code: protected final int nullCheck(int id, int s) {
          int k = stk;
          while (true) {
              k--;
              StackEntry e = stack[k];
  
              if (e.type == NULL_CHECK_START) {
                  if (e.getNullCheckNum() == id) {
                      return e.getNullCheckPStr() == s ? 1 : 0;
                  }
              }
          }
      }
docs: int for consistency with other null check routines