code: public String getExpressionString() {
          StringBuffer buf = new StringBuffer();
          buf.append("(");
          if (mOperator != null) {
              buf.append(mOperator.getOperatorSymbol());
              buf.append(" ");
          } else {
              for (int i = 0; i < mOperators.size(); i++) {
                  UnaryOperator operator = (UnaryOperator) mOperators.get(i);
                  buf.append(operator.getOperatorSymbol());
                  buf.append(" ");
              }
          }
          buf.append(mExpression.getExpressionString());
          buf.append(")");
          return buf.toString();
      }
docs: Returns the expression in the expression language syntax