使用Java Streams和Scanners時，我遇到了意想不到的行為

Question

我最近看到一個Uni課程的主題，這個課程是由一位被指示以某種方式完成的朋友進行的。 我以為我會借此機會參加這項任務。

我像這樣創建了一個Book類：

class Book
{
    private String author, title;

    public Book setAuthor(String a)
    {
        author = a;
        return this;
    }

    public Book setTitle(String t)
    {
        title = t;
        return this;
    }

    public String getAuthor() 
    {
        return author;
    }

    public String getTitle()
    {
        return title;
    }
}

這個概念是用戶可以在程序開始時創建多本書，然后搜索作者：

private final static int BOOK_NO = 3;
private final static SO instance = new SO(); // This is whatever you called the class

public static void main(String[] args)
{
    Book[] books = new Book[BOOK_NO];
    Scanner kybd = new Scanner(System.in);

    for(int i = 0; i < BOOK_NO; i++)
    {
        books[i] = instance.addBook(kybd, new Book());
    }

    Arrays.stream(instance.findBook(kybd, books)).forEach(o -> {
        System.out.println(o.getTitle() + " by " + o.getAuthor());
    });
}

public Book addBook(Scanner s, Book b)
{

    System.out.println("Enter the Author of this book:");
    b.setAuthor(s.next());

    System.out.println("Enter the Title of this book:");
    b.setTitle(s.next());

    return b;
}

public Book[] findBook(Scanner s, Book[] bs)
{
    System.out.println("Search a book by author:");

    List<Book> finding = Arrays .stream(bs)
            .filter(o -> o.getAuthor().equalsIgnoreCase(s.next()))
            .collect(Collectors.toList());

    System.out.println("Found " + finding.size() + " matches.");

    Book[] output = new Book[finding.size()];
    output = finding.toArray(output);
    return output;
}

現在整個程序運行正常，但是在搜索書籍時，我遇到了Scanner的意外行為。 這是我遇到的直接輸入/輸出行為：

Enter the Author of this book:
Foo
Enter the Title of this book:
Bar
Enter the Author of this book:
Foo
Enter the Title of this book:
FooBar
Enter the Author of this book:
Bar
Enter the Title of this book:
Foo
Search a book by author:
Foo
Foo
Foo
Found 2 matches.
Bar by Foo
FooBar by Foo

正如您所看到的，我必須在獲得任何結果之前將該書的作者鍵入掃描儀3次。 我該如何緩解這種情況？ 是什么導致這種情況發生？

Answer 1

這是因為在您的Stream調用next() ，因此對於Stream中的每個Book對象，將對調用filter中的Predicate應用於它，並調用next() 。 將其解析為變量，因此不會多次調用它：

String book = s.next();
List<Book> finding = Arrays.stream(bs)
                           .filter(o -> o.getAuthor().equalsIgnoreCase(book))
                           .collect(Collectors.toList());

filter()接受Predicate ，在這種情況下將是：

Predicate<String> pred = str -> str.equalsIgnoreCase(s.next());

所以每次應用時，都會調用next()