code: @Override
      public void visitClassContext(ClassContext classContext) {
          try {
              currentClass = classContext.getJavaClass();
              stack = new OpcodeStack();
              returnTypes = new HashMap<>();
              super.visitClassContext(classContext);
          } finally {
              currentClass = null;
              stack = null;
              returnTypes = null;
          }
      }
docs: implements the visitor to create and destroy the stack and return types
  
  @param classContext
  the context object of the currently parsed class