java: Declare the java toolchain in .tool-versions

java/.gitignore

@@ -0,0 +1,2 @@
+# Maven build output
+target/

java/.tool-versions

@@ -0,0 +1,2 @@
+java corretto-21
+maven 3

java/README.md

@@ -2,10 +2,30 @@
 
 Skeleton project for Java using Maven.
 
+## Toolchain
+
+The toolchain is declared in [`.tool-versions`](.tool-versions):
+
+```
+java corretto-21
+maven 3
+```
+
+Install it however you prefer — [mise](https://mise.jdx.dev) is the easiest route:
+
+```bash
+mise install
+```
+
+`./script/setup` does not install the toolchain; it expects the tools above to be on your `PATH`.
+
 ## Usage
 
 Feel free to run the main class and test suites directly through your IDE of choice. If you want to run from the terminal, you can use these commands:
 
-- `./script/setup` to attempt to install required packages (Java and Maven)
+- `./script/setup` to download dependencies
 - `./script/run` to run the main class
 - `./script/test` to run the test suite
+
+The example test passes out of the box. A good way to get your bearings is to break it
+deliberately, watch it fail, then put it back.

java/script/run

@@ -1,3 +1,7 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-mvn exec:java
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")/.."
+
+mvn --quiet exec:java

java/script/setup

@@ -1,9 +1,7 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-set -e
+set -euo pipefail
 
-if which brew >/dev/null 2>&1 ; then
-  brew install maven
-else
-  echo "You don't have brew installed, so we can't automatically install Java and Maven. Please install them yourself."
-fi
+cd "$(dirname "${BASH_SOURCE[0]}")/.."
+
+mvn --quiet dependency:resolve

java/script/test

@@ -1,3 +1,7 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-mvn test
+set -euo pipefail
+
+cd "$(dirname "${BASH_SOURCE[0]}")/.."
+
+mvn test