列表到TreeSet的转换产生：“ java.lang.ClassCastException：MyClass无法转换为java.lang.Comparable”

Question

List<MyClass> myclassList = (List<MyClass>) rs.get();

TreeSet<MyClass> myclassSet = new TreeSet<MyClass>(myclassList);

I don't understand why this code generates this:

java.lang.ClassCastException: MyClass cannot be cast to java.lang.Comparable

MyClass does not implement Comparable. I just want to use a Set to filter the unique elements of the List since my List contains unncessary duplicates.

Answer 1

Does MyClass implements Comparable<MyClass> or anything like that?

If not, then that's why.

For TreeSet , you either have to make the elements Comparable , or provide a Comparator . Otherwise TreeSet can't function since it wouldn't know how to order the elements.

Remember, TreeMap implements SortedSet , so it has to know how to order the elements one way or another.

You should familiarize yourself with how implementing Comparable defines natural ordering for objects of a given type.

The interface defines one method, compareTo , that must return a negative integer, zero, or a positive integer if this object is less than, equal to, or greater than the other object respectively.

The contract requires that:

• sgn(x.compareTo(y)) == -sgn(y.compareTo(x))
• it's transitive: x.compareTo(y)>0 && y.compareTo(z)>0 implies x.compareTo(z)>0
• x.compareTo(y)==0 implies that sgn(x.compareTo(z)) == sgn(y.compareTo(z)) for all z

Additionally, it recommends that:

• (x.compareTo(y)==0) == (x.equals(y)) , ie "consistent with equals

This may seem like much to digest at first, but really it's quite natural with how one defines total ordering.

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If your objects can not be ordered one way or another, then a TreeSet wouldn't make sense. You may want to use a HashSet instead, which have its own contracts. You are likely to be required to @Override hashCode() and equals(Object) as appropriate for your type (see: Overriding equals and hashCode in Java )

Answer 2

If you don't pass an explicit Comparator to a TreeSet , it will try to compare the objects (by assuming they are Comparable ). And if they aren't Comparable , it cannot compare them, so this exception is thrown!
TreeSets are sorted sets and require either objects to be Comparable or a Comparator to be passed in to determine how to sort the objects in the Set .

Answer 3

If you just want the set to remove duplicates, used a HashSet , although that will shuffle the order of the objects returned by the Iterator in ways that appear random.
But if you want to preserve the order somewhat, use LinkedHashSet , that will at least preserve the insertion order of the list.

TreeSet is only appropriate if you need the Set sorted, either by the Object's implementation of Comparable or by a custom Comparator passed to the TreeSet's constructor.