Data-oriented programming: sealed + records + pattern matching switch

FirstSteps/README.md

@@ -39,6 +39,10 @@
 - [Packages](src/test/java/pl/mperor/lab/java/lang/element/PackageTest.java)
 - [Optional](src/test/java/pl/mperor/lab/java/lang/element/OptionalTest.java)
 
+### Idioms 💡
+
+- [Data-Oriented Programming](src/test/java/pl/mperor/lab/java/idiom/DataOrientedProgrammingTest.java)
+
 ### Quirks 🤔
 
 - [Integer Cache](src/test/java/pl/mperor/lab/java/quirk/IntegerCacheTest.java)

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

@@ -0,0 +1,47 @@
+package pl.mperor.lab.java.idiom;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class DataOrientedProgrammingTest {
+
+    @Test
+    public void testEvalExhaustiveSwitchOverSealedRecords() {
+        Expr sum = new Add(new Const(3), new Const(4));
+        Expr product = new Mul(new Const(2), sum);
+
+        Assertions.assertEquals(7, eval(sum));
+        Assertions.assertEquals(14, eval(product));
+    }
+
+    @Test
+    public void testGuardShortCircuitsMultiplicationByZero() {
+        Expr neverEvaluated = new Add(new Const(1), new Const(1));
+        Expr product = new Mul(new Const(0), neverEvaluated);
+
+        Assertions.assertEquals(0, eval(product), "the `when c.value() == 0` guard matches before the generic Mul case");
+    }
+
+    // Exhaustive switch over a sealed hierarchy: the compiler proves every Expr
+    // subtype is handled here, so a `default` branch would be dead code.
+    private static double eval(Expr expr) {
+        return switch (expr) {
+            case Const(double value) -> value;
+            case Mul(Const c, Expr _) when c.value() == 0 -> 0;
+            case Mul(Expr left, Expr right) -> eval(left) * eval(right);
+            case Add(Expr left, Expr right) -> eval(left) + eval(right);
+        };
+    }
+
+    sealed interface Expr permits Const, Add, Mul {}
+
+    record Const(double value) implements Expr {}
+
+    record Add(Expr left, Expr right) implements Expr {}
+
+    // Try adding `record Sub(Expr left, Expr right) implements Expr {}` to the
+    // `permits` list above: eval()'s switch immediately stops compiling because
+    // it no longer covers every Expr subtype. The sealed hierarchy turns a
+    // missed case into a build failure instead of a silent runtime bug.
+    record Mul(Expr left, Expr right) implements Expr {}
+}