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collectMap

As explained in Spring in Action, Sixth Edition

collectMap is a Project Reactor operator that collects source elements into a Map whose keys are computed by a function and whose values are the elements.

collectMap collects every element emitted by a Flux into a Mono containing a Map. A supplied function computes each key, while the element becomes its value. section 11.3.3

Group animal names by first letter

A Flux containing "aardvark", "elephant", "koala", "eagle", and "kangaroo" becomes a Mono<Map<Character, String>> with collectMap(a -> a.charAt(0)). The final entries are a→aardvark, e→eagle, and k→kangaroo. When keys repeat, the last element replaces the earlier entry.

How Spring in Action, Sixth Edition covers it

How deep it goes
Defines the operator and demonstrates its result with a StepVerifier test, including duplicate-key replacement.
Examples
Uses animal names keyed by their first characters, including repeated initials that demonstrate last-entry replacement.
What you can do after
Apply it via Project Reactor to produce a Mono containing a Map keyed by a computed element characteristic.
How it connects
Connects Flux, Mono, Map, Function, and collectList through reactive stream transformation and collection.
Cover of Spring in Action, Sixth Edition
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