ava图书馆通过多态实现计算罚金代码，分为儿童书籍，成人书籍，音像制品，读者可一次借阅多本书籍和音像制品代码

非常感谢您的提问。以下是 Java 图书馆通过多态实现计算罚金的代码：

public abstract class Item {
    protected String title;
    protected int playingTime;
    protected boolean gotIt;
    protected int daysOverdue;

    public Item(String title, int playingTime) {
        this.title = title;
        this.playingTime = playingTime;
        this.gotIt = false;
        this.daysOverdue = 0;
    }

    public void setGotIt(boolean gotIt) {
        this.gotIt = gotIt;
    }

    public void setDaysOverdue(int daysOverdue) {
        this.daysOverdue = daysOverdue;
    }

    public abstract double calculateFine();

    public String toString() {
        String result = title + " (" + playingTime + " mins)";
        if (gotIt) {
            result += "*";
        }
        return result;
    }
}

public class Book extends Item {
    private String author;

    public Book(String title, String author, int playingTime) {
        super(title, playingTime);
        this.author = author;
    }

    public double calculateFine() {
        double fine = 0.0;
        if (daysOverdue > 0) {
            fine = daysOverdue * 0.25;
        }
        return fine;
    }

    public String toString() {
        return super.toString() + " by " + author;
    }
}

public class Audio extends Item {
    private String artist;

    public Audio(String title, String artist, int playingTime) {
        super(title, playingTime);
        this.artist = artist;
    }

    public double calculateFine() {
        double fine = 0.0;
        if (daysOverdue > 0) {
            fine = daysOverdue * 0.50;
        }
        return fine;
    }

    public String toString() {
        return super.toString() + " by " + artist;
    }
}

public class Library {
    private ArrayList<Item> items;

    public Library() {
        items = new ArrayList<Item>();
    }

    public void addItem(Item item) {
        items.add(item);
    }

    public void printCatalog() {
        for (Item item : items) {
            System.out.println(item);
        }
    }

    public void calculateFine() {
        for (Item item : items) {
            double fine = item.calculateFine();
            if (fine > 0.0) {
                System.out.println(item + " fine: $" + fine);
            }
        }
    }
}

读者可以一次借阅多本书籍和音像制品，可以在 Library 类中添加一个 ArrayList<Loan> loans，Loan 类中包含 Item 和借阅日期等信息。calculateFine() 方法中需要遍历 loans，计算每个 Loan 的罚金。