Java

Default: the current LTS JDK, Gradle with the wrapper (./gradlew) and Kotlin DSL, picocli for CLIs (Spring Boot when you need a long-running service), JUnit 5 + AssertJ, application plugin for a runnable distribution.

Setup

1. gradle init --type java-application --dsl kotlin, commit the wrapper (gradlew, gradle/wrapper/*).
2. Set a toolchain in build.gradle.kts so Gradle downloads or selects the right JDK regardless of the machine's default:

   java { toolchain { languageVersion = JavaLanguageVersion.of(21) } }
   

3. Enable -Xlint:all -Werror for compileJava.

Modelling with modern Java

• record for immutable data (record Payment(String payer, long amountMinor, Set<String> sharedBy)).
• sealed interface + switch pattern matching for closed sets of cases; the compiler checks exhaustiveness.
• long minor units or BigDecimal with an explicit RoundingMode for money; never double.

Example: settling shared expenses

1. Compute each person's balance = paid − owed.
2. Distribute rounding remainders deterministically (e.g. to the people with the largest fractional parts, ties by name) and test that balances sum to exactly zero.
3. Greedy matching of the largest debtor with the largest creditor gives at most n−1 transfers; that is good enough in practice (the true minimum is NP-hard in general).
4. Print transfers in a stable order so output is diffable.

CLI with picocli

• Annotate a Callable<Integer> with @Command(mixinStandardHelpOptions = true); return the exit code.
• Keep parsing in the command class and logic in plain classes.
• Package with ./gradlew installDist (scripts + jars) or a fat jar if a single file is required.

Testing

• JUnit 5 @ParameterizedTest for tables of inputs; property checks (e.g. with jqwik) for invariants like "balances sum to zero".
• ./gradlew test should not need network access after the first dependency download.

Verification loop

1. ./gradlew build (compiles with lint as errors, runs tests).
2. ./gradlew run --args="--help" and a real example.
3. Run the distribution from build/install/ on a machine with only a JRE.

Pitfalls

• equals on BigDecimal compares scale (2.0 ≠ 2.00); use compareTo for numeric equality.
• Integer caching: == on boxed integers works only for small values; always use equals or unbox.
• Default charset and locale: pass StandardCharsets.UTF_8 and Locale.ROOT for files and formatting.
• Gradle daemons and caches can hide configuration problems; reproduce CI failures with --no-daemon --no-build-cache.
• Records with mutable components (lists) are only shallowly immutable; copy with List.copyOf in the compact constructor.