如何重構 Java 中的多個 if-else 語句？

Question

我如何重構所有這些看起來重復且太長的代碼，有沒有辦法讓它更短？

if (typeOfData.equals("Book data")) 
{
   System.out.println(lineOfText);   
   Scanner scanner2 = new Scanner(lineOfText); 
   LibraryItem libraryItem = new Book();
   libraryItem.readData(scanner2);
   storeItem(libraryItem);
   scanner2.close(); // ends scanner2                        
}
else if (typeOfData.equals("Periodical data"))
{
   System.out.println(lineOfText);                  
   Scanner scanner2 = new Scanner(lineOfText); 
   LibraryItem libraryItem = new Periodical(); // LibrayItem => Periodical(subtype)
   libraryItem.readData(scanner2);
   storeItem(libraryItem);
   scanner2.close(); // ends scanner2 
}
else if (typeOfData.equals("CD data"))
{
   System.out.println(lineOfText);                  
   Scanner scanner2 = new Scanner(lineOfText); 
   LibraryItem libraryItem = new CD(); // LibrayItem => CD(subtype)
   libraryItem.readData(scanner2);
   storeItem(libraryItem);
   scanner2.close(); // ends scanner2                     
} 
else if (typeOfData.equals("DVD data"))
{
   System.out.println(lineOfText);                  
   Scanner scanner2 = new Scanner(lineOfText); 
   LibraryItem libraryItem = new DVD();
   libraryItem.readData(scanner2);
   storeItem(libraryItem);
   scanner2.close(); // ends scanner2 
}
else if (typeOfData.equals("Library User data"))
{
   System.out.println(lineOfText);
   Scanner scanner2 = new Scanner(lineOfText);
   LibraryUser libraryUser = new LibraryUser();
   libraryUser.readData(scanner2);
   storeUser(libraryUser);
   scanner2.close(); // ends scanner2 
}

我曾嘗試使用 Switch 語句，但在這種情況下不起作用。

“typeOfData”變量包含一個用於匹配相關行的字符串。

Answer 1

簡化

您可以在ifs之前或之后提取公共行

System.out.println(lineOfText);
Scanner scanner2 = new Scanner(lineOfText);

if (typeOfData.equals("Book data")) {
    LibraryItem libraryItem = new Book();
    libraryItem.readData(scanner2);
    storeItem(libraryItem);
} else if (typeOfData.equals("Periodical data")) {
    LibraryItem libraryItem = new Periodical(); // LibrayItem => Periodical(subtype)
    libraryItem.readData(scanner2);
    storeItem(libraryItem);
} else if (typeOfData.equals("CD data")) {
    LibraryItem libraryItem = new CD(); // LibrayItem => CD(subtype)
    libraryItem.readData(scanner2);
    storeItem(libraryItem);
} else if (typeOfData.equals("DVD data")) {
    LibraryItem libraryItem = new DVD();
    libraryItem.readData(scanner2);
    storeItem(libraryItem);
} else if (typeOfData.equals("Library User data")) {
    LibraryUser libraryUser = new LibraryUser();
    libraryUser.readData(scanner2);
    storeUser(libraryUser);
}

scanner2.close(); // ends scanner2 

---

提升

您可以想象構造函數將Scanner作為參數，例如

public Book(Scanner sc) {
    readData(sc);
}

那么ifs就變成了

if (typeOfData.equals("Book data")) {
    storeItem(new Book(scanner2));
} else if (typeOfData.equals("Periodical data")) {
    storeItem(new Periodical(scanner2));
} else if (typeOfData.equals("CD data")) {
    storeItem(new CD(scanner2));
} else if (typeOfData.equals("DVD data")) {
    storeItem(new DVD(scanner2));
} else if (typeOfData.equals("Library User data")) {
    storeUser(new LibraryUser(scanner2));
}

或者一個switch

switch (typeOfData) {
    case "Book data"            -> storeItem(new Book(scanner2));
    case "Periodical data"      -> storeItem(new Periodical(scanner2));
    case "CD data"              -> storeItem(new CD(scanner2));
    case "DVD data"             -> storeItem(new DVD(scanner2));
    case "Library User data"    -> storeUser(new LibraryUser(scanner2));
}

Answer 2

這是我能得到的最接近的：

        System.out.println(lineOfText);
        Scanner scanner2 = new Scanner(lineOfText);
        if (typeOfData.equals("Book data"))
        {
            LibraryItem libraryItem = new Book();
            libraryItem.readData(scanner2);
            storeItem(libraryItem);
        }
        else if (typeOfData.equals("Periodical data"))
        {
            LibraryItem libraryItem = new Periodical(); // LibrayItem => Periodical(subtype)
            libraryItem.readData(scanner2);
            storeItem(libraryItem);
        }
        else if (typeOfData.equals("CD data"))
        {
            LibraryItem libraryItem = new CD(); // LibrayItem => CD(subtype)
            libraryItem.readData(scanner2);
            storeItem(libraryItem);
        }
        else if (typeOfData.equals("DVD data"))
        {
            LibraryItem libraryItem = new DVD();
            libraryItem.readData(scanner2);
            storeItem(libraryItem);
        }
        else if (typeOfData.equals("Library User data"))
        {
            LibraryUser libraryUser = new LibraryUser();
            libraryUser.readData(scanner2);
            storeUser(libraryUser);
        }
        scanner2.close(); // ends scanner2

Answer 3

System.out.println(lineOfText);   
Scanner scanner2 = new Scanner(lineOfText);
LibraryItem libraryItem = null;
if (typeOfData.equals("Book data")) 
{
   LibraryItem libraryItem = new Book();                   
}
else if (typeOfData.equals("Periodical data"))
{
   LibraryItem libraryItem = new Periodical();
}
else if (typeOfData.equals("CD data"))
{
   LibraryItem libraryItem = new CD();                     
} 
else if (typeOfData.equals("DVD data"))
{
   LibraryItem libraryItem = new DVD();
}
else if (typeOfData.equals("Library User data"))
{
   LibraryUser libraryUser = new LibraryUser();
}
if(libraryItem != null){
    libraryItem.readData(scanner2);
    storeItem(libraryItem);
}

scanner2.close(); 

Answer 4

也許您可以在 LibraryItem class 中使用工廠方法。

class LibraryItem {

    public static LibraryItem from(String typeOfData) {
        if (typeOfData.equals("Book data")) {
            return new Book();
        }
        if (typeOfData.equals("Periodical data")) {
            return new Periodical();
        }
        if (typeOfData.equals("CD data")) {
            return new CD();
        }
        if (typeOfData.equals("DVD data")) {
            return new DVD();
        }
        if (typeOfData.equals("Library User data")) {
            return new LibraryUser();
        } 
        
        throw new IllegalArgumentException();
    }
}

接着

System.out.println(lineOfText);   
Scanner scanner2 = new Scanner(lineOfText); 
LibraryItem libraryItem = LibraryItem.from(typeOfData);
libraryItem.readData(scanner2);
storeItem(libraryItem);
scanner2.close(); // ends scanner2 

編輯

我剛剛看到可能 LibraryUser 不擴展 LibraryItem。 但也許你可以為方法 readData(Scanner s) 提取一個接口並應用相同的模式