Eclipse Collections Now Supports Indirect Sorting of Primitive Lists

Sometimes the best way to get results is to be indirect

Vladimir Zakharov
Jul 30 · 2 min read
Matryoshka dolls arranged by size
Matryoshka dolls arranged by size

I have recently implemented indirect sort functionality on primitive list types, which are some of many collection types supported by the Eclipse Collections project.

There was a sort functionality on the primitive collections before, the sortThis() method, which is a wrapper around the Java JDK Arrays.sort method. The limitation, which was carried over into the primitive list implementation, is that it could only sort an array by its elements and by the elements’ natural order, rather than by a function of its elements and/or some custom comparator.

Why would you want to sort by anything other than by the natural order of value of the element? Here are some possible reasons:

  • Reverse sorting
  • Sorting by absolute value
  • Building an index into an ordered data structure that is impossible or impractical to reorder (e.g., a tabular data set implemented as in in-memory columnar store)

This also supports the functionality described in an old open feature request for the JDK (see “

There are three new versions of the sortThis method: the regular one supporting comparator, one supporting the “by” pattern that takes an extractor function, and a combination of both. This fills the symmetry gap that existed between primitive and object collection APIs.

If the words ‘“by” pattern’ and ‘symmetry gap’ make little sense to you, please see these blogs by Donald Raab for explanation:

Here are the three new sort API methods on MutableIntList listed after the old sortThis() method:

Equivalent APIs have been introduced for all other mutable primitive list types with the exception of MutableBooleanList, which does not support sorting.

These are some examples of the usage of the new APIs:

The new sort methods are available starting with the release 10.3 of Eclipse Collections.


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