通用 class 定义

class ArrayListConverter<T> {
  List<T> cast;
  public ArrayListConverter(List<T> typeList){
   cast = new ArrayList<>();
   for (Object addType: typeList) {
      cast.add((T)addType);
   }

  }
  public List<T> getList(){
     return cast;
  }

}

只需这样做：

ArrayListConverter<Librarian> conv = new ArrayListConverter<>(libList);
ArrayList<Librarian> libListOg  = conv.getList();

Answer 3

像这样从文件中读取对象很少是个好主意。也就是说，您真正需要做的就是将 oIS.readObject() 的结果转换为 ArrayList<Librarian> 而不是将其传送到 ArrayList<Object> （就像您现在所做的那样），然后修改 return 类型 getLibrarianRecords。哦，自然还有局部变量的类型 fromFile.

    public static ArrayList<Librarian> getLibrarianRecords(String filename){
            ArrayList<Librarian> fromFile = new ArrayList<>(); //Array of
            // existing librarians

            try{
                    FileInputStream fIS =
                            new FileInputStream(SYSTEM_PATH + filename);
                    ObjectInputStream oIS = new ObjectInputStream(fIS);
                    fromFile = (ArrayList<Librarian>)oIS.readObject();
            } catch (IOException ex){
                    System.out.println("Failed to read from file " + ex.getMessage());
                    ex.printStackTrace(); //Catches an IO exception.
            } catch (ClassNotFoundException ex){
                    System.out.println("Error class not found" + ex.getMessage());
                    ex.printStackTrace(); //Catches a class not found
                    // exception.
            }

            return fromFile; //Returns the array list.
    }

这样就不需要遍历列表来实际地逐个元素地进行类型转换。

将 ArrayList 从对象更改为更具体的类型

Change ArrayList from an Object to more specific type

java

object

arraylist

file-handling

object-type