diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/api/SequentialAddAll.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/api/SequentialAddAll.java
index d7db6796..0a92d558 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/api/SequentialAddAll.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/api/SequentialAddAll.java
@@ -62,7 +62,7 @@ static Optional<AddInvocation> of(CtStatement ctStatement) {
                 return Optional.empty();
             }
 
-            return Optional.of(new AddInvocation(collection, executableReference, ctInvocation.getArguments().get(0)));
+            return Optional.of(new AddInvocation(collection, executableReference, ctInvocation.getArguments().getFirst()));
         }
     }
 
@@ -91,7 +91,7 @@ private void reportProblem(CtVariableReference<?> ctVariable, List<? extends CtE
 
         if (ctEnum != null && UseEnumValues.checkEnumValues(ctEnum, TypeUtil.isOrderedCollection(ctVariable.getType()), fieldReads)) {
             addLocalProblem(
-                values.get(0).getParent(CtStatement.class),
+                values.getFirst().getParent(CtStatement.class),
                 new LocalizedMessage(
                     "common-reimplementation",
                     Map.of(
@@ -138,12 +138,12 @@ private void reportProblem(CtVariableReference<?> ctVariable, List<? extends CtE
         }
 
         addLocalProblem(
-            values.get(0).getParent(CtStatement.class),
+            values.getFirst().getParent(CtStatement.class),
             new LocalizedMessage(
                 "common-reimplementation",
                 Map.of(
                     "suggestion", "private static final List<%s> SOME_GOOD_NAME = List.of(%s); /* ... */ %s.addAll(SOME_GOOD_NAME)".formatted(
-                        ExpressionUtil.getExpressionType(values.get(0)),
+                        ExpressionUtil.getExpressionType(values.getFirst()),
                         values.stream()
                             .map(CtElement::toString)
                             .collect(Collectors.joining(", ")),
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/comment/UnnecessaryComment.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/comment/UnnecessaryComment.java
index 27e74d7c..7be14144 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/comment/UnnecessaryComment.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/comment/UnnecessaryComment.java
@@ -42,11 +42,7 @@ private void checkComments(Collection<? extends CtComment> comments) {
     }
 
     private static boolean isStandaloneComment(CtComment ctComment) {
-        var parent = ctComment.getParent();
-        if (parent == null) {
-            return false;
-        }
-        return !parent.getComments().contains(ctComment);
+        return ctComment.getParent() instanceof CtElement ctElement && !ctElement.getComments().contains(ctComment);
     }
 
     @Override
@@ -75,7 +71,7 @@ public void process(CtElement element) {
                         .map(CtComment.class::cast)
                         .collect(Collectors.toCollection(ArrayList::new));
 
-                    followingComments.add(0, ctComment);
+                    followingComments.addFirst(ctComment);
 
                     checkComments(followingComments);
                     return;
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/LeakedCollectionCheck.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/LeakedCollectionCheck.java
index c23265df..8f748cda 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/LeakedCollectionCheck.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/oop/LeakedCollectionCheck.java
@@ -241,9 +241,7 @@ private static List<CtExpression<?>> findPreviousAssignee(CtVariableRead<?> ctVa
 
         boolean foundPreviousAssignment = false;
         CtStatement currentStatement = ctVariableRead.getParent(CtStatement.class);
-        var reversedStatements = new ArrayList<>(StatementUtil.getEffectiveStatements(ctExecutable.getBody()));
-        Collections.reverse(reversedStatements);
-        for (CtStatement ctStatement : reversedStatements) {
+        for (CtStatement ctStatement : StatementUtil.getEffectiveStatements(ctExecutable.getBody()).reversed()) {
             if (!foundPreviousAssignment) {
                 if (ctStatement == currentStatement) {
                     foundPreviousAssignment = true;
@@ -282,7 +280,7 @@ && isParameterOf(ctVariableDeclaration, ctExecutable)) {
             List<CtExpression<?>> previousAssignees = findPreviousAssignee(ctVariableRead);
 
             if (!previousAssignees.isEmpty()) {
-                return findParameterReference(previousAssignees.get(0), ctExecutable);
+                return findParameterReference(previousAssignees.getFirst(), ctExecutable);
             }
 
             return Option.some((CtParameter<?>) ctVariableDeclaration);
@@ -445,11 +443,13 @@ private <T> void checkCtType(CtType<T> ctType) {
                         ctTypeMember = fixRecordAccessor(ctRecord, ctMethod);
                     }
 
-                    if (ctTypeMember instanceof CtConstructor<?> ctConstructor) {
-                        checkCtExecutableAssign(ctConstructor);
-                    } else if (ctTypeMember instanceof CtMethod<?> ctMethod) {
-                        checkCtExecutableReturn(ctMethod);
-                        checkCtExecutableAssign(ctMethod);
+                    switch (ctTypeMember) {
+                        case CtConstructor<?> ctConstructor -> checkCtExecutableAssign(ctConstructor);
+                        case CtMethod<?> ctMethod -> {
+                            checkCtExecutableReturn(ctMethod);
+                            checkCtExecutableAssign(ctMethod);
+                        }
+                        default -> {}
                     }
                 }
             }
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/check/utils/Option.java b/autograder-core/src/main/java/de/firemage/autograder/core/check/utils/Option.java
index ba0598cf..edb044bc 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/check/utils/Option.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/check/utils/Option.java
@@ -33,8 +33,8 @@ default boolean isSome() {
     }
 
     default <U> Option<U> map(Function<T, U> function) {
-	    if (this instanceof Some<T> someValue) {
-            return new Some<>(function.apply(someValue.value));
+        if (this instanceof Some<T>(T value)) {
+            return new Some<>(function.apply(value));
         } else if (this instanceof None<T>) {
             return new None<>();
         }
@@ -56,8 +56,8 @@ default T nullable() {
     }
 
     default Stream<T> stream() {
-        if (this instanceof Some<T> someValue) {
-            return Stream.of(someValue.value);
+        if (this instanceof Some<T>(T value)) {
+            return Stream.of(value);
         } else if (this instanceof None<T>) {
             return Stream.empty();
         }
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/ExpressionUtil.java b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/ExpressionUtil.java
index ef3784ee..bda24efa 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/ExpressionUtil.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/ExpressionUtil.java
@@ -10,7 +10,6 @@
 import spoon.reflect.code.CtBinaryOperator;
 import spoon.reflect.code.CtConditional;
 import spoon.reflect.code.CtExpression;
-import spoon.reflect.code.CtFieldRead;
 import spoon.reflect.code.CtInvocation;
 import spoon.reflect.code.CtLiteral;
 import spoon.reflect.code.CtTextBlock;
@@ -20,8 +19,6 @@
 import spoon.reflect.code.LiteralBase;
 import spoon.reflect.code.UnaryOperatorKind;
 import spoon.reflect.declaration.CtElement;
-import spoon.reflect.declaration.CtEnum;
-import spoon.reflect.declaration.CtEnumValue;
 import spoon.reflect.declaration.CtTypedElement;
 import spoon.reflect.declaration.CtVariable;
 import spoon.reflect.eval.PartialEvaluator;
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/TypeUtil.java b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/TypeUtil.java
index 56557f0d..3129d3ad 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/TypeUtil.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/TypeUtil.java
@@ -137,6 +137,10 @@ public static boolean isTypeEqualTo(CtTypeReference<?> ctType, CtTypeReference<?
     public static boolean isSubtypeOf(CtTypeReference<?> ctTypeReference, Class<?> expected) {
         CtType<?> expectedType = ctTypeReference.getFactory().Type().get(expected);
 
+        return isSubtypeOf(ctTypeReference, expectedType);
+    }
+
+    public static boolean isSubtypeOf(CtTypeReference<?> ctTypeReference, CtType<?> expectedType) {
         if (ctTypeReference.getTypeDeclaration() == null || ctTypeReference instanceof CtTypeParameterReference) {
             return ctTypeReference.isSubtypeOf(expectedType.getReference());
         }
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/UsesFinder.java b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/UsesFinder.java
index 3be22dbb..32a83697 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/UsesFinder.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/UsesFinder.java
@@ -38,6 +38,7 @@
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.SequencedSet;
 import java.util.Set;
 import java.util.stream.Stream;
 
@@ -78,16 +79,13 @@ private static UsesFinder getFor(FactoryAccessor factoryAccessor) {
      */
     @SuppressWarnings("rawtypes")
     public static CtElementStream<CtElement> getAllUses(CtNamedElement element) {
-        if (element instanceof CtVariable variable) {
-            return UsesFinder.variableUses(variable).asUntypedStream();
-        } else if (element instanceof CtTypeParameter typeParameter) {
-            return UsesFinder.typeParameterUses(typeParameter).asUntypedStream();
-        } else if (element instanceof CtExecutable executable) {
-            return UsesFinder.executableUses(executable).asUntypedStream();
-        } else if (element instanceof CtType type) {
-            return UsesFinder.typeUses(type).asUntypedStream();
-        }
-        throw new IllegalArgumentException("Unsupported element: " + element.getClass().getName());
+        return switch (element) {
+            case CtVariable variable -> UsesFinder.variableUses(variable).asUntypedStream();
+            case CtTypeParameter typeParameter -> UsesFinder.typeParameterUses(typeParameter).asUntypedStream();
+            case CtExecutable executable -> UsesFinder.executableUses(executable).asUntypedStream();
+            case CtType type -> UsesFinder.typeUses(type).asUntypedStream();
+            default -> throw new IllegalArgumentException("Unsupported element: " + element.getClass().getName());
+        };
     }
 
     public static CtElementStream<CtVariableAccess<?>> variableUses(CtVariable<?> variable) {
@@ -183,7 +181,10 @@ public static boolean isAccessingVariable(CtVariable<?> ctVariable, CtVariableAc
     }
 
     public static CtVariable<?> getDeclaredVariable(CtVariableAccess<?> ctVariableAccess) {
-        return UsesFinder.getFor(ctVariableAccess).scanner.variableAccessDeclarations.getOrDefault(ctVariableAccess, null);
+        return UsesFinder.getFor(ctVariableAccess).scanner.variableAccessDeclarations.getOrDefault(
+            ctVariableAccess,
+            VariableUtil.getVariableDeclaration(ctVariableAccess.getVariable())
+        );
     }
     /**
      * The scanner searches for uses of supported code elements in a single pass over the entire model.
@@ -198,7 +199,7 @@ private static class UsesScanner extends CtScanner {
         private final Map<CtTypeParameter, List<CtTypeParameterReference>> typeParameterUses = new IdentityHashMap<>();
         private final Map<CtExecutable, List<CtElement>> executableUses = new IdentityHashMap<>();
         private final Map<CtType, List<CtTypeReference>> typeUses = new IdentityHashMap<>();
-        private final Map<CtType, Set<CtType>> subtypes = new IdentityHashMap<>();
+        private final Map<CtType, SequencedSet<CtType>> subtypes = new IdentityHashMap<>();
 
         // Caches the current instanceof pattern variables, since Spoon doesn't track them yet
         // We are conservative: A pattern introduces a variable until the end of the current block
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralElement.java b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralElement.java
index 3d066483..4d8dc244 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralElement.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralElement.java
@@ -12,11 +12,11 @@ public boolean equals(Object otherObject) {
         if (this == otherObject) {
             return true;
         }
-        if (!(otherObject instanceof StructuralElement<?> otherStructuralElement)) {
+        if (!(otherObject instanceof StructuralElement<?>(var otherElement))) {
             return false;
         }
 
-        return StructuralEqualsVisitor.equals(this.element, otherStructuralElement.element);
+        return StructuralEqualsVisitor.equals(this.element, otherElement);
     }
 
     @Override
diff --git a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralEqualsVisitor.java b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralEqualsVisitor.java
index f57f65d3..9de6e57a 100644
--- a/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralEqualsVisitor.java
+++ b/autograder-core/src/main/java/de/firemage/autograder/core/integrated/structure/StructuralEqualsVisitor.java
@@ -15,6 +15,7 @@
 import spoon.support.visitor.equals.EqualsVisitor;
 
 import java.util.LinkedHashSet;
+import java.util.SequencedSet;
 import java.util.Set;
 
 public final class StructuralEqualsVisitor extends EqualsVisitor {
@@ -25,7 +26,7 @@ public final class StructuralEqualsVisitor extends EqualsVisitor {
         CtRole.COMMENT, CtRole.COMMENT_CONTENT, CtRole.COMMENT_TAG, CtRole.COMMENT_TYPE
     );
 
-    private final Set<Difference> differences;
+    private final SequencedSet<Difference> differences;
 
     public record Difference(CtRole role, Object left, Object right) {}
 
@@ -117,7 +118,7 @@ protected boolean fail(CtRole role, Object element, Object other) {
      *
      * @return the differences
      */
-    public Set<Difference> differences() {
+    public SequencedSet<Difference> differences() {
         return new LinkedHashSet<>(this.differences);
     }
 }
diff --git a/pom.xml b/pom.xml
index 72fa78be..b82893b4 100644
--- a/pom.xml
+++ b/pom.xml
@@ -38,7 +38,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 
-        <java.version>17</java.version>
+        <java.version>21</java.version>
         <maven.compiler.source>${java.version}</maven.compiler.source>
         <maven.compiler.target>${java.version}</maven.compiler.target>
         <maven.compiler.release>${java.version}</maven.compiler.release>