Java – why does TreeSet throw ClassCastException

In the following code, I try to add two employee objects

Set<Employee> s = new TreeSet<Employee>();
s.add(new Employee(1001));
s.add(new Employee(1002));

But the result is Java lang.ClassCastException：

Exception in thread "main" java.lang.ClassCastException: Employee cannot be cast to java.lang.Comparable
    at java.util.TreeMap.put(TreeMap.java:542)
    at java.util.TreeSet.add(TreeSet.java:238)
    at MyClient.main(MyClient.java:9)

But if I change

Set<Employee> s = new TreeSet<Employee>();
s.add(new Employee(1001));

or

Set<Employee> s = new HashSet<Employee>();
s.add(new Employee(1001));
s.add(new Employee(1002));

Then success is no exception I didn't do any class conversion in the above code Please help me find out the reason and put forward my solution

Solution

Employees must implement comparable, or provide a comparator when creating TreeSet

This is described in detail in the sortedset documentation:

If you do not meet these requirements, the sort set will not know how to compare its elements and will not work properly