How to determine if a List is sorted in Java?

java generics sorting wildcard
Eric Wilson picture Eric Wilson · Jun 15, 2010 · Viewed 85.3k times · Source

I would like a method that takes a List<T> where T implements Comparable and returns true or false depending on whether the list is sorted or not.

What is the best way to implement this in Java? It's obvious that generics and wildcards are meant to be able to handle such things easily, but I'm getting all tangled up.

It would also be nice to have an analogous method to check if the list is in reverse order.

Answer

ColinD picture ColinD · Jun 15, 2010

Guava provides this functionality through its awesome Ordering class. An Ordering is a Comparator++. In this case, if you have a list of some type that implements Comparable, you could write:

boolean sorted = Ordering.natural().isOrdered(list);

This works for any Iterable, not just List, and you can handle nulls easily by specifying whether they should come before or after any other non-null elements:

Ordering.natural().nullsLast().isOrdered(list);

Also, since you mentioned that you'd like to be able to check for reverse order as well as normal, that would be done as:

Ordering.natural().reverse().isOrdered(list);

Java 8 users: Use the equivalent Comparators#isInOrder(Iterable) instead, since the rest of Ordering is mostly obsolete (as explained in the class documentation).

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