code: @Override
      public void visitClassContext(ClassContext classContext) {
          try {
              JavaClass cls = classContext.getJavaClass();
              if (cls.getMajor() >= Const.MAJOR_1_5) {
                  stack = new OpcodeStack();
                  checkedFields = new HashSet<>();
                  enumRegs = new HashMap<>();
                  enumFields = new HashMap<>();
                  super.visitClassContext(classContext);
              }
          } finally {
              stack = null;
              checkedFields = null;
              enumRegs = null;
              enumFields = null;
          }
      }
docs: implements the visitor to check that the class is greater or equal than 1.5, and set and clear the stack
  
  @param classContext
  the context object for the currently parsed class