Apex Callouts: Salesforce-to-Salesforce Integration

public with sharing class UEMSCallouts {

    private static String getAccessToken() {

        String clientId     = EncodingUtil.urlEncode(Label.UEMS_CLIENT_ID,'UTF-8');
        String clientSecret = EncodingUtil.urlEncode(Label.UEMS_CLIENT_SECRET,'UTF-8');
        String username     = EncodingUtil.urlEncode(Label.UEMS_USERNAME,'UTF-8');
        String password     = EncodingUtil.urlEncode(Label.UEMS_PASSWORD,'UTF-8');
        String reqbody      = 'grant_type=password&client_id='+clientId+'&client_secret='+clientSecret+'&username='+username+'&password='+password;
        String result       = '';

        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint(Label.UEMS_token_ENDPOINT);
        request.setMethod('POST');
        request.setHeader('Content-Type','application/x-www-form-urlencoded');
        request.setBody(reqbody);
        HttpResponse response = http.send(request);

        if (response.getStatusCode() == 200) {
            Map<String, Object> resMap = (Map<String, Object>)JSON.deserializeUntyped(response.getBody());
            result = (String)resMap.get('access_token');
        } else {
            CalloutException e = new CalloutException();
            e.setMessage('Error retrieving access token!' + response.getBody());
            throw e;
        }

        return result;
    }

    public static List<Map<String, String>> getLeadSources() {
        String accessToken = getAccessToken();
        List<Map<String, String>> result = new List<Map<String, String>>();

        if (accessToken == '') return result;

        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint(Label.UEMS_GetLeadSource_ENDPOINT);
        request.setMethod('GET');
        request.setHeader('Authorization', 'Bearer ' + accessToken);
        HttpResponse response = http.send(request);

        if (response.getStatusCode() == 200) {
            List<Object> resList = (List<Object>)JSON.deserializeUntyped(response.getBody());
            for (Object item : resList) {
                Map<String, Object> tempMap = (Map<String, Object>)item;
                Map<String, String> itemMap = new Map<String, String>();
                for (String key : tempMap.keySet())
                    itemMap.put(key, (String)tempMap.get(key));
                result.add(itemMap);
            }
        } else {
            CalloutException e = new CalloutException();
            e.setMessage('Error fetching lead sources!' + response.getBody());
            throw e;
        }

        return result;
    }

    public static void createNewLead(Map<String, String> record) {
        String accessToken = getAccessToken();

        if (accessToken == '') return;

        Map<String, List<Map<String, String>>> dataMap = new Map<String, List<Map<String, String>>>{
            'record' => new List<Map<String, String>>{record}
        };

        Http http = new Http();
        HttpRequest request = new HttpRequest();
        request.setEndpoint(Label.UEMS_CreateLead_ENDPOINT);
        request.setMethod('POST');
        request.setHeader('Authorization', 'Bearer ' + accessToken);
        request.setHeader('Content-Type', 'application/json;charset=UTF-8');
        request.setBody(JSON.serialize(dataMap));
        HttpResponse response = http.send(request);

        if (response.getStatusCode() != 200) {
            CalloutException e = new CalloutException();
            e.setMessage('Error creating lead!');
            throw e;
        }
    }
}