一種可以從父數組返回子對象的方法？

Question

我正在尋找一種可以從父數組返回子對象的方法 - 這意味着如何使方法返回子對象？

問題是我有這個方法可以從數組中返回一個父對象，但是在程序中我需要一個具有自己屬性的子對象，並且該數組有 3 種不同類型的子對象

這是我到目前為止：

class Store {
    private BookParents store[];
    private int ind, max;

    public Store() {
         ind=0; //Begin 0 and change with the method AddBooks;
         max=100;
         store = new BookParents[100];
    } 
public String AddBooks(BooksChild1 a){
    if(ind<max){
        store[ind++]=a;
        return "TheBooksChild added correctly";
    }
      return "Full store";
    }
public String AddBooks(BooksChild2 b){
    if(ind<max){
        store[ind++]=b;
        return "TheBooksChild added correctly";
    }
      return "Full store";
    }
public String AddBooks(BooksChild3 c){
    if(ind<max){
        store[ind++]=c;
        return "TheBooksChild added correctly";
    }
      return "Full store";
    }

    public BooksParents SearchBook(String c) {
        AnyBookChild null1 = new Book1("Unknown", 0,"Unknown","Unknown","Unknown","Unknown","Unknown");

        if(ind!=0){
            for(int i=0 ;i<ind;i++){                    
                if(c.compareTo(store[i].getName(store[i]) )==0)
                    System.out.println(store[i].PrintlnBook());                            
                    return store[i]; 
            }
            System.out.println("Book didn't find, try another name.");
            return null;
         } else {
             System.out.println("There is not books in the store");
             return null;
         }
    }
}

Answer 1

由於您有多個 BooksChild 類，因此您將需要 BooksChild 類擴展的 BookParents 類。 您仍然可以使用 BookParents 數組：

store = new BookParents[100];

在每個 BooksChild 中，您將使用一個如下所示的構造函數：

public class BooksChild1 extends BookParents{
     String myISBN, myTitle, myAuthor;

     public BooksChild1(String isbn, String title, String author){ 
         super(1, isbn, title, author); //1 denotes the class type 
     } 
 }

在 BookParents 類中，您將擁有另一個構造函數：

public class BookParents{
     int myType;
     String myISBN, myTitle, myAuthor;

     public BookParents(int type, String isbn, String title, String author){ 
         //when a new BooksChild1 is created, it will set the myType equal to 1
         myType   = type; 
         myISBN   = isbn;
         myTitle  = title;
         myAuthor = author; 
    }

    //this accessor will allow you to find the type of BooksChild 
    public int getMyType(){ 
        return myType;
    }
}

`

現在對於商店部分，將 BooksChild 添加到 BooksParent 數組：

public class Store{
    //these declarations must be public if you want to use them in a public methods
    public BookParents store[];
    public int ind, max;

    public Store() {
        ind=0; //Begin 0 and change with the method AddBooks;
        max=100;
        store = new BookParents[100];
    }

    public String addBook(BookParnets bp){
        /*must be max-1 or otherwise it will try 
          to push a final ind of 100 which is out of bounds*/
        if(ind<max-1){ 
           store[ind++]=bp;
           return "TheBooksChild added correctly";
        }
        return "Full store";
    }
}

當您聲明 BooksChild 類型對象以將 BooksChild1 插入您的商店時，最后一部分將存在於您的驅動程序類中，BooksChild1 的聲明語句如下：

BookParents b = new BooksChild1("isbn", "title", "author");

如果您需要在您的商店數組中查找 BooksChild 的類型，您可以使用以下命令返回類型（讓 store[0] 包含一個未知類型的 BooksChild）：

 store[0].getMyType();