Pattern Matching in Java

Article

The code refers to the following articles:

• English: Pattern Matching in Java: instanceof, switch, and Record Patterns
• German: Pattern Matching in Java: instanceof, switch und Record Patterns

Contents

Every listing of the article is a compact source file of its own and prints the output shown there. The files need Java 25 or newer – compact source files and the IO class are final since Java 25 – even though the features they show are older. Run one directly from the repository root:

java src/main/java/Intro.java

mvn compile compiles all of them at once.

The folders, in the order of the article:

• src/main/java/ – the runnable listings, e.g. InstanceofPattern (Pattern Matching for instanceof), SwitchTypePatterns and SwitchCaseNull (Pattern Matching for switch), ExhaustiveSealed (a switch over a sealed interface without default), NestedRecordPattern and RecordPatternNull (record patterns), and Expressions (an expression tree of records with eval(), format() and simplify())
• does-not-compile/ – the listings the article shows with a compiler error; the first line of each file names the error. Compile one with javac -d /tmp does-not-compile/ScopeWithOr.java to see it. The messages in the article come from javac 27; up to Java 26, the "missing patterns" lines are not printed.
• preview/ – primitive type patterns, a preview feature in Java 23 to 27: java --enable-preview preview/PrimitiveTypePatterns.java

Two files end with an exception on purpose, because the article shows it: InstanceofWrongCast with a ClassCastException (the test checks for Number, the cast assumes Integer), and SwitchWithoutCaseNull with a NullPointerException (a switch without case null receives null).

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