为什么TreeSet抛出ClassCastException？ [英] Why does TreeSet throw a ClassCastException?

问题描述

我正在尝试将两个 Employee对象添加到TreeSet中：

I am trying to add two 'Employee' objects to a TreeSet:

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

但是它抛出ClassCastException：

But it throws a 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)

但是如果我仅向TreeSet添加一个对象：

But if I add only one object to the TreeSet:

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

或者如果我改用HashSet：

Or if I use a HashSet instead:

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

然后成功。

推荐答案

任一 Employee 必须实现 可比较 ，或者您需要在创建 时提供比较器 TreeSet 。

Either Employee has to implement Comparable, or you need to provide a comparator when creating the TreeSet.

这在 SortedSet ：

插入排序集中的所有元素必须实现 Comparable 接口（或被指定的比较器接受）。此外，所有这些元素必须相互可比较： e1.compareTo（e2）（或 comparator.compare（e1，e2））不得为任何元素 e1 和 e2 ClassCastException $ c>在排序集中。尝试违反此限制将导致违规方法或构造函数调用引发 ClassCastException 。

All elements inserted into a sorted set must implement the Comparable interface (or be accepted by the specified comparator). Furthermore, all such elements must be mutually comparable: e1.compareTo(e2) (or comparator.compare(e1, e2)) must not throw a ClassCastException for any elements e1 and e2 in the sorted set. Attempts to violate this restriction will cause the offending method or constructor invocation to throw a ClassCastException.

如果您不满足这些要求，则排序后的集合将不知道如何比较其元素并且将无法运行。

If you don't fulfil these requirements, the sorted set won't know how to compare its elements and won't be able to function.

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