RetrieveMetadata.java

Map<String, List<String>> map = // some info about types and members of them you'd like to retrieve
RetrieveRequest retrieveRequest = createRetrieveRequest(map);
AsyncResult asyncResult = connection.retrieve(retrieveRequest);
RetrieveResult retrieveResult = waitForRetrieveCompletion(asyncResult, connection);
// Parse result...

private RetrieveRequest createRetrieveRequest(Map<String, List<String>> map) throws ConnectionException {    
    RetrieveRequest retrieveRequest = new RetrieveRequest();
    retrieveRequest.setUnpackaged(createPackage(map));
    return retrieveRequest;
}