Hi
I am trying to understand basics of displaying data in visual force page.
Below is the code:
public class HttpExample3
{
   public string result{get;set;}
   public Double grand_total{get;set;}
   public InvoiceList inv{get;set;}
   
   public string callservice()
   {
     Http p = new Http();
     
     HttpRequest request = new HttpRequest();
     
     request.setEndPoint('https://docsample.herokuapp.com/jsonSample');
     
     request.setHeader('Content-type', 'application/json');      
     
     request.setMethod('GET');
     
     HttpResponse response=p.send(request);
     
     return response.getBody();
               
    }
    
    public void GetTotalPrice()
    {
      grand_total=InvoiceList.TotalPrice(callservice());
    }
 }

<apex:page controller="HttpExample3">
<apex:form>
<apex:pageBlock title="GetResponse">

<apex:pageblockSection>

<apex:pageblockSectionItem>
<apex:commandButton value="GetResponse"
                      action="{!callservice}"/>


</apex:pageblockSectionItem>
</apex:pageblockSection>

</apex:pageBlock>

<apex:pageBlock title="TotalPrice"> 

<apex:pageblockSection>

<apex:pageblockSectionItem>
<apex:commandButton value="GetTotalPrice"
                      action="{!GetTotalPrice}"/>

 </apex:pageblockSectionItem>

</apex:pageblockSection>
 {!grand_total}
</apex:pageBlock>

</apex:form>

</apex:page>

public class InvoiceList
{
        public static Double totalPrice{get;set;} 
        public DateTime statementDate {get;set;} 
        public List<LineItems> lineItems {get;set;} 
        public Integer invoiceNumber {get;set;} 
        public static double grand_total=0.0; 
                       
        public static double TotalPrice(string jsonString)
        {
          System.JSONParser jp=JSON.createParser(jsonString);
          
          while (jp.nextToken() != Null)
          {
             if ((jp.getCurrentToken() == JSONToken.FIELD_NAME) && jp.getText() == 'totalPrice')
             {
               jp.nextToken();
               grand_total+=jp.getDoubleValue();     
             }
          }
          
         return grand_total;
       }

       public class LineItems
       {
        public Double UnitPrice {get;set;} 
        public Double Quantity {get;set;} 
        public String ProductName {get;set;} 
       }
 }

I am making callservice method have return type because I need the response in other calculations.
code works fine when I click Get Total price button , but it fails when I click callservice button.
I get error as : he name can only contain underscores and alphanumeric characters. It must begin with a letter and be unique, and must not include spaces, end with an underscore, or contain two consecutive underscores. "

Please help me .

divya