Collections

emptyList

返回一个空的，不可变的List

• 避免空指针异常
• 语义清晰

singletonList

返回一个单个元素、不可变的list

• 只传递包含一个元素的list，且不希望接收方修改它
• 需要高性能的不可变的单个元素的list

Collections.SingletonList("hello")

map

不可变map

只有值是不可变对象的时候 map才是真正的不可变 否则原对象改变的时候map还是可变的

• Collections.unmodifiableMap(map);
• ImmutableMap.copyOf(map); // since jdk 10
• Map.copyOf(map);

@Slf4j
public class ShallowImmutableMap {
    public static void main(String[] args) {
        final HashMap<String, Person> map = new HashMap<>();
        final Person jasper = new Person(1, "jasper");
        final Person cliff = new Person(2, "cliff");
        map.put("1",jasper);
        map.put("2",cliff);

        jasper.setName("jasper1");

        final Map<String, Person> collectionsMap = Collections.unmodifiableMap(map);
        log.info("collectionMap:{}", collectionsMap);
        final ImmutableMap<String, Person> guavaMap = ImmutableMap.copyOf(map);
        log.info("guavaMap:{}", guavaMap);
        // since jdk 10
        final Map<String, Person> mapMap = Map.copyOf(map);
        log.info("mapMap:{}", mapMap);
    }
}