import java.io.Serializable; import java.util.EnumMap; import java.util.Map; import java.util.Set; public class ClassAccess implements Serializable { private static final long serialVersionUID = 2269507042140353173L; enum TestEnum {} // at one point javac generated a synthetic field for .class accesses with a name starting with the string "class$" Map<TestEnum, Set<String>> testField = new EnumMap<TestEnum, Set<String>>(TestEnum.class); // Map<TestEnum, Set<String>> testField = new HashMap<>(); }