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Add lazy-constants module: LazyConstant (JEP 531) against the holder idiom and double-checked locking

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Basics, failure semantics on 26 vs 27, lazy collections, API across 25/26/27, preview-flag traps and a JMH read-path benchmark (jars fetched and sha1-checked, not committed).

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a
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Claude
2026-09-24 12:27:27 +00:00
parent af44e59e7f
commit 814c32b5ce
25 changed files with 606 additions and 0 deletions
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lazy-constants/src/LazyCollections.java
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import java.util.List;
import java.util.Map;
import java.util.Set;
// List.ofLazy, Map.ofLazy (JDK 26 and 27) and Set.ofLazy (new in JDK 27): each element is computed on first access.
public class LazyCollections {
public static void main(String[] args) {
List<String> list = List.ofLazy(3, i -> { System.out.println(" computing list element " + i); return "v" + i; });
System.out.println("list created, size " + list.size() + " (nothing computed yet)");
System.out.println("list.get(1) = " + list.get(1));
System.out.println("list.get(1) again = " + list.get(1));
Map<String, Integer> map = Map.ofLazy(Set.of("a", "bb", "ccc"), k -> { System.out.println(" computing map value for " + k); return k.length(); });
System.out.println("map created, size " + map.size() + " (nothing computed yet)");
System.out.println("map.get(\"bb\") = " + map.get("bb"));
System.out.println("map.get(\"bb\") again = " + map.get("bb"));
Set<Integer> set = Set.ofLazy(Set.of(1, 2, 3), n -> { System.out.println(" testing " + n); return n > 1; });
System.out.println("set created");
System.out.println("set.contains(2) = " + set.contains(2));
System.out.println("set.contains(1) = " + set.contains(1));
System.out.println("set.contains(9) = " + set.contains(9));
System.out.println("set.size() = " + set.size());
}
}