[Java] How to compare two string dates in Java?

Scenario

The format of string date is yyyyMMdd, ex. 20180806

How-To
Here has sample code:

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@PostMapping(value = "/{utterTemplateId}/utterances/save") public @ResponseBody List<Utterance> saveUtterance(@PathVariable Integer utterTemplateId, @RequestBody Utterance utterance, Alerter alerter) { Boolean hasStartDate = !Strings.isNullOrEmpty(utterance.getStartDate()); Boolean hasEndDate = !Strings.isNullOrEmpty(utterance.getEndDate()); if ((hasStartDate && !hasEndDate) || (!hasStartDate && hasEndDate)) { throw new RuntimeException("請輸入完整上下架日期"); } if (hasStartDate && hasEndDate) { String startDate = utterance.getStartDate(); String endDate = utterance.getEndDate(); DateFormat dateFormat = new SimpleDateFormat("yyyyMMdd"); try { if (dateFormat.parse(startDate).after(dateFormat.parse(endDate))) { throw new RuntimeException("下架日期不可早於上架日期"); } } catch (ParseException e) { throw new RuntimeException("上下架日期解析發生錯誤, 錯誤原因: " + e.getMessage(), e); } } // ignore some code }