Salesforce Authentication Failing

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I am trying to use OAuth authentication to get the Salesforce Authentication Token, so I referred wiki docs, but after getting authorization code, when I make a Post request with 5 required parameters, I’m getting following exception

{"error":"invalid_grant","error_description":"authentication failure"} CODE 400
JSON = {"error":"invalid_grant","error_description":"authentication failure"}

which is I guess a bad request.

PostMethod post = new PostMethod("https://login.salesforce.com/services/oauth2/token");
post.addParameter("code",##############);
post.addParameter("grant_type","authorization_code");
post.addParameter("redirect_uri","#################");  
post.addParameter("client_id",this.client_id);
post.addParameter("client_secret",this.client_secret);
httpclient.executeMethod(post);
String responseBody = post.getResponseBodyAsString();
System.out.println(responseBody+" CODE "+post.getStatusCode());

Kindly reply, if exception known?