diff --git a/README.md b/README.md
index 332fb30..d1c1cf9 100644
--- a/README.md
+++ b/README.md
@@ -65,6 +65,7 @@ These modules keep only sources, a `run.sh` and their transcripts (`output/`). T
 | [`jep512/`](jep512) | Compact Source Files and Instance Main Methods in Java 25 (JEP 512) | `java Hello.java` on JDK 21 vs 25 vs 27, the class `javac` builds around `void main()`, implicit `java.base` imports, `java.lang.IO`, the measured launch order, a private-constructor difference between 25 and 27 |
 | [`jep513/`](jep513) | Flexible Constructor Bodies in Java 25 (JEP 513): Validate Before super() | statements before `super(...)`/`this(...)`, the parent-calls-child bug and its fix, twelve compile errors that remain with the 25 and 27 wordings side by side, `--release` rejection |
 | [`jep511/`](jep511) | Module Import Declarations in Java 25 (JEP 511) | `import module java.base;` against seven single imports, ambiguity errors and how single-type, on-demand and same-package names beat a module import, what one module import does and does not bring in (`java.sql` does not give `java.base`; `java.se` needs `--add-modules`), named modules, JEP 512 compact files, jshell's default imports, identical bytecode |
+| [`patterns/`](patterns) | Pattern Matching in Java: switch, Record Patterns and Primitive Patterns (JDK 21 to 27) | type and record patterns, guards and dominance errors, sealed exhaustiveness (with JDK 27's new `missing patterns` lines), `MatchException` from a throwing accessor and from separate compilation, unnamed patterns, primitive patterns with `--enable-preview` and the preview class-file trap |
 
 ## Captured output (`docs/output/`)
 
diff --git a/patterns/README.md b/patterns/README.md
new file mode 100644
index 0000000..2bb5ff6
--- /dev/null
+++ b/patterns/README.md
@@ -0,0 +1,15 @@
+# patterns — Pattern matching from JDK 21 to 27
+
+Companion code for the ankurm.com article **Pattern Matching in Java: switch, Record Patterns and Primitive Patterns (JDK 21 to 27)**. The explanation
+lives in the article; this folder holds the runnable sources and the transcripts they produced.
+
+```bash
+JDK25=/path/to/jdk-25 JDK27=/path/to/jdk-27 [JDK21=/path/to/jdk-21] ./run.sh    # regenerates output/*.txt
+```
+
+- `src/` compiles and runs on 21, 25 and 27 (except `Unnamed.java`, which needs 22 or later).
+- `broken/` fails to compile on purpose.
+- `preview/` uses primitive patterns (JEP 532, preview in 27) and needs `--enable-preview --release N`.
+- `sep/` is a sealed hierarchy compiled in two versions, to show the one runtime failure an exhaustive `switch` cannot prevent.
+
+Tested on Temurin 25.0.4.1+1 and 27+35, and OpenJDK 21.0.10 for the baseline.
diff --git a/patterns/broken/Dominated.java b/patterns/broken/Dominated.java
new file mode 100644
index 0000000..d62a26c
--- /dev/null
+++ b/patterns/broken/Dominated.java
@@ -0,0 +1,10 @@
+public class Dominated {
+    static String f(Object o) {
+        return switch (o) {
+            case CharSequence cs -> "chars";
+            case String s -> "string";      // String is a CharSequence, so this can never be reached
+            default -> "other";
+        };
+    }
+    public static void main(String[] args) { System.out.println(f("x")); }
+}
diff --git a/patterns/broken/GuardBeforeGeneral.java b/patterns/broken/GuardBeforeGeneral.java
new file mode 100644
index 0000000..e553811
--- /dev/null
+++ b/patterns/broken/GuardBeforeGeneral.java
@@ -0,0 +1,10 @@
+public class GuardBeforeGeneral {
+    static String f(Object o) {
+        return switch (o) {
+            case String s -> "any string";
+            case String s when s.isEmpty() -> "empty string";   // guarded case after the unguarded one
+            default -> "other";
+        };
+    }
+    public static void main(String[] args) { System.out.println(f("")); }
+}
diff --git a/patterns/broken/NotExhaustive.java b/patterns/broken/NotExhaustive.java
new file mode 100644
index 0000000..268e706
--- /dev/null
+++ b/patterns/broken/NotExhaustive.java
@@ -0,0 +1,15 @@
+public class NotExhaustive {
+    sealed interface Shape permits Circle, Square, Rect {}
+    record Circle(double r) implements Shape {}
+    record Square(double side) implements Shape {}
+    record Rect(double w, double h) implements Shape {}
+
+    static double area(Shape s) {
+        return switch (s) {
+            case Circle(double r) -> Math.PI * r * r;
+            case Square(double side) -> side * side;
+            // Rect is missing
+        };
+    }
+    public static void main(String[] args) { System.out.println(area(new Circle(1))); }
+}
diff --git a/patterns/broken/NotExhaustiveObject.java b/patterns/broken/NotExhaustiveObject.java
new file mode 100644
index 0000000..dfb054e
--- /dev/null
+++ b/patterns/broken/NotExhaustiveObject.java
@@ -0,0 +1,9 @@
+public class NotExhaustiveObject {
+    static String f(Object o) {
+        return switch (o) {
+            case String s -> "string";
+            case Integer i -> "int";
+        };
+    }
+    public static void main(String[] args) { System.out.println(f("x")); }
+}
diff --git a/patterns/output/01-type-patterns.txt b/patterns/output/01-type-patterns.txt
new file mode 100644
index 0000000..e736be5
--- /dev/null
+++ b/patterns/output/01-type-patterns.txt
@@ -0,0 +1,23 @@
+$ javac src/TypePatterns.java && java TypePatterns     (21.0.10+7-Ubuntu-124.04)
+string of length 5 / string of length 5
+3 -1
+int 42
+string JAVA
+int array of 3
+other Double
+
+$ javac src/TypePatterns.java && java TypePatterns     (25.0.4.1+1-LTS)
+string of length 5 / string of length 5
+3 -1
+int 42
+string JAVA
+int array of 3
+other Double
+
+$ javac src/TypePatterns.java && java TypePatterns     (27+35)
+string of length 5 / string of length 5
+3 -1
+int 42
+string JAVA
+int array of 3
+other Double
diff --git a/patterns/output/02-guards-and-dominance.txt b/patterns/output/02-guards-and-dominance.txt
new file mode 100644
index 0000000..35316ae
--- /dev/null
+++ b/patterns/output/02-guards-and-dominance.txt
@@ -0,0 +1,39 @@
+$ javac src/Guards.java && java Guards     (25.0.4.1+1-LTS)
+empty string
+short string: abc
+long string: abcdefgh
+negative -4
+int 12
+other
+
+$ javac src/Guards.java && java Guards     (27+35)
+empty string
+short string: abc
+long string: abcdefgh
+negative -4
+int 12
+other
+
+$ javac broken/Dominated.java     (25.0.4.1+1-LTS)
+broken/Dominated.java:5: error: this case label is dominated by a preceding case label
+            case String s -> "string";      // String is a CharSequence, so this can never be reached
+                 ^
+1 error
+
+$ javac broken/Dominated.java     (27+35)
+broken/Dominated.java:5: error: this case label is dominated by a preceding case label
+            case String s -> "string";      // String is a CharSequence, so this can never be reached
+                 ^
+1 error
+
+$ javac broken/GuardBeforeGeneral.java     (25.0.4.1+1-LTS)
+broken/GuardBeforeGeneral.java:5: error: this case label is dominated by a preceding case label
+            case String s when s.isEmpty() -> "empty string";   // guarded case after the unguarded one
+                 ^
+1 error
+
+$ javac broken/GuardBeforeGeneral.java     (27+35)
+broken/GuardBeforeGeneral.java:5: error: this case label is dominated by a preceding case label
+            case String s when s.isEmpty() -> "empty string";   // guarded case after the unguarded one
+                 ^
+1 error
diff --git a/patterns/output/03-record-patterns.txt b/patterns/output/03-record-patterns.txt
new file mode 100644
index 0000000..ead4310
--- /dev/null
+++ b/patterns/output/03-record-patterns.txt
@@ -0,0 +1,31 @@
+$ javac src/RecordPatterns.java && java RecordPatterns     (21.0.10+7-Ubuntu-124.04)
+point at 3,4
+not a point
+7
+box of string hi
+box of int 7
+box of 2.5
+
+$ javac src/RecordPatterns.java && java RecordPatterns     (25.0.4.1+1-LTS)
+point at 3,4
+not a point
+7
+box of string hi
+box of int 7
+box of 2.5
+
+$ javac src/RecordPatterns.java && java RecordPatterns     (27+35)
+point at 3,4
+not a point
+7
+box of string hi
+box of int 7
+box of 2.5
+
+$ javac src/MatchExceptionDemo.java && java MatchExceptionDemo     (25.0.4.1+1-LTS)
+MatchException: java.lang.IllegalStateException: accessor blew up
+cause: java.lang.IllegalStateException: accessor blew up
+
+$ javac src/MatchExceptionDemo.java && java MatchExceptionDemo     (27+35)
+MatchException: java.lang.IllegalStateException: accessor blew up
+cause: java.lang.IllegalStateException: accessor blew up
diff --git a/patterns/output/04-exhaustiveness.txt b/patterns/output/04-exhaustiveness.txt
new file mode 100644
index 0000000..1b08012
--- /dev/null
+++ b/patterns/output/04-exhaustiveness.txt
@@ -0,0 +1,54 @@
+$ javac src/SealedExhaustive.java && java SealedExhaustive     (21.0.10+7-Ubuntu-124.04)
+Circle[r=1.0] -> 3.14
+Square[side=2.0] -> 4.00
+Rect[w=2.0, h=3.0] -> 6.00
+
+$ javac src/SealedExhaustive.java && java SealedExhaustive     (25.0.4.1+1-LTS)
+Circle[r=1.0] -> 3.14
+Square[side=2.0] -> 4.00
+Rect[w=2.0, h=3.0] -> 6.00
+
+$ javac src/SealedExhaustive.java && java SealedExhaustive     (27+35)
+Circle[r=1.0] -> 3.14
+Square[side=2.0] -> 4.00
+Rect[w=2.0, h=3.0] -> 6.00
+
+$ javac broken/NotExhaustive.java     (21.0.10+7-Ubuntu-124.04)
+broken/NotExhaustive.java:8: error: the switch expression does not cover all possible input values
+        return switch (s) {
+               ^
+1 error
+
+$ javac broken/NotExhaustive.java     (25.0.4.1+1-LTS)
+broken/NotExhaustive.java:8: error: the switch expression does not cover all possible input values
+        return switch (s) {
+               ^
+1 error
+
+$ javac broken/NotExhaustive.java     (27+35)
+broken/NotExhaustive.java:8: error: the switch expression does not cover all possible input values
+        return switch (s) {
+               ^
+  missing patterns:
+      Rect _
+1 error
+
+$ javac broken/NotExhaustiveObject.java     (21.0.10+7-Ubuntu-124.04)
+broken/NotExhaustiveObject.java:3: error: the switch expression does not cover all possible input values
+        return switch (o) {
+               ^
+1 error
+
+$ javac broken/NotExhaustiveObject.java     (25.0.4.1+1-LTS)
+broken/NotExhaustiveObject.java:3: error: the switch expression does not cover all possible input values
+        return switch (o) {
+               ^
+1 error
+
+$ javac broken/NotExhaustiveObject.java     (27+35)
+broken/NotExhaustiveObject.java:3: error: the switch expression does not cover all possible input values
+        return switch (o) {
+               ^
+  missing patterns:
+      Object _
+1 error
diff --git a/patterns/output/05-separate-compilation.txt b/patterns/output/05-separate-compilation.txt
new file mode 100644
index 0000000..f99eea0
--- /dev/null
+++ b/patterns/output/05-separate-compilation.txt
@@ -0,0 +1,32 @@
+$ java -cp old Client     (Client and Shape compiled together, v1)     (25.0.4.1+1-LTS)
+circle: 3.141592653589793
+square: 4.0
+
+$ java -cp new:old Main     (Shape gained Triangle; Client.class NOT recompiled)     (25.0.4.1+1-LTS)
+Exception in thread "main" java.lang.MatchException
+	at Client.area(Client.java:4)
+	at Main.main(Main.java:4)
+
+$ javac -cp new sep/client/Client.java     (now recompile Client against v2)     (25.0.4.1+1-LTS)
+sep/client/Client.java:4: error: the switch expression does not cover all possible input values
+        return switch (s) {
+               ^
+1 error
+
+$ java -cp old Client     (Client and Shape compiled together, v1)     (27+35)
+circle: 3.141592653589793
+square: 4.0
+
+$ java -cp new:old Main     (Shape gained Triangle; Client.class NOT recompiled)     (27+35)
+Exception in thread "main" java.lang.MatchException
+	at Client.area(Client.java:4)
+	at Main.main(Main.java:4)
+
+$ javac -cp new sep/client/Client.java     (now recompile Client against v2)     (27+35)
+sep/client/Client.java:4: error: the switch expression does not cover all possible input values
+        return switch (s) {
+               ^
+  missing patterns:
+      Triangle _
+1 error
+
diff --git a/patterns/output/06-null-and-unnamed.txt b/patterns/output/06-null-and-unnamed.txt
new file mode 100644
index 0000000..58949a9
--- /dev/null
+++ b/patterns/output/06-null-and-unnamed.txt
@@ -0,0 +1,26 @@
+$ javac src/NullCase.java && java NullCase     (25.0.4.1+1-LTS)
+without case null: NullPointerException
+with case null: null!
+case null, default: null or something else
+instanceof with null: false
+
+$ javac src/NullCase.java && java NullCase     (27+35)
+without case null: NullPointerException
+with case null: null!
+case null, default: null or something else
+instanceof with null: false
+
+$ javac src/Unnamed.java && java Unnamed     (21.0.10+7-Ubuntu-124.04)
+src/Unnamed.java:9: error: unnamed variables are a preview feature and are disabled by default.
+            case Circle _ -> "round";          // the type matters, the value does not
+                        ^
+  (use --enable-preview to enable unnamed variables)
+1 error
+
+$ javac src/Unnamed.java && java Unnamed     (25.0.4.1+1-LTS)
+round angular
+5
+
+$ javac src/Unnamed.java && java Unnamed     (27+35)
+round angular
+5
diff --git a/patterns/output/07-primitive-patterns.txt b/patterns/output/07-primitive-patterns.txt
new file mode 100644
index 0000000..220a799
--- /dev/null
+++ b/patterns/output/07-primitive-patterns.txt
@@ -0,0 +1,87 @@
+$ javac preview/PrimitiveInstanceof.java     (25.0.4.1+1-LTS)
+preview/PrimitiveInstanceof.java:3: error: primitive patterns are a preview feature and are disabled by default.
+        if (i instanceof byte b) return i + " fits in a byte: " + b;
+            ^
+  (use --enable-preview to enable primitive patterns)
+1 error
+
+$ javac preview/PrimitiveInstanceof.java     (27+35)
+preview/PrimitiveInstanceof.java:3: error: primitive patterns are a preview feature and are disabled by default.
+        if (i instanceof byte b) return i + " fits in a byte: " + b;
+            ^
+  (use --enable-preview to enable primitive patterns)
+1 error
+
+$ javac --enable-preview --release 25 preview/PrimitiveInstanceof.java && java --enable-preview PrimitiveInstanceof     (25.0.4.1+1-LTS)
+100 fits in a byte: 100
+1000 fits in a short: 1000
+100000 needs an int
+0.5 converts to float 0.5 without loss
+0.1 does NOT convert to float without loss
+
+$ javac --enable-preview --release 27 preview/PrimitiveInstanceof.java && java --enable-preview PrimitiveInstanceof     (27+35)
+100 fits in a byte: 100
+1000 fits in a short: 1000
+100000 needs an int
+0.5 converts to float 0.5 without loss
+0.1 does NOT convert to float without loss
+
+$ javac --enable-preview --release 25 preview/PrimitiveSwitch.java && java --enable-preview PrimitiveSwitch     (25.0.4.1+1-LTS)
+OK | other success 204 | client error 404 | something else 500
+yes no
+zero one many (9223372036854775807)
+
+$ javac --enable-preview --release 27 preview/PrimitiveSwitch.java && java --enable-preview PrimitiveSwitch     (27+35)
+OK | other success 204 | client error 404 | something else 500
+yes no
+zero one many (9223372036854775807)
+
+$ javac --enable-preview --release 25 preview/PrimitiveInRecord.java && java --enable-preview PrimitiveInRecord     (25.0.4.1+1-LTS)
+whole degrees: 21
+fractional: 21.5
+not a reading
+
+$ javac --enable-preview --release 27 preview/PrimitiveInRecord.java && java --enable-preview PrimitiveInRecord     (27+35)
+whole degrees: 21
+fractional: 21.5
+not a reading
+
+$ javac --enable-preview --release 25 preview/PrimitiveEdges.java && java --enable-preview PrimitiveEdges     (25.0.4.1+1-LTS)
+0.0          int? true  float? true 
+-0.0         int? false float? true 
+NaN          int? false float? true 
+1.6777217E7  int? true  float? false
+1.0E10       int? false float? true 
+3.0          int? true  float? true 
+1099511627776 instanceof int? false
+Object holding Integer matches int pattern: 42
+Short in Object does not match int
+65 as char: A
+
+$ javac --enable-preview --release 27 preview/PrimitiveEdges.java && java --enable-preview PrimitiveEdges     (27+35)
+0.0          int? true  float? true 
+-0.0         int? false float? true 
+NaN          int? false float? true 
+1.6777217E7  int? true  float? false
+1.0E10       int? false float? true 
+3.0          int? true  float? true 
+1099511627776 instanceof int? false
+Object holding Integer matches int pattern: 42
+Short in Object does not match int
+65 as char: A
+
+$ javac --enable-preview --release 25 preview/BoxedToNarrower.java && java --enable-preview BoxedToNarrower     (25.0.4.1+1-LTS)
+preview/BoxedToNarrower.java:4: error: incompatible types: Integer cannot be converted to byte
+        if (boxed instanceof byte b) System.out.println("fits a byte: " + b);   // Integer to byte is not allowed
+            ^
+1 error
+Error: Could not find or load main class BoxedToNarrower
+Caused by: java.lang.ClassNotFoundException: BoxedToNarrower
+
+$ javac --enable-preview --release 27 preview/BoxedToNarrower.java && java --enable-preview BoxedToNarrower     (27+35)
+preview/BoxedToNarrower.java:4: error: incompatible types: Integer cannot be converted to byte
+        if (boxed instanceof byte b) System.out.println("fits a byte: " + b);   // Integer to byte is not allowed
+            ^
+1 error
+Error: Could not find or load main class BoxedToNarrower
+Caused by: java.lang.ClassNotFoundException: BoxedToNarrower
diff --git a/patterns/output/08-preview-class-files.txt b/patterns/output/08-preview-class-files.txt
new file mode 100644
index 0000000..86da886
--- /dev/null
+++ b/patterns/output/08-preview-class-files.txt
@@ -0,0 +1,11 @@
+$ javap -v PrimitiveSwitch | grep -E 'minor|major'     (compiled by 25.0.4.1+1-LTS with --enable-preview)
+  minor version: 65535
+  major version: 69
+
+$ java --enable-preview PrimitiveSwitch     (class compiled on 25, run on 27+35)
+Error: LinkageError occurred while loading main class PrimitiveSwitch
+	java.lang.UnsupportedClassVersionError: PrimitiveSwitch (class file version 69.65535) was compiled with preview features that are unsupported. This version of the Java Runtime only recognizes preview features for class file version 71.65535
+
+$ java PrimitiveSwitch     (same class, no flag, run on 25.0.4.1+1-LTS)
+Error: LinkageError occurred while loading main class PrimitiveSwitch
+	java.lang.UnsupportedClassVersionError: Preview features are not enabled for PrimitiveSwitch (class file version 69.65535). Try running with '--enable-preview'
diff --git a/patterns/preview/BoxedToNarrower.java b/patterns/preview/BoxedToNarrower.java
new file mode 100644
index 0000000..15f07b6
--- /dev/null
+++ b/patterns/preview/BoxedToNarrower.java
@@ -0,0 +1,6 @@
+public class BoxedToNarrower {
+    public static void main(String[] args) {
+        Integer boxed = 300;
+        if (boxed instanceof byte b) System.out.println("fits a byte: " + b);   // Integer to byte is not allowed
+    }
+}
diff --git a/patterns/preview/PrimitiveEdges.java b/patterns/preview/PrimitiveEdges.java
new file mode 100644
index 0000000..109cb7d
--- /dev/null
+++ b/patterns/preview/PrimitiveEdges.java
@@ -0,0 +1,16 @@
+public class PrimitiveEdges {
+    public static void main(String[] args) {
+        double[] ds = {0.0, -0.0, Double.NaN, 16777217.0, 1e10, 3.0};
+        for (double d : ds) {
+            System.out.printf("%-12s int? %-5b float? %-5b%n", d, d instanceof int, d instanceof float);
+        }
+        long big = 1L << 40;
+        System.out.println(big + " instanceof int? " + (big instanceof int));
+        Object o = 42;
+        if (o instanceof int i) System.out.println("Object holding Integer matches int pattern: " + i);
+        Object s = (short) 7;
+        if (s instanceof int i) System.out.println("Short in Object matches int? " + i); else System.out.println("Short in Object does not match int");
+        int x = 65;
+        if (x instanceof char c) System.out.println("65 as char: " + c);
+    }
+}
diff --git a/patterns/preview/PrimitiveInRecord.java b/patterns/preview/PrimitiveInRecord.java
new file mode 100644
index 0000000..a8ff24a
--- /dev/null
+++ b/patterns/preview/PrimitiveInRecord.java
@@ -0,0 +1,17 @@
+public class PrimitiveInRecord {
+    record Reading(double celsius) {}
+
+    static String classify(Object o) {
+        return switch (o) {
+            case Reading(int whole) -> "whole degrees: " + whole;     // double component matched against an int pattern
+            case Reading(double d) -> "fractional: " + d;
+            default -> "not a reading";
+        };
+    }
+
+    public static void main(String[] args) {
+        System.out.println(classify(new Reading(21.0)));
+        System.out.println(classify(new Reading(21.5)));
+        System.out.println(classify("x"));
+    }
+}
diff --git a/patterns/preview/PrimitiveInstanceof.java b/patterns/preview/PrimitiveInstanceof.java
new file mode 100644
index 0000000..145fd50
--- /dev/null
+++ b/patterns/preview/PrimitiveInstanceof.java
@@ -0,0 +1,20 @@
+public class PrimitiveInstanceof {
+    static String describe(int i) {
+        if (i instanceof byte b) return i + " fits in a byte: " + b;
+        if (i instanceof short s) return i + " fits in a short: " + s;
+        return i + " needs an int";
+    }
+
+    static void floats(double d) {
+        if (d instanceof float f) System.out.println(d + " converts to float " + f + " without loss");
+        else System.out.println(d + " does NOT convert to float without loss");
+    }
+
+    public static void main(String[] args) {
+        System.out.println(describe(100));
+        System.out.println(describe(1000));
+        System.out.println(describe(100000));
+        floats(0.5);
+        floats(0.1);
+    }
+}
diff --git a/patterns/preview/PrimitiveSwitch.java b/patterns/preview/PrimitiveSwitch.java
new file mode 100644
index 0000000..3b988b2
--- /dev/null
+++ b/patterns/preview/PrimitiveSwitch.java
@@ -0,0 +1,31 @@
+public class PrimitiveSwitch {
+    static String httpFamily(int status) {
+        return switch (status) {
+            case 200 -> "OK";
+            case int i when i >= 200 && i < 300 -> "other success " + i;
+            case int i when i >= 400 && i < 500 -> "client error " + i;
+            case int i -> "something else " + i;
+        };
+    }
+
+    static String flag(boolean b) {
+        return switch (b) {
+            case true -> "yes";
+            case false -> "no";
+        };
+    }
+
+    static String big(long n) {
+        return switch (n) {
+            case 0L -> "zero";
+            case 1L -> "one";
+            case long l -> "many (" + l + ")";
+        };
+    }
+
+    public static void main(String[] args) {
+        System.out.println(httpFamily(200) + " | " + httpFamily(204) + " | " + httpFamily(404) + " | " + httpFamily(500));
+        System.out.println(flag(true) + " " + flag(false));
+        System.out.println(big(0) + " " + big(1) + " " + big(Long.MAX_VALUE));
+    }
+}
diff --git a/patterns/run.sh b/patterns/run.sh
new file mode 100755
index 0000000..6668a49
--- /dev/null
+++ b/patterns/run.sh
@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# Pattern matching from JDK 21 to 27: type/record patterns, guards, exhaustiveness, unnamed patterns (final),
+# and primitive patterns (JEP 532, preview in 27; also preview on 25 with --enable-preview).
+# Regenerates every file in output/.   JDK25=/path JDK27=/path [JDK21=/path] ./run.sh
+set -uo pipefail
+unset JAVA_TOOL_OPTIONS
+HERE="$(cd "$(dirname "$0")" && pwd)"
+JDK25="${JDK25:-/opt/jdks/jdk-25.0.4.1+1}"; JDK27="${JDK27:-/opt/jdks/jdk27}"; JDK21="${JDK21:-}"
+OUT="$HERE/output"; mkdir -p "$OUT"; B="$(mktemp -d)"; trap 'rm -rf "$B"' EXIT
+cd "$HERE"
+t() { "$1/bin/java" -version 2>&1 | sed -n 2p | sed 's/.*(build \(.*\))/\1/'; }
+rel() { case "$1" in "$JDK25") echo 25;; "$JDK27") echo 27;; *) echo 21;; esac; }
+mk() { mktemp -d -p "$B"; }
+# compile + run (no flags)
+cr() { local jdk="$1" src="$2" cls="$3" o; o="$(mk)"
+  echo "\$ javac $src && java $cls     ($(t "$jdk"))"
+  "$jdk/bin/javac" -d "$o" "$src" 2>&1 && "$jdk/bin/java" -cp "$o" "$cls" 2>&1; }
+# compile only, show diagnostics
+co() { local jdk="$1" src="$2" o; o="$(mk)"
+  echo "\$ javac $src     ($(t "$jdk"))"
+  "$jdk/bin/javac" -d "$o" "$src" 2>&1 | grep -v '^Note:'; }
+# preview compile + run
+crp() { local jdk="$1" src="$2" cls="$3" o r; o="$(mk)"; r="$(rel "$jdk")"
+  echo "\$ javac --enable-preview --release $r $src && java --enable-preview $cls     ($(t "$jdk"))"
+  "$jdk/bin/javac" --enable-preview --release "$r" -d "$o" "$src" 2>&1 | grep -v '^Note:'
+  "$jdk/bin/java" --enable-preview -cp "$o" "$cls" 2>&1; }
+both() { local fn="$1"; shift; "$fn" "$JDK25" "$@"; echo; "$fn" "$JDK27" "$@"; }
+all3() { local fn="$1"; shift; [ -n "$JDK21" ] && { "$fn" "$JDK21" "$@"; echo; }; "$fn" "$JDK25" "$@"; echo; "$fn" "$JDK27" "$@"; }
+
+# 01 - type patterns
+{ all3 cr src/TypePatterns.java TypePatterns; } > "$OUT/01-type-patterns.txt"
+
+# 02 - guards and dominance
+{ both cr src/Guards.java Guards; echo; both co broken/Dominated.java; echo; both co broken/GuardBeforeGeneral.java; } > "$OUT/02-guards-and-dominance.txt"
+
+# 03 - record patterns, and what happens when an accessor throws
+{ all3 cr src/RecordPatterns.java RecordPatterns; echo; both cr src/MatchExceptionDemo.java MatchExceptionDemo; } > "$OUT/03-record-patterns.txt"
+
+# 04 - exhaustiveness: the compiler's check, and what separate compilation can still break
+{ all3 cr src/SealedExhaustive.java SealedExhaustive; echo
+  all3 co broken/NotExhaustive.java; echo
+  all3 co broken/NotExhaustiveObject.java; } > "$OUT/04-exhaustiveness.txt"
+{ for j in "$JDK25" "$JDK27"; do
+    o="$(mk)"; mkdir "$o/old" "$o/new"
+    "$j/bin/javac" -d "$o/old" sep/v1/*.java sep/client/Client.java 2>&1
+    "$j/bin/javac" -cp "$o/old" -d "$o/new" sep/v2/*.java 2>&1
+    echo "\$ java -cp old Client     (Client and Shape compiled together, v1)     ($(t "$j"))"; "$j/bin/java" -cp "$o/old" Client 2>&1; echo
+    echo "\$ java -cp new:old Main     (Shape gained Triangle; Client.class NOT recompiled)     ($(t "$j"))"; "$j/bin/java" -cp "$o/new:$o/old" Main 2>&1 | head -4; echo
+    echo "\$ javac -cp new sep/client/Client.java     (now recompile Client against v2)     ($(t "$j"))"
+    "$j/bin/javac" -cp "$o/new" -d "$(mk)" sep/client/Client.java 2>&1; echo
+  done; } > "$OUT/05-separate-compilation.txt"
+
+# 06 - null and unnamed patterns
+{ both cr src/NullCase.java NullCase; echo; all3 cr src/Unnamed.java Unnamed; } > "$OUT/06-null-and-unnamed.txt"
+
+# 07 - primitive patterns (preview)
+{ for f in PrimitiveInstanceof; do both co preview/$f.java; done; echo
+  for f in PrimitiveInstanceof PrimitiveSwitch PrimitiveInRecord PrimitiveEdges; do both crp preview/$f.java $f; echo; done
+  both crp preview/BoxedToNarrower.java BoxedToNarrower; } > "$OUT/07-primitive-patterns.txt"
+{ o="$(mk)"; "$JDK25/bin/javac" --enable-preview --release 25 -d "$o" preview/PrimitiveSwitch.java 2>&1 | grep -v '^Note:'
+  echo "\$ javap -v PrimitiveSwitch | grep -E 'minor|major'     (compiled by $(t "$JDK25") with --enable-preview)"
+  "$JDK25/bin/javap" -v -cp "$o" PrimitiveSwitch | grep -E 'minor|major'; echo
+  echo "\$ java --enable-preview PrimitiveSwitch     (class compiled on 25, run on $(t "$JDK27"))"; "$JDK27/bin/java" --enable-preview -cp "$o" PrimitiveSwitch 2>&1 | head -2; echo
+  echo "\$ java PrimitiveSwitch     (same class, no flag, run on $(t "$JDK25"))"; "$JDK25/bin/java" -cp "$o" PrimitiveSwitch 2>&1 | head -2
+} > "$OUT/08-preview-class-files.txt"
+echo "wrote $OUT"
diff --git a/patterns/sep/client/Client.java b/patterns/sep/client/Client.java
new file mode 100644
index 0000000..27a217e
--- /dev/null
+++ b/patterns/sep/client/Client.java
@@ -0,0 +1,14 @@
+public class Client {
+    // Compiled against Shape v1, where the only permitted subtypes are Circle and Square.
+    public static double area(Shape s) {
+        return switch (s) {
+            case Circle c -> Math.PI * c.r() * c.r();
+            case Square q -> q.side() * q.side();
+        };
+    }
+
+    public static void main(String[] args) {
+        System.out.println("circle: " + area(new Circle(1)));
+        System.out.println("square: " + area(new Square(2)));
+    }
+}
diff --git a/patterns/sep/v1/Circle.java b/patterns/sep/v1/Circle.java
new file mode 100644
index 0000000..54aa892
--- /dev/null
+++ b/patterns/sep/v1/Circle.java
@@ -0,0 +1 @@
+public record Circle(double r) implements Shape {}
diff --git a/patterns/sep/v1/Shape.java b/patterns/sep/v1/Shape.java
new file mode 100644
index 0000000..9ff8813
--- /dev/null
+++ b/patterns/sep/v1/Shape.java
@@ -0,0 +1 @@
+public sealed interface Shape permits Circle, Square {}
diff --git a/patterns/sep/v1/Square.java b/patterns/sep/v1/Square.java
new file mode 100644
index 0000000..4f65bd7
--- /dev/null
+++ b/patterns/sep/v1/Square.java
@@ -0,0 +1 @@
+public record Square(double side) implements Shape {}
diff --git a/patterns/sep/v2/Circle.java b/patterns/sep/v2/Circle.java
new file mode 100644
index 0000000..54aa892
--- /dev/null
+++ b/patterns/sep/v2/Circle.java
@@ -0,0 +1 @@
+public record Circle(double r) implements Shape {}
diff --git a/patterns/sep/v2/Main.java b/patterns/sep/v2/Main.java
new file mode 100644
index 0000000..7254c68
--- /dev/null
+++ b/patterns/sep/v2/Main.java
@@ -0,0 +1,6 @@
+public class Main {
+    public static void main(String[] args) {
+        // Triangle exists only in v2. Client was compiled before it did.
+        System.out.println("triangle: " + Client.area(new Triangle(3, 4)));
+    }
+}
diff --git a/patterns/sep/v2/Shape.java b/patterns/sep/v2/Shape.java
new file mode 100644
index 0000000..0a87ca8
--- /dev/null
+++ b/patterns/sep/v2/Shape.java
@@ -0,0 +1 @@
+public sealed interface Shape permits Circle, Square, Triangle {}
diff --git a/patterns/sep/v2/Square.java b/patterns/sep/v2/Square.java
new file mode 100644
index 0000000..4f65bd7
--- /dev/null
+++ b/patterns/sep/v2/Square.java
@@ -0,0 +1 @@
+public record Square(double side) implements Shape {}
diff --git a/patterns/sep/v2/Triangle.java b/patterns/sep/v2/Triangle.java
new file mode 100644
index 0000000..7f1ddbd
--- /dev/null
+++ b/patterns/sep/v2/Triangle.java
@@ -0,0 +1 @@
+public record Triangle(double base, double height) implements Shape {}
diff --git a/patterns/src/Guards.java b/patterns/src/Guards.java
new file mode 100644
index 0000000..d9fb88b
--- /dev/null
+++ b/patterns/src/Guards.java
@@ -0,0 +1,16 @@
+public class Guards {
+    static String size(Object o) {
+        return switch (o) {
+            case String s when s.isEmpty() -> "empty string";
+            case String s when s.length() < 5 -> "short string: " + s;
+            case String s -> "long string: " + s;
+            case Integer i when i < 0 -> "negative " + i;
+            case Integer i -> "int " + i;
+            default -> "other";
+        };
+    }
+
+    public static void main(String[] args) {
+        for (Object o : new Object[]{"", "abc", "abcdefgh", -4, 12, 1.5}) System.out.println(size(o));
+    }
+}
diff --git a/patterns/src/MatchExceptionDemo.java b/patterns/src/MatchExceptionDemo.java
new file mode 100644
index 0000000..32e48de
--- /dev/null
+++ b/patterns/src/MatchExceptionDemo.java
@@ -0,0 +1,15 @@
+public class MatchExceptionDemo {
+    record Fragile(int v) {
+        public int v() { throw new IllegalStateException("accessor blew up"); }
+    }
+
+    public static void main(String[] args) {
+        Object o = new Fragile(1);
+        try {
+            if (o instanceof Fragile(int v)) System.out.println("matched " + v);
+        } catch (MatchException e) {
+            System.out.println("MatchException: " + e.getMessage());
+            System.out.println("cause: " + e.getCause());
+        }
+    }
+}
diff --git a/patterns/src/NullCase.java b/patterns/src/NullCase.java
new file mode 100644
index 0000000..97fcb32
--- /dev/null
+++ b/patterns/src/NullCase.java
@@ -0,0 +1,31 @@
+public class NullCase {
+    static String withoutCaseNull(Object o) {
+        return switch (o) {
+            case String s -> "string";
+            default -> "other";
+        };
+    }
+
+    static String withCaseNull(Object o) {
+        return switch (o) {
+            case null -> "null!";
+            case String s -> "string";
+            default -> "other";
+        };
+    }
+
+    static String combined(Object o) {
+        return switch (o) {
+            case String s -> "string";
+            case null, default -> "null or something else";
+        };
+    }
+
+    public static void main(String[] args) {
+        try { System.out.println(withoutCaseNull(null)); }
+        catch (NullPointerException e) { System.out.println("without case null: NullPointerException"); }
+        System.out.println("with case null: " + withCaseNull(null));
+        System.out.println("case null, default: " + combined(null));
+        System.out.println("instanceof with null: " + (null instanceof String));
+    }
+}
diff --git a/patterns/src/RecordPatterns.java b/patterns/src/RecordPatterns.java
new file mode 100644
index 0000000..31ec063
--- /dev/null
+++ b/patterns/src/RecordPatterns.java
@@ -0,0 +1,39 @@
+public class RecordPatterns {
+    record Point(int x, int y) {}
+    record Line(Point from, Point to) {}
+    record Box<T>(T content) {}
+
+    // Deconstruct a record: the components are bound to x and y.
+    static String where(Object o) {
+        if (o instanceof Point(int x, int y)) {
+            return "point at " + x + "," + y;
+        }
+        return "not a point";
+    }
+
+    // Patterns nest: take a Line apart into its four coordinates in one step.
+    static int length1(Line line) {
+        if (line instanceof Line(Point(var x1, var y1), Point(var x2, var y2))) {
+            return Math.abs(x2 - x1) + Math.abs(y2 - y1);
+        }
+        return -1;
+    }
+
+    // A nested pattern can also test the component's type.
+    static String box(Box<Object> b) {
+        return switch (b) {
+            case Box<Object>(String s) -> "box of string " + s;
+            case Box<Object>(Integer i) -> "box of int " + i;
+            case Box<Object>(var other) -> "box of " + other;
+        };
+    }
+
+    public static void main(String[] args) {
+        System.out.println(where(new Point(3, 4)));
+        System.out.println(where("nope"));
+        System.out.println(length1(new Line(new Point(0, 0), new Point(3, 4))));
+        System.out.println(box(new Box<>("hi")));
+        System.out.println(box(new Box<>(7)));
+        System.out.println(box(new Box<>(2.5)));
+    }
+}
diff --git a/patterns/src/SealedExhaustive.java b/patterns/src/SealedExhaustive.java
new file mode 100644
index 0000000..ed580fd
--- /dev/null
+++ b/patterns/src/SealedExhaustive.java
@@ -0,0 +1,21 @@
+public class SealedExhaustive {
+    sealed interface Shape permits Circle, Square, Rect {}
+    record Circle(double r) implements Shape {}
+    record Square(double side) implements Shape {}
+    record Rect(double w, double h) implements Shape {}
+
+    // No default: the compiler checks that every permitted subtype is covered.
+    static double area(Shape s) {
+        return switch (s) {
+            case Circle(double r) -> Math.PI * r * r;
+            case Square(double side) -> side * side;
+            case Rect(double w, double h) -> w * h;
+        };
+    }
+
+    public static void main(String[] args) {
+        for (Shape s : new Shape[]{new Circle(1), new Square(2), new Rect(2, 3)}) {
+            System.out.printf("%s -> %.2f%n", s, area(s));
+        }
+    }
+}
diff --git a/patterns/src/TypePatterns.java b/patterns/src/TypePatterns.java
new file mode 100644
index 0000000..ab51a24
--- /dev/null
+++ b/patterns/src/TypePatterns.java
@@ -0,0 +1,45 @@
+public class TypePatterns {
+    // The old way: test, cast, then use.
+    static String oldStyle(Object o) {
+        if (o instanceof String) {
+            String s = (String) o;
+            return "string of length " + s.length();
+        }
+        return "something else";
+    }
+
+    // The pattern way: test and bind in one step.
+    static String newStyle(Object o) {
+        if (o instanceof String s) {
+            return "string of length " + s.length();
+        }
+        return "something else";
+    }
+
+    // The binding variable stays in scope wherever the test is known to have succeeded.
+    static int lengthOrMinusOne(Object o) {
+        if (!(o instanceof String s)) {
+            return -1;
+        }
+        return s.length();          // s is in scope here: we only get here if the test succeeded
+    }
+
+    // A switch can test many types in a row.
+    static String describe(Object o) {
+        return switch (o) {
+            case Integer i -> "int " + (i + 1);
+            case String s -> "string " + s.toUpperCase();
+            case int[] arr -> "int array of " + arr.length;
+            default -> "other " + o.getClass().getSimpleName();
+        };
+    }
+
+    public static void main(String[] args) {
+        System.out.println(oldStyle("hello") + " / " + newStyle("hello"));
+        System.out.println(lengthOrMinusOne("abc") + " " + lengthOrMinusOne(42));
+        System.out.println(describe(41));
+        System.out.println(describe("java"));
+        System.out.println(describe(new int[]{1, 2, 3}));
+        System.out.println(describe(3.5));
+    }
+}
diff --git a/patterns/src/Unnamed.java b/patterns/src/Unnamed.java
new file mode 100644
index 0000000..c72282f
--- /dev/null
+++ b/patterns/src/Unnamed.java
@@ -0,0 +1,23 @@
+public class Unnamed {
+    record Point(int x, int y) {}
+    sealed interface Shape permits Circle, Square {}
+    record Circle(double r) implements Shape {}
+    record Square(double side) implements Shape {}
+
+    static String kind(Shape s) {
+        return switch (s) {
+            case Circle _ -> "round";          // the type matters, the value does not
+            case Square _ -> "angular";
+        };
+    }
+
+    static int xOnly(Object o) {
+        if (o instanceof Point(int x, _)) return x;    // do not care about y
+        return -1;
+    }
+
+    public static void main(String[] args) {
+        System.out.println(kind(new Circle(1)) + " " + kind(new Square(1)));
+        System.out.println(xOnly(new Point(5, 9)));
+    }
+}