code: @Override
      public void visitClassContext(ClassContext classContext) {
          try {
              JavaClass cls = classContext.getJavaClass();
              if (cls.getMajor() >= Const.MAJOR_1_4) {
                  stack = new OpcodeStack();
                  regValueType = new HashMap<Integer, State>();
                  super.visitClassContext(classContext);
              }
          } finally {
              stack = null;
              regValueType = null;
          }
      }
docs: implements the visitor to make sure the class is at least java 1.4 and to reset the opcode stack