Create test class for Java 21 ... (#21)

- Virtual Threads
- Sequenced Collections
- Record Patterns
- Pattern Matching for switch
- Updated README.md

Author: mperor

README.md

@@ -31,5 +31,7 @@ This project includes unit tests for key functionalities introduced in each Java
 - [Java 17](src/test/java/pl/mperor/lab/java/Java17.java)
 - [Java 18](src/test/java/pl/mperor/lab/java/Java18.java)
 - [Java 19](src/test/java/pl/mperor/lab/java/Java19.java)
+- [Java 20](src/test/java/pl/mperor/lab/java/Java20.java)
+- [Java 21](src/test/java/pl/mperor/lab/java/Java21.java)
 
 For detailed examples and tests of each feature, please refer to the individual source files linked above.

src/test/java/pl/mperor/lab/java/Java21.java

@@ -0,0 +1,98 @@
+package pl.mperor.lab.java;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+import pl.mperor.lab.TestUtils;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.SequencedCollection;
+
+/// Java 21™ (September 2023)
+/// [JDK 21](https://openjdk.org/projects/jdk/21)
+///
+/// - STANDARD FEATURES:
+///     - 444:	Virtual Threads
+///     - 431:	Sequenced Collections
+///     - 440:	Record Patterns
+///     - 441:	Pattern Matching for switch
+///     - 452:	Key Encapsulation Mechanism API
+///     - 439:	Generational ZGC
+///     - 451:	Prepare to Disallow the Dynamic Loading of Agents
+///     - 449:	Deprecate the Windows 32-bit x86 Port for Removal
+///
+/// - PREVIEW & INCUBATOR:
+///     - 430:	String Templates (Preview)
+///     - 442:	Foreign Function & Memory API (Third Preview)
+///     - 443:	Unnamed Patterns and Variables (Preview)
+///     - 445:	Unnamed Classes and Instance Main Methods (Preview)
+///     - 446:	Scoped Values (Preview)
+///     - 453:	Structured Concurrency (Preview)
+///     - 448:	Vector API (Sixth Incubator)
+public class Java21 {
+
+    @Test
+    public void testVirtualThreads() throws InterruptedException {
+        TestUtils.ReadableOut out = TestUtils.setTempSystemOut();
+        Thread virtualThread = Thread.startVirtualThread(() ->
+                System.out.print("Hello from Virtual Thread!")
+        );
+        virtualThread.join();
+        TestUtils.resetSystemOut();
+
+        Assertions.assertTrue(virtualThread.isDaemon() && virtualThread.isVirtual());
+        Assertions.assertEquals(Thread.NORM_PRIORITY, virtualThread.getPriority());
+        Assertions.assertEquals("Hello from Virtual Thread!", out.all());
+    }
+
+    @Test
+    public void testSequencedCollections() {
+        List<String> letters = List.of("one", "two", "three");
+        Assertions.assertInstanceOf(SequencedCollection.class, letters);
+        Assertions.assertEquals("one", letters.getFirst());
+        Assertions.assertEquals("three", letters.getLast());
+        Assertions.assertThrows(NoSuchElementException.class, Collections.emptyList()::getFirst);
+    }
+
+    @Test
+    public void testRecordPatternDeconstruct() {
+        record Point(int x, int y) {}
+        record LineSegment(Point start, Point end) {}
+        Object obj = new LineSegment(new Point(0, 1), new Point(1, 2));
+
+        if (obj instanceof LineSegment(Point(int x, int y), Point end)) {
+            Assertions.assertEquals(0, x);
+            Assertions.assertEquals(1, y);
+            Assertions.assertEquals(new Point(1, 2), end);
+        }
+    }
+
+    @Test
+    public void testSwitchPatternMatching() {
+        Assertions.assertEquals("String: Hello", switchOverClasses("Hello"));
+        Assertions.assertEquals("int: 1", switchOverClasses(1));
+        Assertions.assertEquals("long: 13", switchOverClasses(13L));
+        Assertions.assertEquals("boolean: true", switchOverClasses(true));
+        Assertions.assertEquals("null", switchOverClasses(null));
+        record Person(String name, String surname) {
+            @Override
+            public String toString() {
+                return "%s %s".formatted(name, surname);
+            }
+        }
+        Assertions.assertEquals("Object: John Doe", switchOverClasses(new Person("John", "Doe")));
+    }
+
+    private static String switchOverClasses(Object obj) {
+        return switch (obj) {
+            case String s -> String.format("String: %s", s);
+            case Integer i -> String.format("int: %d", i);
+            case Long l -> String.format("long: %d", l);
+            case Boolean b -> String.format("boolean: %s", b);
+            case null -> "null";
+            default -> "Object: " + obj;
+        };
+    }
+
+}