Java如何判断存储Book父类对象的ArrayList中元素所属的子类

以下是两种常用实现方式，按需选择即可：

方案1：使用instanceof直接判断类型（最小改动现有代码） #

步骤1：修改bookSearch方法返回匹配的书籍对象 #

原方法仅返回布尔值，调整为找到匹配书籍就返回对应的Book实例，找不到返回null：

// 修改LibraryMain中的bookSearch方法
public Book bookSearch(String bookTittle)
{
    for (int i = 0;i<LibBook.size();i++)
    {
        Book book = LibBook.get(i);
        if(book.getTittle().equalsIgnoreCase(bookTittle)){
            return book;
        }
    }
    return null;
}

步骤2：修改main方法的搜索逻辑，判断返回对象的类型 #

public static void main(String[] args) {
    LibraryMain mainLib = new LibraryMain();
    Scanner in = new Scanner(System.in);
    System.out.println("--Search book--");
    System.out.print("Book tittle: ");
    String tittle = in.nextLine();
    Book result = mainLib.bookSearch(tittle);
    if(result != null){
        String type = "";
        if(result instanceof Science){
            type = "science";
        }else if(result instanceof Fiction){
            type = "fiction";
        }
        System.out.printf("Book Found! %s is a %s book%n", result.getTittle(), type);
    }
    else{
        System.out.println("Not Found");
    }
}

方案2：父类新增类型方法（更符合面向对象设计，扩展性强） #

如果后续会新增更多书籍子类，推荐用这种方式，避免不断新增instanceof判断分支。

步骤1：修改Book类，新增获取书籍类型的方法 #

public class Book {
    private String bookTittle;
    private String bookAuthor;

    public Book(String tittle, String author)
    {
        this.bookTittle = tittle;
        this.bookAuthor = author;
    }

    public String getTittle()
    {
        return bookTittle;
    }
    
    // 新增方法，子类重写即可返回对应分类
    public String getBookType(){
        return "unknown";
    }

    // other getter and setter
}

步骤2：分别在两个子类重写getBookType方法 #

Science类新增重写逻辑：

@Override
public String getBookType() {
    return "science";
}

Fiction类新增重写逻辑：

@Override
public String getBookType() {
    return "fiction";
}

步骤3：修改main方法逻辑直接调用方法即可 #

public static void main(String[] args) {
    LibraryMain mainLib = new LibraryMain();
    Scanner in = new Scanner(System.in);
    System.out.println("--Search book--");
    System.out.print("Book tittle: ");
    String tittle = in.nextLine();
    Book result = mainLib.bookSearch(tittle);
    if(result != null){
        System.out.printf("Book Found! %s is a %s book%n", result.getTittle(), result.getBookType());
    }
    else{
        System.out.println("Not Found");
    }
}

内容的提问来源于stack exchange，提问作者Deni