Add java21-features module: runnable demos and captured output for the ankurm.com java21-features article

java21-features/broken/DominatedCase.java

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+/** Fails to compile on purpose: a more general pattern before a specific one makes the later case unreachable. */
+public class DominatedCase {
+    static String f(Object o) { return switch (o) { case CharSequence cs -> "cs"; case String s -> "s"; default -> "d"; }; }
+    public static void main(String[] args) { }
+}

java21-features/broken/NonExhaustive.java

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+/** Fails to compile on purpose: a pattern switch over a sealed type must cover every permitted subtype. */
+public class NonExhaustive {
+    sealed interface S permits A, B {}
+    record A() implements S {}
+    record B() implements S {}
+    static String f(S s) { return switch (s) { case A a -> "a"; }; }
+    public static void main(String[] args) { }
+}

java21-features/broken/PreviewWithoutFlag.java

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+/** Fails to compile on purpose: string templates are a preview feature in 21 and are off by default. */
+public class PreviewWithoutFlag {
+    public static void main(String[] args) { String n = "x"; System.out.println(STR."hi \{n}"); }
+}