Why does Collections.sort() apply only for lists a

问题:

Why does Collections.sort() apply only for Lists and not for Sets? Is there any particular reason?

回答1:

Most (but not all) Set implementations do not have a concept of order, so Collections.sort does not support them as a whole. If you want a set with a concept of order, you can use something like a TreeSet:

A NavigableSet implementation based on a TreeMap. The elements are ordered using their natural ordering, or by a Comparator provided at set creation time, depending on which constructor is used.

Or a LinkedHashSet:

Hash table and linked list implementation of the Set interface, with predictable iteration order. This implementation differs from HashSet in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which elements were inserted into the set (insertion-order)

回答2:

A Set, by definition, has no order.

回答3:

A Set is not a List. While a List, by contract, is supposed to retain insertion order (otherwise, methods such as .get(someindex) would not make any sense), this is not the case for a Set. You have no method to get an element at a particular index in a Set! Neither do you have methods to insert at a particular position etc.

More specifically, the ordering of Set is undefined; however, implementations of Set can add ordering constraints.

For instance:

• LinkedHashSet retains insertion ordering;
• TreeSet maintains natural ordering of its elements, either because its elements implement Comparable, or because you supply a Comparator.

If you sorted a LinkedHashSet, you would break its insertion ordering guarantee!

回答4:

A set is not ordered. You can use SortedSet. Or you can create a List from the set and sort it.

回答5:

List is an ordered set of elements while Set is not which implies that none of Set elements will have any sequence number. So you can't sort it.