Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
2026-09-21 15:08:50 +00:00
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+# 5. Primitive types in patterns (JEP 532, fifth preview)
+
+Prev: [4. Post-quantum TLS](04-post-quantum-tls.md) &middot; Next: [6. Lazy constants](06-lazy-constants.md)
+
+Compile and run with `--enable-preview --release 27`. Source: [`PrimitivePatterns`](../jep-tour/src/PrimitivePatterns.java), transcript [23](output/23-primitive-patterns.txt).
+
+## The idea in one sentence
+
+A pattern may now name a primitive type, and the test is **"can this value be converted to that type without losing information?"**. It is an exactness test, not a cast.
+
+```java
+int small = 100, big = 300;
+small instanceof byte   // true  : 100 fits
+big   instanceof byte   // false : a byte holds -128..127
+16_777_216 instanceof float  // true
+16_777_217 instanceof float  // false : a float has a 24-bit significand
+```
+
+You can also `switch` on a `long`, `float`, `double` or `boolean`, use `case int` against an `Integer`, and put a narrowing primitive in a record pattern:
+`Reading(byte c)` matches `Reading[celsius=36]` but not `Reading[celsius=4000]` ([23](output/23-primitive-patterns.txt)).
+
+## The compile error you will meet
+
+Dominance rules apply. Put `case Integer i` before `case int primitive` and the second label can never match:
+
+```
+error: this case label is dominated by a preceding case label
+```
+
+The real text is in [20-broken-dominated.txt](output/20-broken-dominated.txt); source in [`Dominated.java`](../jep-tour/src/broken/Dominated.java).
+
+Prev: [4. Post-quantum TLS](04-post-quantum-tls.md) &middot; Next: [6. Lazy constants](06-lazy-constants.md)