How to initialize immutable collection in Java 9?-javaTutorial-php.cn

在Java 9中如何初始化不可变集合？

Java 9 providesfactory methodsto createimmutablelists,collectionsandmap. It can be used to createemptyornon-emptycollection objects. In Java 8 and previous versions, we can use utility methods of collection classes such asunmodifiableXXXto createimmutablecollection objects. If we need to create an immutable list, we can use theCollections.unmodifiableList()method.

These factory methods allow us to easily initializeimmutablecollections, whether they areemptyornon-empty.

Initialization of immutable list:

List immutableEmptyList = List.of();

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In the above code, we initialize an emptyimmutableList.

Initializing an immutable collection:

Set immutableEmptySet = Set.of();

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In the above code, we initialize an emptyimmutableSet.

Initialize immutable map:

Map immutableEmptyMap = Map.of();

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In the above, we have initialized an empty,immutableMap.

Example

import java.util.ArrayList; import java.util.Collections; import java.util.List; import java.util.Map; import java.util.Set; public class ImmutableCollectionTest { public static void main(String args[]) { List list8 = new ArrayList(); list8.add("INDIA"); list8.add("AUSTRALIA"); list8.add("ENGLAND"); list8.add("NEWZEALAND"); List immutableList8 = Collections.unmodifiableList(list8); immutableList8.forEach(System.out::println); System.out.println(); List immutableList = List.of("INDIA", "AUSTRALIA", "ENGLAND", "NEWZEALAND"); immutableList.forEach(System.out::println); System.out.println(); Set immutableSet = Set.of("INDIA", "AUSTRALIA", "ENGLAND", "NEWZEALAND"); immutableSet.forEach(System.out::println); System.out.println(); Map immutableMap = Map.of("INDIA", "India", "AUSTRALIA", "Australia", "ENGLAND", "England", "NEWZEALAND", "Newzealand"); immutableMap.forEach((key, value) -> System.out.println(key + " : " + value)); System.out.println(); } }

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Output

INDIA AUSTRALIA ENGLAND NEWZEALAND INDIA AUSTRALIA ENGLAND NEWZEALAND AUSTRALIA ENGLAND NEWZEALAND INDIA AUSTRALIA : Australia ENGLAND : England NEWZEALAND : Newzealand INDIA : India

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