public Map<String, String> getProperties(EObject eObject) {
     // ...
}

[query public getProperties(obj : OclAny) : EMap =
  invoke(
    '...service.CommonService',
    'getProperties(org.eclipse.emf.ecore.EObject)',
    Sequence{obj})/]

[query public keys(arg : EMap) : Set(String) =
  invoke('...service.CommonService',
    'getStringMapKeys(java.util.Map)',
    Sequence{arg})/]

[query public get(arg : EMap, key : String) : String =
  invoke('...service.CommonService',
    'getStringMapValue(java.util.Map, java.lang.String)',
    Sequence{arg, key})/]

[query public merge(arg : Sequence(EMap)) : EMap =
  invoke('...service.CommonService',
    'mergeStringMaps(java.util.List)',
    Sequence{arg})/]

    public Set<String> getStringMapKeys(Map<String, String> obj) {
        return obj.keySet();
    }

    public String getStringMapValue(Map<String, String> obj, String key) {
        return obj.get(key);
    }
    
    public Map<String, String> mergeStringMaps(List<Map<String, String>> objs) {
        Map<String, String> result = new HashMap<>();
        for (Map<String, String> obj : objs) {
            obj.forEach(result::putIfAbsent);
        }
        return result;
    }