Question

我想简化这种方法。你能告诉我怎么做吗？我以某种方式尝试过，但是结果很差。 Books列表长度未知。可以是10，也可以是0。如果有的话，我需要三本书。

public List<Book> findTheLatest3Books() {
    List<Book> books = new ArrayList<>(bookRepository.findAllByOrderByDateOfCreation());
    List<Book> listOf3LatestBooks = new ArrayList<>();
    if (books.size() >= 3) {
        for (int i = 0; i < 3; i++) {
            if (books.get(i).isAvailable()) {
                listOf3LatestBooks.add(books.get(i));
            }
        }
    }
    if (books.size() == 2) {
        for (int i = 0; i < 2; i++) {
            if (books.get(i).isAvailable()) {
                listOf3LatestBooks.add(books.get(i));
            }
        }
    }
    if (books.size() == 1) {
        for (int i = 0; i < 1; i++) {
            if (books.get(i).isAvailable()) {
                listOf3LatestBooks.add(books.get(i));
            }
        }
    }
    if (books.size() == 0) {
        throw new IllegalArgumentException("No books in DB");
    }
    return listOf3LatestBooks;
}

Answer 1

通过流：

List<Book> books = ...    
if (books.isEmpty()) throw new IllegalArgumentException("No books in DB");            
return books.stream() // Stream<Book>
            .limit(3) // truncated to be no longer than 3 in length.
            .filter(Book::isAvailable) // keep only books that are available               
            .collect(Collectors.toCollection(ArrayList::new));

这将使用截断长度不超过3的流，然后进行过滤以保留可用的书。

Answer 2

如果使用books.size() < 3，请选择大小，否则使用3。

if (books.isEmpty()) {
    throw new IllegalArgumentException("No books in DB");
}

final int n = books.size() >= 3 ? 3 : books.size();

for (int i = 0; i < n; ++i) {
    if (books.get(i).isAvailable()) {
        listOf3LatestBooks.add(books.get(i));
    }
}

您似乎在使用自定义的Spring JPA存储库方法。您只能获取前3条记录，by limiting query results.

Answer 3

您可以在i < 3 && i < books.size()语句中应用条件for：

public List<Book> findTheLatest3Books() {
    List<Book> books = new ArrayList<Book>(bookRepository.findAllByOrderByDateOfCreation());
    List<Book> listOf3LatestBooks = new ArrayList<>();

    for (int i = 0; i < 3 && i < books.size(); i++) {
        if (books.get(i).isAvailable()) {
            listOf3LatestBooks.add(books.get(i));
        }
    }

    if (books.size() == 0) {
        throw new IllegalArgumentException("No books in DB");
    }
    return listOf3LatestBooks;
}

如何使用if语句缩短方法？

3 个答案: