使用Java Streams和Scanners时，我遇到了意想不到的行为

Question

I recently saw a topic of some Uni coursework which was being conducted by a friend whom was instructed to do it a certain way. I thought I'd take the opportunity to jump in on the task.

I created a Book class like so:

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;
    }
}

The concept is that a user can create multiple books at the start of the program and then search for an author:

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;
}

Now the whole program works fine, however I am experience unexpected behaviour with the Scanner when it comes to searching for a book. Here is a direct input/output behaviour I am experiencing:

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

As you can see, I am having to type the author of the book into the scanner 3 times before getting any result. How can I mitigate this? What is causing this to happen?

Answer 1

This is because in your Stream you call next() , so for every Book object in the Stream , the Predicate in the call to filter is applied to it, and next() will be called. Resolve it to a variable so it isn't called more than once:

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

filter() accepts a Predicate , which in this case will be something like:

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

So every time it is applied, next() will be called