收集通過流

Question

通過鍵排序我有一個簡單地用三個值類：

class Book{ 
    private String title; 
    private String author; 
    private double price; 
} 

title是唯一的（總是）。系統中有一個選定的標題列表（如List<String>）和每個Book的列表。我想從標題列表中獲得所有Book。我的代碼是：

List<Book> allBooks = getAllBooks(); 
List<String> selectedTitles = getSelectedTitles(); 
List<Book> selectedBooks = allBooks.stream(). 
    filter(x -> selectedTitles.contains(x.getTitle())). 
    collect(Collectors.toList()); 

它的工作原理，但selectedBooks被下令allBook insted的的selectedTitles。例如：？

allBooks(only title) = {"A","B","C","D"} 
selectedTitles = {"B","A","D"} 
selectedBooks(only title) = {"A","B","D"} 

怎麼可能（通過stream()得到由selectedTitles排序（在這個例子應該是{"B","D","A"}

Answer 1

不要排序，只要查看已經選擇的書籍：

List<Book> allBooks = getAllBooks(); 
List<String> selectedTitles = getSelectedTitles(); 

Map<String,Book> titleToBook = allBooks.stream() 
    .collect(Collectors.toMap(Book::getTitle, Function.identity())); 

List<Book> selectedBooks = selectedTitles.stream() 
    .map(titleToBook::get).filter(Objects::nonNull) 
    .collect(Collectors.toList()); 

Answer 2

這是你應該做的：？

selectedTitles.stream() 
     .map(x -> b.stream() 
       .filter(y -> y.getName().equals(x)) 
       .findFirst() 
       .orElse(null)) 
     .collect(Collectors.toList());