Ever since the launch of ChatGPT in 2022, a rising concern is whether ChatGPT will replace programmers and kill jobs. Motivated by this widespread concern, we conducted an empirical study to systematically compare ChatGPT against programmers in question-answering and software-maintaining. We reused a dataset introduced by prior work, which includes 130 StackOverflow (SO) discussion threads referred to by the Java developers of 357 GitHub projects. We mainly investigated three research questions (RQs). First, how does ChatGPT compare with programmers when answering technical questions? Second, how do developers perceive the differences between ChatGPT's answers and SO answers? Third, how does ChatGPT compare with humans when revising code for maintenance requests? For RQ1, we provided the 130 SO questions to ChatGPT, and manually compared ChatGPT answers with the accepted/most popular SO answers in terms of relevance, readability, informativeness, comprehensiveness, and reusability. For RQ2, we conducted a user study with 30 developers, asking each developer to assess and compare 10 pairs of answers, without knowing the information source (i.e., ChatGPT or SO). For RQ3, we distilled 48 software maintenance tasks from 48 GitHub projects citing the studied SO threads. We queried ChatGPT to revise a given Java file, and to incorporate the code implementation for any prescribed maintenance requirement. Our study reveals interesting phenomena: For the majority of SO questions (97/130), ChatGPT provided better answers; in 203 of 300 ratings, developers preferred ChatGPT answers to SO answers; ChatGPT revised code correctly for 22 of the 48 tasks. Our research will expand people's knowledge of ChatGPT capabilities, and shed light on future adoption of ChatGPT by the software industry.