Add records-in-depth idiom test: validation, defensive copy, withers, array pitfall

Author: claude

FirstSteps/README.md

@@ -42,6 +42,7 @@
 ### Idioms 💡
 
 - [Data-Oriented Programming](src/test/java/pl/mperor/lab/java/idiom/DataOrientedProgrammingTest.java)
+- [Records in Depth](src/test/java/pl/mperor/lab/java/idiom/RecordsInDepthTest.java)
 
 ### Quirks 🤔
 

FirstSteps/src/test/java/pl/mperor/lab/java/idiom/RecordsInDepthTest.java

@@ -0,0 +1,69 @@
+package pl.mperor.lab.java.idiom;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+public class RecordsInDepthTest {
+
+    @Test
+    public void testCompactConstructorValidatesAndNormalizesName() {
+        var team = new Team("  Falcons  ", List.of("Ann", "Bob"));
+        Assertions.assertEquals("Falcons", team.name(), "the compact constructor strips surrounding whitespace");
+
+        Assertions.assertThrows(NullPointerException.class, () -> new Team(null, List.of("Ann")));
+        Assertions.assertThrows(IllegalArgumentException.class, () -> new Team("   ", List.of("Ann")));
+    }
+
+    @Test
+    public void testCompactConstructorDefensiveCopyOfMembers() {
+        var mutableMembers = new ArrayList<>(List.of("Ann", "Bob"));
+        var team = new Team("Falcons", mutableMembers);
+
+        mutableMembers.add("Cid");
+        Assertions.assertEquals(2, team.members().size(), "the record keeps its own copy, unaffected by later mutation of the source list");
+        Assertions.assertThrows(UnsupportedOperationException.class, () -> team.members().add("Dee"), "List.copyOf() returns an immutable list");
+    }
+
+    @Test
+    public void testWitherReturnsModifiedCopyAndLeavesOriginalUnchanged() {
+        var falcons = new Team("Falcons", List.of("Ann", "Bob"));
+        var eagles = falcons.withName("Eagles");
+
+        Assertions.assertEquals("Eagles", eagles.name());
+        Assertions.assertEquals("Falcons", falcons.name(), "withName() returns a new instance; the original record is unchanged");
+        Assertions.assertNotEquals(falcons, eagles);
+        Assertions.assertSame(falcons.members(), eagles.members(), "the untouched component is shared, not re-copied");
+    }
+
+    record Team(String name, List<String> members) {
+        Team {
+            Objects.requireNonNull(name, "name must not be null");
+            if (name.isBlank()) {
+                throw new IllegalArgumentException("name must not be blank");
+            }
+            name = name.strip();
+            members = List.copyOf(members);
+        }
+
+        Team withName(String newName) {
+            return new Team(newName, members);
+        }
+    }
+
+    @Test
+    public void testArrayComponentBreaksGeneratedEqualsAndHashCode() {
+        var badge1 = new Badge("A1", new byte[] {1, 2, 3});
+        var badge2 = new Badge("A1", new byte[] {1, 2, 3});
+
+        // Generated equals()/hashCode() compare array components by reference,
+        // not content, so two "equal looking" badges are reported as different.
+        Assertions.assertNotEquals(badge1, badge2);
+        Assertions.assertArrayEquals(badge1.data(), badge2.data(), "the array contents are in fact identical");
+    }
+
+    record Badge(String id, byte[] data) {}
+}