javademos/docs/05-primitive-patterns.md

5. Primitive types in patterns (JEP 532, fifth preview)

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Compile and run with --enable-preview --release 27. Source: PrimitivePatterns, transcript 23.

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.

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).

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; source in Dominated.java.

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