Java:如何将addUp BigDecimal值存储在映射中

Question

任务是练习收集而不使用流。有一些类似于图书馆的基本功能:我们有关于借阅的读者、书籍和信息。有4类/记录如下：

public record Book(String title, String author, BigDecimal priceForKeepingLonger) {} 
public record Reader(String name, String lastName, int age, BigDecimal fineAmount) {}
public record Borrowing(Book book, Reader reader, LocalDate borrowingDate) {}
psvm

任务：

• 创建了一张地图，其键为: Reader :用于保持书籍长度的罚款金额(超过14天)。
• 一个用户可以预订超过1本书。

问题：

• 我必须增加每个用户的罚款，但我无法做到这一点。我有一种方法可以分别计算每本书的数量..

你能帮帮我吗。下面的主要课程(我在这里很新鲜，所以希望足够多的数据/代码)：

public class Main {

static BigDecimal countFine(Borrowing borrowing){

    if(borrowing == null){
        throw new IllegalArgumentException("Borrowing collections is null");
    }
    LocalDate todaysDate = LocalDate.now();
    BigDecimal sum = BigDecimal.ZERO;
    long howManyDaysBookIsBorrowedAlready = -ChronoUnit.DAYS.between(todaysDate, borrowing.borrowingDate());
    BigDecimal amountOfDaysMinus14 = BigDecimal.valueOf(howManyDaysBookIsBorrowedAlready).subtract(BigDecimal.valueOf(14));
    if (howManyDaysBookIsBorrowedAlready <=14){
        sum = BigDecimal.ZERO;
    }
    else{
        sum = sum.add(BigDecimal.valueOf(5).multiply(amountOfDaysMinus14));
        if(borrowing.reader().age() <18){
            sum = sum.divide(BigDecimal.valueOf(2));
        }
    }
    return sum;
}


public static Map<Reader, BigDecimal> createMapReaderWithAMountOfFines(Set<Borrowing> borrowings){

    if(borrowings == null){
        throw new IllegalArgumentException("Borrowing set collections cannot be null");
    }
    Map<Reader, BigDecimal> readerWithAMountOfFines = new HashMap<>();

    borrowings.forEach(borrowing -> {

        if(readerWithAMountOfFines.containsKey(borrowing.reader())){

            readerWithAMountOfFines.get(borrowing.reader()).add(countFine(borrowing));
        }
        else {
            readerWithAMountOfFines.put(borrowing.reader(),countFine(borrowing));
        }
    } );
    return readerWithAMountOfFines;
}

public static void main(String[] args) {
    Reader r1 = new Reader("Tomasz", "Knapik", 21, BigDecimal.ZERO);
    Reader r2 = new Reader("Adrian", "Knapik", 16, BigDecimal.ZERO);
    Reader r3 = new Reader("Tomasz", "Kamel", 29, BigDecimal.ZERO);
    Reader r4 = new Reader("Ala", "Drag", 18, BigDecimal.ZERO);
    Reader r5 = new Reader("Ola", "Was", 28, BigDecimal.ZERO);
    Reader r6 = new Reader("Ola", "Chrzaszcz", 35, BigDecimal.ZERO);
    HashSet<Reader> readers = new HashSet<>();
    Collections.addAll(readers,r1,r2,r3,r4,r5,r6);

    Book b1 = new Book("Przedwiosnie", "Zeromski",new BigDecimal("5"));
    Book b2 = new Book("Pan Tadeusz", "Mickiewicz",new BigDecimal("8"));
    Book b3 = new Book("Kordian", "Mickiewicz",new BigDecimal("5"));
    Book b4 = new Book("Potop", "Sienkiewicz",new BigDecimal("10"));
    Book b5 = new Book("Lalka", "Prus",new BigDecimal("5"));
    HashSet<Book> books = new HashSet<>();
    Collections.addAll(books,b1,b2,b3,b4,b5);

    Borrowing w1 = new Borrowing(b1,r4, LocalDate.of(2020,7,1));
    Borrowing w2 = new Borrowing(b1,r1, LocalDate.of(2020,8,1));
    Borrowing w3 = new Borrowing(b2,r2, LocalDate.of(2020,7,20));
    Borrowing w4 = new Borrowing(b5,r2, LocalDate.of(2020,7,1));
    Borrowing w5 = new Borrowing(b5,r3, LocalDate.of(2020,8,1));
    Borrowing w6 = new Borrowing(b4,r2, LocalDate.of(2020,7,20));
    Borrowing w7 = new Borrowing(b4,r4, LocalDate.of(2020,6,1));
    Borrowing w8 = new Borrowing(b3,r5, LocalDate.of(2020,7,18));
    HashSet<Borrowing> wypozyczenia = new HashSet<>();
    Collections.addAll(wypozyczenia,w1,w2,w3,w4,w5,w6,w7,w8);

    Map<Reader, BigDecimal> readerWithAMountOfFines = createMapReaderWithAMountOfFines(wypozyczenia);
    readerWithAMountOfFines.forEach((k,v) -> System.out.println(k + " " + v));





}

Answer 1

在为读者添加罚款时，您需要记住，-- BigDecimal.add方法返回和，它不修改现有值。

因此，要将新的罚款添加到现有的罚款中，需要检索现有的罚款，添加新的罚款，并将结果存储在readerWithAMountOfFines中。

BigDecimal existingFineAmount = readerWithAMountOfFines.get(borrowing.reader());
BigDecimal newFineAmount = existingFineAmount.add(countFine(borrowing));
readerWithAMountOfFines.put(borrowing.reader(), newFineAmount);

Answer 2

我在这里找到了另一个使用合并方法的解决方案

public static Map<Reader, BigDecimal> createMapReaderWithAMountOfFines(Set<Borrowing> borrowings){

    if(borrowings == null){
        throw new IllegalArgumentException("Borrowing set collections cannot be null");
    }
    Map<Reader, BigDecimal> readerWithAMountOfFines = new HashMap<>();

    borrowings.forEach(borrowing -> {

        if(readerWithAMountOfFines.containsKey(borrowing.reader())){
            
            readerWithAMountOfFines.merge(borrowing.reader(),countFine(borrowing),BigDecimal::add);
        }
        else {
            readerWithAMountOfFines.put(borrowing.reader(),countFine(borrowing));
        }
    } );
    return readerWithAMountOfFines;
}