The Java Version Almanac
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Collection Constants

The collection interfaces Set, List and Map have several static of(...) methods to create constant instances of the respective collection types. Collections created this way do not allow null elements and any mutation method throws an UnsupportedOperationException. Also as these methods are designed to declare constants duplicate set entries or map keys are considered as an error.

Since Java 9

import java.awt.Color; import java.time.DayOfWeek; import java.util.List; import java.util.Map; import java.util.Set; public class CollectionConstants { static final Set<Color> BAVARIAN_COLORS = Set.of(Color.WHITE, Color.BLUE); static final List<DayOfWeek> LONG_WEEKEND = List.of(DayOfWeek.FRIDAY, DayOfWeek.SATURDAY, DayOfWeek.SUNDAY); static final Map<String, Integer> PEAK_ELEVATIONS = Map.of( // "Mauna Key", 4205, // "Matterhorn", 4478, // "Makalu", 8485); public static void main(String... args) { System.out.println("The elevation of Matterhorn is " + PEAK_ELEVATIONS.get("Matterhorn")); // Invalid usages: try { BAVARIAN_COLORS.add(Color.BLACK); } catch (UnsupportedOperationException e) { System.out.println("Constant collections cannot be modified"); } try { List.of("null", null); } catch (NullPointerException e) { System.out.println("null is not allowed in constant collections"); } try { Set.of("same", "same"); } catch (IllegalArgumentException e) { System.out.println("Duplicate entries are not allowed for constant sets"); } try { Map.of("key", 1, "key", 2); } catch (IllegalArgumentException e) { System.out.println("Duplicate keys are not allowed for constant maps"); } } }

This snippet at GitHub