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异常消息提示重复键但显示值的原因是什么？
List<Employee> employees = new ArrayList<>();
employees.add(new Employee("John", 40));
employees.add(new Employee("John", 30));
Map<String, Integer> map = employees.stream()
.collect(Collectors.toMap(Employee::getName, Employee::getAge));
不是将“John”显示为重复键，而是显示“40”
Exception in thread "main" java.lang.IllegalStateException: Duplicate key 40
(...)