Effective Java

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ITEM 37: USE ENUMMAP INSTEAD OF ORDINAL INDEXING
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parameter form of 
Collectors.groupingBy
, which allows the caller to specify
the map implementation using the 
mapFactory
parameter:
// Using a stream and an EnumMap to associate data with an enum
System.out.println(Arrays.stream(garden)
.collect(groupingBy(p -> p.lifeCycle,
() -> new EnumMap<>(LifeCycle.class)
, toSet())));
This optimization would not be worth doing in a toy program like this one but
could be critical in a program that made heavy use of the map.
The behavior of the stream-based versions differs slightly from that of the
EmumMap
version. The 
EnumMap
version always makes a nested map for each plant
lifecycle, while the stream-based versions only make a nested map if the garden
contains one or more plants with that lifecycle. So, for example, if the garden con-
tains annuals and perennials but no biennials, the size of 
plantsByLifeCycle
will
be three in the 
EnumMap
version and two in both of the stream-based versions.
You may see an array of arrays indexed (twice!) by ordinals used to represent
a mapping from two enum values. For example, this program uses such an array to
map two phases to a phase transition (liquid to solid is freezing, liquid to gas is
boiling, and so forth):