“…To address this problem, unsupervised domain adaptation methods can be applied. While existing methods focus on traditional cross-domain sentiment classification to learn shared representations for sentences or documents, including pivot-based methods (Blitzer et al, 2007;Pan et al, 2010;Bollegala et al, 2013;Yu and Jiang, 2016), auto-encoders (Glorot et al, 2011;Chen et al, 2012;Zhou et al, 2016), domain adversarial networks (Ganin et al, 2016;Li et al, , 2018c, or semi-supervised methods (He et al, 2018a). Due to the difficulties in fine-grained adaptation, there exist very few methods for cross-domain aspect extraction Ding et al, 2017), which acts as a sub-task of E2E-ABSA, or aspect and opinion co-extraction (Li et al, 2012;Wang and Pan, 2018) that focuses on detecting aspect and opinion words, while E2E-ABSA needs to analyze more complicated correspondences between them.…”