Add java17-features module: runnable demos and captured output for the ankurm.com java17-features article

README.md

@@ -102,3 +102,4 @@ MIT licensed, see `LICENSE`.
 Modules for the Java 8 / 11 / 17 / 21 feature articles, one folder each, each with `src/`, `broken/`, `output/` and `run.sh`:
 - `java8-features/` — see its README
 - `java11-features/` — see its README
+- `java17-features/` — see its README

java17-features/README.md

@@ -0,0 +1,13 @@
+# java17-features — Java 17 features with examples
+
+Companion code for the ankurm.com article **Java 17 Features with Examples: Records, Sealed Classes, Text Blocks, Switch Expressions**. The explanation lives in the article; this folder holds the runnable sources and the transcripts they produced. There is no `docs/` folder: the depth is inside the article itself.
+
+```bash
+JDK11=/path/to/jdk11 JDK17=/path/to/jdk17 ./run.sh      # regenerates output/*.txt
+```
+
+- `src/` compiles and runs unchanged on the JDK named in each output header.
+- `broken/` fails to compile on purpose; the compiler's own message is committed in `output/`.
+- `output/` is real captured output, regenerated by `run.sh`.
+
+Tested on: Temurin 17.0.20.1, plus 11.0.32.1 for the strong-encapsulation comparison.

java17-features/broken/RecordFieldMutation.java

@@ -0,0 +1,5 @@
+/** Fails to compile on purpose: record fields are final. */
+public class RecordFieldMutation {
+    record Point(int x, int y) { void move() { this.x = 10; } }
+    public static void main(String[] args) { }
+}

java17-features/broken/SealedViolation.java

@@ -0,0 +1,7 @@
+/** Fails to compile on purpose: a class that is not in the permits list cannot extend a sealed type. */
+public class SealedViolation {
+    sealed interface Shape permits Circle {}
+    record Circle(double r) implements Shape {}
+    record Triangle(double b, double h) implements Shape {}
+    public static void main(String[] args) { }
+}

java17-features/broken/SwitchPatternOn17.java

@@ -0,0 +1,5 @@
+/** Fails on 17 without --enable-preview: pattern matching for switch is only a preview feature in 17 (JEP 406). */
+public class SwitchPatternOn17 {
+    static String f(Object o) { return switch (o) { case String s -> "string"; default -> "other"; }; }
+    public static void main(String[] args) { System.out.println(f("x")); }
+}

java17-features/output/01-records.txt

@@ -0,0 +1,6 @@
+$ javac src/RecordsDemo.java && java RecordsDemo     (17.0.20.1)
+toString        : Point[x=3, y=4]
+equals          : true   hashCode equal: true
+accessor        : p.x() = 3, distance = 5.0
+compact ctor    : negative coordinate: -1,2
+shallow         : Team[name=core, members=[a, b]]   <- the record's list changed after construction

java17-features/output/02-sealed.txt

@@ -0,0 +1,6 @@
+$ javac src/SealedDemo.java && java SealedDemo     (17.0.20.1)
+Circle[r=1.0]              area = 3.142
+Square[side=2.0]           area = 4.000
+Rect[w=2.0, h=3.0]         area = 6.000
+permitted       : [Circle, Square, Rect]
+isSealed        : true

java17-features/output/03-text-blocks.txt

@@ -0,0 +1,8 @@
+$ javac src/TextBlocksDemo.java && java TextBlocksDemo     (17.0.20.1)
+{
+  "name": "Ankur",
+  "city": "Mumbai"
+}
+lines           : 4
+backslash-newline: joined on one line
+formatted       : Hello, reader!

java17-features/output/04-switch-expressions.txt

@@ -0,0 +1,5 @@
+$ javac src/SwitchExprDemo.java && java SwitchExprDemo     (17.0.20.1)
+MON -> 9 working hours
+SAT -> 0 working hours
+WED -> 8 working hours
+old fall-through: one two

java17-features/output/05-instanceof-pattern.txt

@@ -0,0 +1,6 @@
+$ javac src/PatternInstanceofDemo.java && java PatternInstanceofDemo     (17.0.20.1)
+long string hello
+other hi
+int 42
+Stream.toList   : [1, 2, 3]
+toList() result is unmodifiable

java17-features/output/06-helpful-npe.txt

@@ -0,0 +1,2 @@
+$ javac src/HelpfulNpeDemo.java && java HelpfulNpeDemo     (17.0.20.1)
+Cannot read field "city" because "u.address" is null

java17-features/output/07-strong-encapsulation.txt

@@ -0,0 +1,13 @@
+$ javac src/EncapsulationDemo.java && java EncapsulationDemo     (11.0.32.1)
+WARNING: An illegal reflective access operation has occurred
+WARNING: Illegal reflective access by EncapsulationDemo (file:/…/) to field java.lang.String.value
+WARNING: Please consider reporting this to the maintainers of EncapsulationDemo
+WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
+WARNING: All illegal access operations will be denied in a future release
+read String.value: byte[]
+
+$ javac src/EncapsulationDemo.java && java EncapsulationDemo     (17.0.20.1)
+java.lang.reflect.InaccessibleObjectException: Unable to make field private final byte[] java.lang.String.value accessible: module java.base does not "opens java.lang" to unnamed module @<hash>
+
+$ java --add-opens java.base/java.lang=ALL-UNNAMED EncapsulationDemo     (17.0.20.1)
+read String.value: byte[]

java17-features/output/08-random-generator.txt

@@ -0,0 +1,4 @@
+$ javac src/RandomDemo.java && java RandomDemo     (17.0.20.1)
+L64X128MixRandom nextInt(100) x3: 98 1 80
+ThreadLocalRandom implements RandomGenerator: true
+algorithms >= 10: true

java17-features/output/09-compile-errors.txt

@@ -0,0 +1,18 @@
+$ javac broken/SealedViolation.java     (17.0.20.1)
+broken/SealedViolation.java:5: error: class is not allowed to extend sealed class: Shape (as it is not listed in its permits clause)
+    record Triangle(double b, double h) implements Shape {}
+    ^
+1 error
+
+$ javac broken/RecordFieldMutation.java     (17.0.20.1)
+broken/RecordFieldMutation.java:3: error: cannot assign a value to final variable x
+    record Point(int x, int y) { void move() { this.x = 10; } }
+                                                   ^
+1 error
+
+$ javac broken/SwitchPatternOn17.java     (17.0.20.1)
+broken/SwitchPatternOn17.java:3: error: patterns in switch statements are a preview feature and are disabled by default.
+    static String f(Object o) { return switch (o) { case String s -> "string"; default -> "other"; }; }
+                                                         ^
+  (use --enable-preview to enable patterns in switch statements)
+1 error

java17-features/run.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+# Java 17 features: every demo compiled and run on a real JDK 17 (and 11 for the encapsulation comparison); regenerates output/*.txt
+#   JDK11=/path JDK17=/path ./run.sh
+set -uo pipefail
+unset JAVA_TOOL_OPTIONS
+HERE="$(cd "$(dirname "$0")" && pwd)"
+JDK11="${JDK11:-/opt/jdks/jdk11}"; JDK17="${JDK17:-/opt/jdks/jdk17}"
+OUT="$HERE/output"; mkdir -p "$OUT"; B="$(mktemp -d)"; trap 'rm -rf "$B"' EXIT
+cd "$HERE"
+ver() { "$1/bin/java" -version 2>&1 | sed -n 1p | sed 's/.*"\(.*\)".*/\1/'; }
+cr() { local jdk="$1" src="$2" cls="$3" o; o="$(mktemp -d -p "$B")"
+  echo "\$ javac $src && java $cls     ($(ver "$jdk"))"
+  "$jdk/bin/javac" ${JFLAGS:-} -encoding UTF-8 -d "$o" "$src" 2>&1 | grep -v '^Note:'; "$jdk/bin/java" -cp "$o" "$cls" 2>&1; }
+co() { local jdk="$1" src="$2" o; o="$(mktemp -d -p "$B")"
+  echo "\$ javac $src     ($(ver "$jdk"))"; local r; r="$("$jdk/bin/javac" -encoding UTF-8 -d "$o" "$src" 2>&1)"; if [ -n "$r" ]; then echo "$r"; else echo "(compiled cleanly)"; fi; }
+cr "$JDK17" src/RecordsDemo.java RecordsDemo               > "$OUT/01-records.txt"
+cr "$JDK17" src/SealedDemo.java SealedDemo                 > "$OUT/02-sealed.txt"
+cr "$JDK17" src/TextBlocksDemo.java TextBlocksDemo         > "$OUT/03-text-blocks.txt"
+cr "$JDK17" src/SwitchExprDemo.java SwitchExprDemo         > "$OUT/04-switch-expressions.txt"
+cr "$JDK17" src/PatternInstanceofDemo.java PatternInstanceofDemo > "$OUT/05-instanceof-pattern.txt"
+JFLAGS=-g cr "$JDK17" src/HelpfulNpeDemo.java HelpfulNpeDemo         > "$OUT/06-helpful-npe.txt"
+{ cr "$JDK11" src/EncapsulationDemo.java EncapsulationDemo; echo; cr "$JDK17" src/EncapsulationDemo.java EncapsulationDemo
+  o="$(mktemp -d -p "$B")"; "$JDK17/bin/javac" -d "$o" src/EncapsulationDemo.java
+  echo; echo "\$ java --add-opens java.base/java.lang=ALL-UNNAMED EncapsulationDemo     ($(ver "$JDK17"))"
+  "$JDK17/bin/java" --add-opens java.base/java.lang=ALL-UNNAMED -cp "$o" EncapsulationDemo 2>&1; } | sed -E 's#\(file:[^)]*\)#(file:/…/)#; s#unnamed module @[0-9a-f]+#unnamed module @<hash>#' > "$OUT/07-strong-encapsulation.txt"
+cr "$JDK17" src/RandomDemo.java RandomDemo                 > "$OUT/08-random-generator.txt"
+{ co "$JDK17" broken/SealedViolation.java; echo; co "$JDK17" broken/RecordFieldMutation.java; echo; co "$JDK17" broken/SwitchPatternOn17.java; } > "$OUT/09-compile-errors.txt"
+exit 0

java17-features/src/EncapsulationDemo.java

@@ -0,0 +1,15 @@
+import java.lang.reflect.Field;
+
+/** JEP 403: strong encapsulation of JDK internals. Same code, different outcome on 11 and 17. */
+public class EncapsulationDemo {
+    public static void main(String[] args) throws Exception {
+        Field f = String.class.getDeclaredField("value");
+        try {
+            f.setAccessible(true);
+            Object v = f.get("abc");
+            System.out.println("read String.value: " + v.getClass().getSimpleName());
+        } catch (RuntimeException e) {
+            System.out.println(e.getClass().getName() + ": " + e.getMessage());
+        }
+    }
+}

java17-features/src/HelpfulNpeDemo.java

@@ -0,0 +1,10 @@
+/** Helpful NullPointerException messages (JEP 358): on by default since 15. */
+public class HelpfulNpeDemo {
+    static class Address { String city; }
+    static class User { Address address; }
+    public static void main(String[] args) {
+        User u = new User();
+        try { System.out.println(u.address.city.length()); }
+        catch (NullPointerException e) { System.out.println(e.getMessage()); }
+    }
+}

java17-features/src/PatternInstanceofDemo.java

@@ -0,0 +1,16 @@
+/** Pattern matching for instanceof (final in 16, JEP 394) and Stream.toList() (16). */
+public class PatternInstanceofDemo {
+    static String describe(Object o) {
+        if (o instanceof String s && s.length() > 3) return "long string " + s;
+        if (o instanceof Integer i) return "int " + (i + 1);
+        return "other " + o;
+    }
+    public static void main(String[] args) {
+        System.out.println(describe("hello"));
+        System.out.println(describe("hi"));
+        System.out.println(describe(41));
+        var list = java.util.stream.Stream.of(3, 1, 2).sorted().toList();
+        System.out.println("Stream.toList   : " + list);
+        try { list.add(4); } catch (UnsupportedOperationException e) { System.out.println("toList() result is unmodifiable"); }
+    }
+}

java17-features/src/RandomDemo.java

@@ -0,0 +1,12 @@
+import java.util.random.RandomGenerator;
+import java.util.random.RandomGeneratorFactory;
+
+/** JEP 356: RandomGenerator interface and named algorithms. Seeded, so the output is stable. */
+public class RandomDemo {
+    public static void main(String[] args) {
+        RandomGenerator g = RandomGeneratorFactory.of("L64X128MixRandom").create(42L);
+        System.out.println("L64X128MixRandom nextInt(100) x3: " + g.nextInt(100) + " " + g.nextInt(100) + " " + g.nextInt(100));
+        System.out.println("ThreadLocalRandom implements RandomGenerator: " + (java.util.concurrent.ThreadLocalRandom.current() instanceof RandomGenerator));
+        System.out.println("algorithms >= 10: " + (RandomGeneratorFactory.all().count() >= 10));
+    }
+}

java17-features/src/RecordsDemo.java

@@ -0,0 +1,21 @@
+/** Records (final in 16, JEP 395): what the compiler generates, compact constructors, and the shallow-immutability caveat. */
+public class RecordsDemo {
+    record Point(int x, int y) {
+        Point { if (x < 0 || y < 0) throw new IllegalArgumentException("negative coordinate: " + x + "," + y); }
+        double distance() { return Math.sqrt(x * x + y * y); }
+    }
+    record Team(String name, java.util.List<String> members) {}
+
+    public static void main(String[] args) {
+        Point p = new Point(3, 4);
+        System.out.println("toString        : " + p);
+        System.out.println("equals          : " + p.equals(new Point(3, 4)) + "   hashCode equal: " + (p.hashCode() == new Point(3, 4).hashCode()));
+        System.out.println("accessor        : p.x() = " + p.x() + ", distance = " + p.distance());
+        try { new Point(-1, 2); } catch (IllegalArgumentException e) { System.out.println("compact ctor    : " + e.getMessage()); }
+
+        java.util.List<String> list = new java.util.ArrayList<>(java.util.List.of("a"));
+        Team t = new Team("core", list);
+        list.add("b");
+        System.out.println("shallow         : " + t + "   <- the record's list changed after construction");
+    }
+}

java17-features/src/SealedDemo.java

@@ -0,0 +1,21 @@
+/** Sealed classes (final in 17, JEP 409): the compiler knows every permitted subtype. */
+public class SealedDemo {
+    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 {}
+
+    // On 17 the switch must still be an if-chain: pattern matching for switch is only a preview here (JEP 406)
+    static double area(Shape s) {
+        if (s instanceof Circle c) return Math.PI * c.r() * c.r();
+        if (s instanceof Square q) return q.side() * q.side();
+        if (s instanceof Rect r) return r.w() * r.h();
+        throw new AssertionError("unreachable for a sealed Shape");
+    }
+    public static void main(String[] args) {
+        for (Shape s : new Shape[] { new Circle(1), new Square(2), new Rect(2, 3) })
+            System.out.printf("%-26s area = %.3f%n", s, area(s));
+        System.out.println("permitted       : " + java.util.Arrays.stream(Shape.class.getPermittedSubclasses()).map(Class::getSimpleName).toList());
+        System.out.println("isSealed        : " + Shape.class.isSealed());
+    }
+}

java17-features/src/SwitchExprDemo.java

@@ -0,0 +1,20 @@
+/** Switch expressions (final in 14, JEP 361): arrow form, no fall-through, yield, and exhaustiveness on enums. */
+public class SwitchExprDemo {
+    enum Day { MON, TUE, WED, THU, FRI, SAT, SUN }
+    public static void main(String[] args) {
+        for (Day d : new Day[] { Day.MON, Day.SAT, Day.WED }) {
+            int len = switch (d) {
+                case SAT, SUN -> 0;
+                case MON, TUE, WED, THU, FRI -> {
+                    int hours = d == Day.MON ? 9 : 8;
+                    yield hours;
+                }
+            };
+            System.out.println(d + " -> " + len + " working hours");
+        }
+        // old style: forgetting break falls through
+        int n = 1; StringBuilder sb = new StringBuilder();
+        switch (n) { case 1: sb.append("one "); case 2: sb.append("two "); break; case 3: sb.append("three"); }
+        System.out.println("old fall-through: " + sb.toString().trim());
+    }
+}

java17-features/src/TextBlocksDemo.java

@@ -0,0 +1,19 @@
+/** Text blocks (final in 15, JEP 378): indentation stripping, trailing-space and escape rules. */
+public class TextBlocksDemo {
+    public static void main(String[] args) {
+        String json = """
+            {
+              "name": "Ankur",
+              "city": "Mumbai"
+            }
+            """;
+        System.out.print(json);
+        System.out.println("lines           : " + json.lines().count());
+        String oneLine = """
+            joined \
+            on one line""";
+        System.out.println("backslash-newline: " + oneLine);
+        System.out.println("formatted       : " + """
+            Hello, %s!""".formatted("reader"));
+    }
+}