Add build-changed task in Java workflow

.github/workflows/java.yml

@@ -11,8 +11,9 @@ on:
   workflow_dispatch:
 
 jobs:
-  build:
+  build-all:
     name: Check if tests compile cleanly with starter sources
+    if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd
@@ -25,6 +26,22 @@ jobs:
         run: ./gradlew compileStarterTestJava --continue
         working-directory: exercises
 
+  build-changed:
+    name: Check if changed exercise tests compile cleanly with starter sources
+    if: github.event_name == 'pull_request'
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd
+        with:
+          fetch-depth: 0
+      - name: Set up JDK 21
+        uses: actions/setup-java@be666c2fcd27ec809703dec50e508c2fdc7f6654
+        with:
+          java-version: 21
+          distribution: "temurin"
+      - name: Check if changed exercise tests compile cleanly
+        run: bin/build-changed-exercise
+
   lint:
     name: Lint Java files using Checkstyle
     runs-on: ubuntu-24.04

bin/build-changed-exercise

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+set -eo pipefail
+
+# Determine the base branch of the PR
+BASE_BRANCH=${GITHUB_BASE_REF:-main}
+
+# Fetch full history for proper diff
+git fetch origin "$BASE_BRANCH"
+
+# Compute merge base
+MERGE_BASE=$(git merge-base HEAD origin/"$BASE_BRANCH")
+
+# Get changed files relative to merge base
+changed_files=$(git diff --name-only "$MERGE_BASE" HEAD)
+
+# If any Gradle build file changed, run the full suite and exit
+if echo "$changed_files" | grep -qE '\.(gradle|gradlew|bat)$|settings\.gradle'; then
+  echo "Gradle build files changed, running full build suite..."
+  cd exercises && ./gradlew compileStarterTestJava --continue
+  exit 0
+fi
+
+# Extract unique exercise directories
+changed_exercises=$(echo "$changed_files" | \
+  grep -E '^exercises/(practice|concept)/[^/]+/.+\.java$' | \
+  cut -d/ -f1-3 | sort -u || true)
+
+if [ -z "$changed_exercises" ]; then
+  echo "No relevant exercises changed, skipping compile checks."
+  exit 0
+fi
+
+# Print exercises
+echo "Changed exercises detected:"
+echo "$changed_exercises"
+echo "----------------------------------------"
+
+# Run build compile checks
+exit_code=0
+for dir in $changed_exercises; do
+  slug=$(basename "$dir")
+
+  echo "========================================"
+  echo "=== Running compileStarterTestJava for $slug ==="
+  echo "========================================"
+
+  if [[ $dir == exercises/practice/* ]]; then
+    ./exercises/gradlew -p exercises ":practice:$slug:compileStarterTestJava" || exit_code=1
+  elif [[ $dir == exercises/concept/* ]]; then
+    ./exercises/gradlew -p exercises ":concept:$slug:compileStarterTestJava" || exit_code=1
+  fi
+done
+
+exit $exit_code