我toBuilder()在對象實例上使用來創建構建器實例，然后使用 build 方法來創建新實例。原始對象有一個列表，新對象是否引用了同一個列表或它的副本？@Getter@Setter@AllArgsConstructorpublic class Library { private List<Book> books; @Builder(toBuilder=true) public Library(final List<Book> books){ this.books = books; }}Library lib2 = lib1.toBuilder().build();lib2 書籍會引用與 lib1 書籍相同的列表嗎？
3 回答
慕勒3428872
TA貢獻1848條經驗 獲得超6個贊
是的，@Builder(toBuilder=true)注解不執行對象的深層復制，只復制字段的引用。
List<Book> books = new ArrayList<>();
Library one = new Library(books);
Library two = one.toBuilder().build();
System.out.println(one.getBooks() == two.getBooks()); // true, same reference
滄海一幻覺
TA貢獻1824條經驗 獲得超5個贊
您可以使用一個簡單的技巧手動制作集合的副本：
List<Book> books = new ArrayList<>();
Library one = new Library(books);
Library two = one.toBuilder()
.books(new ArrayList<>(one.getBooks))
.build();
System.out.println(one.getBooks() == two.getBooks()); // false, different refs
慕神8447489
TA貢獻1780條經驗 獲得超1個贊
實際上你可以做的是使用其他映射工具從現有對象創建一個新對象。
例如com.fasterxml.jackson.databind.ObjectMapper
@AllArgsConstructor
public static class Book
{
private String title;
}
@NoArgsConstructor
@AllArgsConstructor
@Getter
public static class Library
{
private List<Book> books;
}
ObjectMapper objectMapper = new ObjectMapper(); //it's configurable
objectMapper.configure( DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false );
objectMapper.configure( SerializationFeature.FAIL_ON_EMPTY_BEANS, false );
List<Book> books = new ArrayList<>();
Library one = new Library( books );
Library two = objectMapper.convertValue( one, Library.class );
System.out.println( one.getBooks() == two.getBooks() ); // false, different refs
它可以很容易地包裝在一些實用方法中，以便在整個項目中使用，比如ConvertUtils.clone(rollingStones, Band.class)
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