8336032: Enforce immutability of Lists used by ClassFile API

Reviewed-by: asotona

src/java.base/share/classes/java/lang/classfile/AttributedElement.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2022, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,6 +25,7 @@
 package java.lang.classfile;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Optional;
 
@@ -83,6 +84,6 @@ public sealed interface AttributedElement extends ClassFileElement
                 list.add(t);
             }
         }
-        return list;
+        return Collections.unmodifiableList(list);
     }
 }

src/java.base/share/classes/java/lang/classfile/ClassSignature.java

@@ -76,7 +76,7 @@ public sealed interface ClassSignature
                                     Signature.ClassTypeSig superclassSignature,
                                     Signature.ClassTypeSig... superinterfaceSignatures) {
         return new SignaturesImpl.ClassSignatureImpl(
-                requireNonNull(typeParameters),
+                List.copyOf(requireNonNull(typeParameters)),
                 requireNonNull(superclassSignature),
                 List.of(superinterfaceSignatures));
     }

src/java.base/share/classes/java/lang/classfile/CompoundElement.java

@@ -25,6 +25,7 @@
 package java.lang.classfile;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Spliterator;
@@ -90,7 +91,7 @@ public sealed interface CompoundElement<E extends ClassFileElement>
                 list.add(e);
             }
         });
-        return list;
+        return Collections.unmodifiableList(list);
     }
 
 }