Hii need help 
i am getting error ..When trying to get the access token 
Here is my code 

error::whenc clicking button error=redirect_uri_mismatch&error_description=redirect_uri%20must%20match%20configuration

///Apex class public class OpportunityTriggerCreateOpp_HandlerClass {    
public static final string client_Id='3MVG9fe4g9fhX0E5D96.SbSaoF1aZrrAtrdkkQOCvZBwTXOX8o.xKXj_VnKMUVbC8_Rl1JBm_Fzfi6xw8W5nBNic';    
public static final string consumer_Secret='121B828EA2A3BA970CCFA6380002E95E06242299329B3C4EB69FF863BB63';    

public static String redirect_URI ='https://sourceorg--ap17--c.visualforce.com/apex/Opportunity_redirectURL';       
 public static PageReference dosubmit(){            
String authorization_endpoint = 'https://ap16-dev-ed.my.salesforce.com/services/oauth2/authorize';         String scope = '';        
String final_EndPoint =  authorization_endpoint+'?client_id='+client_Id+'&redirect_uri='+redirect_URI+'&response_type=code';          
PageReference pageRef = new PageReference(final_EndPoint);        
return pageRef;    
}    
public static void doFetchAccessToken(){            
String encodedString = EncodingUtil.base64Encode(Blob.valueOf(client_Id+':'+consumer_Secret));         String endPoint = 'https://ap16-dev-ed.my.salesforce.com/oauth2/v1/tokens/bearer';                 String oAuthCode = ApexPages.currentPage().getParameters().get('code');         String requestBody = 'grant_type=authorization_code&code='+oAuthCode+'&redirect_uri='+redirect_URI;       
 String errorMessage ='';            
    HttpRequest httpReq = new HttpRequest();         HttpResponse httpRes = new HttpResponse();        
Http http = new Http();        
​​​​​​ httpReq.setMethod('POST');        httpReq.setEndPoint(endPoint);         httpReq.setHeader('Authorization' , 'Basic '+encodedString);       
httpReq.setHeader('Content-Type' , 'application/json');         httpReq.setBody(requestBody);        
httpRes = http.send(httpReq);         system.debug(httpRes.getStatusCode());         system.debug(httpRes.getBody());  
  } }

////vf page
<apex:page Controller="OpportunityTriggerCreateOpp_HandlerClass"  >     <apex:form >        
         <apex:pageBlock >            
              <apex:commandButton action="{!dosubmit}" value="Do Submit"/>      
  </apex:pageBlock>    
</apex:form>
</apex:page>