How to use JUnit and Mockito to mock inner logic

I'm king of new to Junit and Mockito and trying to understand how to mock parts of logic of a class.

I have a method which does a http post and get call, but it might be a DB lookup. How do I mock the response from the HTTP call in this method

  BufferedReader in = new BufferedReader(new InputStreamReader(httpsConnection.getInputStream()));
String inputLine;
StringBuilder response = new StringBuilder();

while ((inputLine = in.readLine()) != null) {
    response.append(inputLine);
}

I mean there must a way to test my logic without doing to real http call or db lookup or whatever.

// HTTP GET request
public String sendGet(URL url) throws Exception {

    s_logger.debug("URL: " + url);
    HttpsURLConnection httpsConnection = (HttpsURLConnection) url.openConnection();

    // Add request header
    httpsConnection.setRequestMethod("GET");
    httpsConnection.setRequestProperty("Content-Type", "application/xml");

    if (sessionId != null) {
        httpsConnection.setRequestProperty("sessionId", sessionId);
    } else {
        httpsConnection.setRequestProperty("username", ACCOUNT_USERNAME);
        httpsConnection.setRequestProperty("password", ACCOUNT_PASSWORD);
    }

    httpsConnection.setInstanceFollowRedirects(false);
    httpsConnection.setUseCaches(false);

    // Print Request Header
    s_logger.debug("Request Header: " + httpsConnection.getRequestProperties());

    int responseCode = httpsConnection.getResponseCode();
    s_logger.debug("Response Code : " + responseCode);

    Map<String, List<String>> headerFields = httpsConnection.getHeaderFields();
    s_logger.debug("Response Header: " + headerFields);

    BufferedReader in = new BufferedReader(new InputStreamReader(httpsConnection.getInputStream()));
    String inputLine;
    StringBuilder response = new StringBuilder();

    while ((inputLine = in.readLine()) != null) {
        response.append(inputLine);
    }
    in.close();

    // Set Session ID
    final String SESSION_KEY = "sessionId";
    if (this.sessionId == null && httpsConnection.getHeaderFields().containsKey(SESSION_KEY)) {
        this.sessionId = httpsConnection.getHeaderFields().get(SESSION_KEY).get(0);
        s_logger.debug("SESSION ID : " + this.sessionId);
    }

    // print result
    s_logger.debug("RESPONSE: " + response.toString());
    return response.toString();
}

Thanks for any help or suggestion.