ignore complex format strings for the formatted lint

autograder-core/src/main/java/de/firemage/autograder/core/check/api/UseStringFormatted.java

@@ -37,6 +37,11 @@ private void checkCtInvocation(CtInvocation<?> ctInvocation) {
         }
 
         CtExpression<?> format = args.remove(0);
+        // skip if the format string is not a string literal (e.g. a complex concatenation)
+        if (SpoonUtil.tryGetStringLiteral(SpoonUtil.resolveConstant(format)).isEmpty()) {
+            return;
+        }
+
         String output = "%s.formatted(%s)".formatted(
             format,
             args.stream().map(CtExpression::toString).reduce((a, b) -> a + ", " + b).orElse("")

autograder-core/src/test/java/de/firemage/autograder/core/check/api/TestUseStringFormatted.java

@@ -0,0 +1,70 @@
+package de.firemage.autograder.core.check.api;
+
+import de.firemage.autograder.core.LinterException;
+import de.firemage.autograder.core.LocalizedMessage;
+import de.firemage.autograder.core.Problem;
+import de.firemage.autograder.core.ProblemType;
+import de.firemage.autograder.core.check.AbstractCheckTest;
+import de.firemage.autograder.core.compiler.JavaVersion;
+import de.firemage.autograder.core.file.StringSourceInfo;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class TestUseStringFormatted extends AbstractCheckTest {
+    private static final List<ProblemType> PROBLEM_TYPES = List.of(ProblemType.USE_STRING_FORMATTED);
+
+    void assertUseFormatString(Problem problem, String suggestion) {
+        assertEquals(
+            this.linter.translateMessage(
+                new LocalizedMessage(
+                    "use-string-formatted",
+                    Map.of("formatted", suggestion)
+                )
+            ),
+            this.linter.translateMessage(problem.getExplanation())
+        );
+    }
+
+    @Test
+    void testComplexFormat() throws LinterException, IOException {
+        ProblemIterator problems = this.checkIterator(StringSourceInfo.fromSourceString(
+            JavaVersion.JAVA_17,
+            "Test",
+            """
+                public class Test {
+                    private static final String INITIAL_FORMAT = "%s %";
+                    private static final String INDEX_FORMAT = "d: %";
+                    private static final int indexWidth = 2;
+                    private static final int maxInstructionWidth = 4;
+                    private static final int maxArgAWidth = 6;
+                    private static final String SECOND_FORMAT = "d: %";
+                    private static final int maxArgBWidth = 6;
+                    private static final String THIRD_FORMAT = "d: %s";
+
+                    public static void validateNumber(String a, int index, int second, String third) {
+                        String format = String.format(INITIAL_FORMAT
+                                + (indexWidth)
+                                + INDEX_FORMAT
+                                + (maxInstructionWidth)
+                                + (maxArgAWidth + 1)
+                                + SECOND_FORMAT
+                                + (maxArgBWidth)
+                                + THIRD_FORMAT,
+                            a,
+                            index,
+                            second,
+                            third
+                        );
+                    }
+                }
+                """
+        ), PROBLEM_TYPES);
+
+        problems.assertExhausted();
+    }
+}