Sorted collection in Java

Question

This comes very late, but there is a class in the JDK just for the purpose of having a sorted list. It is named (somewhat out of order with the other Sorted* interfaces) "java.util.PriorityQueue". It can sort either Comparable<?>s or using a Comparator.

The difference with a List sorted using Collections.sort(...) is that this will maintain a partial order at all times, with O(log(n)) insertion performance, by using a heap data structure, whereas inserting in a sorted ArrayList will be O(n) (i.e., using binary search and move).

However, unlike a List, PriorityQueue does not support indexed access (get(5)), the only way to access items in a heap is to take them out, one at a time (thus the name PriorityQueue).

TreeMap and TreeSet will give you an iteration over the contents in sorted order. Or you could use an ArrayList and use Collections.sort() to sort it. All those classes are in java.util

If you want to maintain a sorted list which you will frequently modify (i.e. a structure which, in addition to being sorted, allows duplicates and whose elements can be efficiently referenced by index), then use an ArrayList but when you need to insert an element, always use Collections.binarySearch() to determine the index at which you add a given element. The latter method tells you the index you need to insert at to keep your list in sorted order.

Use Google Guava's TreeMultiset class. Guava has a spectacular collections API.

One problem with providing an implementation of List that maintains sorted order is the promise made in the JavaDocs of the add() method.

You want the SortedSet implementations, namely TreeSet.

There are a few options. I'd suggest TreeSet if you don't want duplicates and the objects you're inserting are comparable.

You can also use the static methods of the Collections class to do this.

See Collections#sort(java.util.List) and TreeSet for more info.