asmhatreandClaude Sonnet 5
f59c1de96d
Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3
33 lines
1.5 KiB
Markdown
33 lines
1.5 KiB
Markdown
# 5. Primitive types in patterns (JEP 532, fifth preview)
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Prev: [4. Post-quantum TLS](04-post-quantum-tls.md) · Next: [6. Lazy constants](06-lazy-constants.md)
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Compile and run with `--enable-preview --release 27`. Source: [`PrimitivePatterns`](../jep-tour/src/PrimitivePatterns.java), transcript [23](output/23-primitive-patterns.txt).
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## The idea in one sentence
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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.
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```java
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int small = 100, big = 300;
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small instanceof byte // true : 100 fits
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big instanceof byte // false : a byte holds -128..127
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16_777_216 instanceof float // true
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16_777_217 instanceof float // false : a float has a 24-bit significand
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```
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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:
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`Reading(byte c)` matches `Reading[celsius=36]` but not `Reading[celsius=4000]` ([23](output/23-primitive-patterns.txt)).
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## The compile error you will meet
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Dominance rules apply. Put `case Integer i` before `case int primitive` and the second label can never match:
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```
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error: this case label is dominated by a preceding case label
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```
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The real text is in [20-broken-dominated.txt](output/20-broken-dominated.txt); source in [`Dominated.java`](../jep-tour/src/broken/Dominated.java).
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Prev: [4. Post-quantum TLS](04-post-quantum-tls.md) · Next: [6. Lazy constants](06-lazy-constants.md)
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