LineBlobClientImpl implements LineBlobClient { @Override public CompletableFuture<MessageContentResponse> getMessageContent(final String messageId) { return toMessageContentResponseFuture(retrofitImpl.getMessageContent(messageId)); }  @Override CompletableFuture<MessageContentResponse> getMessageContent(final String messageId); @Override CompletableFuture<MessageContentResponse> getRichMenuImage(final String richMenuId); @Override CompletableFuture<BotApiResponse> setRichMenuImage(
            final String richMenuId, final String contentType, final byte[] content);  }
@Test public void getMessageContentTest() throws Exception { whenCall(retrofitMock.getMessageContent(any()), ResponseBody.create(MediaType.parse("image/jpeg"), ZERO_BYTES)); final MessageContentResponse contentResponse = target.getMessageContent("ID").get(); verify(retrofitMock, only()).getMessageContent("ID"); assertThat(contentResponse.getLength()).isEqualTo(0); assertThat(contentResponse.getMimeType()).isEqualTo("image/jpeg"); }