如何在 Java 中合并两个不可修改的集合？

Question

如何合并两个 unmodifiable static final 集合？

public static final Set<Long> ORG_SUBSCRIBER_ALLOWED_NUMBER_CD = Set.of(COMPANY_GST, GOVERNMENT_BODY_GST);

public static final Set<Long> INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD = Set.of(BUSINESS_PAN, INDIVIDUAL_PAN);

我想把上面的static final sets合并成一个set（one-statement initialization），因为它是一个Class变量

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = ?

Answer 1

为什么你不能创建第三个集合而只添加两个最终集合？

Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = new HashSet<>();
SUBSCRIBER_ALLOWED_NUMBER_CD.addAll(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD);
SUBSCRIBER_ALLOWED_NUMBER_CD.addAll(INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD);

前两组是final只是意味着你不能修改那组。它不会阻止您将集合读入另一个新集合。

Answer 2

A one-statement 初始化，如果首选：

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = 
     Collections.unmodifiableSet(
        Stream.of(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD, 
                  INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
              .flatMap(Set::stream)
              .collect(Collectors.toSet()));

或者，也许更具可读性：

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD;
static {
    Set<Long> all = new HashSet<>();
    all.addAll(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD);
    all.addAll(INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD);
    
    SUBSCRIBER_ALLOWED_NUMBER_CD = Collections.unmodifiableSet(all);
}

如果第三组预计不会不可修改，请忽略 Collections.unmodifiableSet 调用。

Answer 3

如果你想使用流：

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD =
    Stream.of(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD, INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
        .flatMap(Set::stream)
        .collect(Collectors.toSet());

或者，在您的情况下，如评论中所述，您可以使用 Collectors.toUnmodifiableSet() :

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD =
    Stream.of(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD, INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
        .flatMap(Set::stream)
        .collect(Collectors.toUnmodifiableSet());

Answer 4

Stream#concat 也很有用

Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD  =  
         Stream.concat(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD.stream(),
                       INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD.stream())
                .collect(Collectors.toSet());

Answer 5

只是要指出，与 JDK-only 解决方案相比，Guava 在处理这种情况时仍然更具可读性（并且可能更高效）：

public static final Set<Long> SUBSCRIBER_ALLOWED_NUMBER_CD = ImmutableSet.builder()
    .addAll(ORG_SUBSCRIBER_ALLOWED_NUMBER_CD)
    .addAll(INDIVIDUAL_SUBSCRIBER_ALLOWED_NUMBER_CD)
    .build();

如何在 Java 中合并两个不可修改的集合？

How to merge two unmodifiable Sets in Java?

java

merge

java-stream

java-11

unmodifiable