This page details the compatibility of different collections releases.

There are two types of compatibility discussed here, source and binary.

• Two versions are source compatible when application code can be compiled against either version successfully. The compilation may result in deprecation warnings but that is perfectly acceptable. When code is source incompatible, it fails to compile, thus this type of incompatibility is easy to find.
• Two versions are binary compatible when application code compiled against one version will run using the other version without recompilation. This is much more difficult to test for, and follows quite complicated rules from the Java language.

Releases of commons collections aim to be source and binary compatible within minor versions, and to avoid breakages as much as possible between major versions.

Commons collections 3.0 is source compatible with version 2.1 and 2.0 with the exception of classes previously deprecated in 2.0/2.1 (which were removed in 3.0).

Commons collections 3.0 is binary compatible with version 2.1 and 2.0 except for certain methods on one class. As the release was a major version, this is permitted, however it was unintentional and an error. The problem is in the IteratorUtils class (see methods below). (Strictly speaking, there are some minor incompatibilities on other classes, but these changes were deliberate and have not caused any known issues.)

It is not possible to make v2.1 and v3.0 compatible without futher binary incompatibility. The chosen solution is to provide a work around by releasing v2.1.1 and v3.1. The following deprecations must be resolved in v2.1.1 to allow compatibiilty with v3.1.

• Deprecated IteratorUtils.arrayIterator(...) - use new ArrayIterator(...) instead
• Deprecated IteratorUtils.singletonIterator(...) - use new SingletonIterator(...) instead
• Deprecated IteratorUtils.emptyIterator() - use EmptyIterator.INSTANCE instead
• Deprecated IteratorUtils.emptyListIterator() - use EmptyListIterator.INSTANCE instead
• Deprecated IteratorUtils.EMPTY_ITERATOR - use EmptyIterator.INSTANCE instead
• Deprecated IteratorUtils.EMPTY_LIST_ITERATOR - use EmptyIterator.INSTANCE instead

For the future, a new tool clirr is being developed to help spot binary incompatibility before release.