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在使用Java流和扫描仪时,我遇到了意想不到的行为

java-stream java

4

Jaquarh · 技术社区 · 6 年前

我最近看到了一个关于大学课程的话题,这个话题是由一位朋友指导的,他被要求以某种方式完成。我想我会抓住这个机会去干这项工作。

我创建了一个Book类,如下所示:

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;
    }
}

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;
}

现在,整个程序运行良好,但当涉及到搜索书籍时,我在扫描仪上遇到了意想不到的行为。以下是我正在经历的直接输入/输出行为:

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

正如你所看到的,在得到任何结果之前,我必须在扫描仪中输入这本书的作者3次。我如何减轻这种影响?是什么导致了这种情况?

1 回复 | 直到 6 年前

1

3

GBlodgett 6 年前

这是因为在你的 Stream 你打电话 next() ,所以每一个 Book 中的对象 流动 这个 Predicate 在中,对筛选器的调用应用于它,并且 下一个()

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

filter() 接受 Predicate ,在这种情况下,将类似于:

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

所以每次应用的时候,, 下一个() 将被称为