Hi Roman; Somewhat unfortunately, "just return null" is what the default and all conforming implementations do of compute do when presented with a Map containing a mapping to null and a mapping function returning null. The specification of the new Java 8 Map methods does not always handle maps containing null values as clean as might be wished. This is mostly to maintain consistent semantics with some pre-java 8 ConcurrentHashMap operations. The chain of debatable decisions all starts with "putIfAbsent()" and then tries to proceed consistently to the other new default operations. In general a null-value mapping in a Map is treated as absent--the same way as CHM would treat a get() returning null. Every attempt I made at improving the behaviour of mapping to null values ended up being weirder and more mysterious than what Java 8 shipped with.
Sorry. Mike On Jul 20 2014, at 15:25 , David Holmes <david.hol...@oracle.com> wrote: > See: > > http://mail.openjdk.java.net/pipermail/core-libs-dev/2013-June/018251.html > > and related bug report. > > David > > On 21/07/2014 6:01 AM, Paul Sandoz wrote: >> Begin forwarded message: >>> -------- Пересылаемое сообщение-------- >>> 17.07.2014, 19:20, "Roman Leventov" <leven...@ya.ru>: >>> >>> Map.compute() Javadoc has the following paragraph: >>> -------- >>> The default implementation is equivalent to performing the following steps >>> for this map, then returning the current value or null if absent: >>> >>> V oldValue = map.get(key); >>> V newValue = remappingFunction.apply(key, oldValue); >>> if (oldValue != null ) { >>> if (newValue != null) >>> map.put(key, newValue); >>> else >>> map.remove(key); >>> } else { >>> if (newValue != null) >>> map.put(key, newValue); >>> else >>> return null; >>> } >>> -------- >>> >>> But this code don't correspond neither verbal description of the behaviour >>> nor the actual default implementation in java.util.Map. If the oldValue is >>> null and newValue is null, map should still remove the key, not to just >>> `return null;` >>> >>
