code: public Observable<Page<KeyItem>> getKeysNextAsync(final String nextPageLink) {
          return getKeysNextWithServiceResponseAsync(nextPageLink)
              .map(new Func1<ServiceResponse<Page<KeyItem>>, Page<KeyItem>>() {
                  @Override
                  public Page<KeyItem> call(ServiceResponse<Page<KeyItem>> response) {
                      return response.body();
                  }
              });
      }
docs: List keys in the specified vault.
  Retrieves a list of the keys in the Key Vault as JSON Web Key structures that contain the public part of a stored key. The LIST operation is applicable to all key types, however only the base key identifier, attributes, and tags are provided in the response. Individual versions of a key are not listed in the response. This operation requires the keys/list permission.
  
  @param nextPageLink The NextLink from the previous successful call to List operation.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the PagedList&lt;KeyItem&gt; object