Add Values to ArrayList Used as Value in HashMap in Java

Covers multiple approaches to storing several values per key in a Java HashMap using ArrayList as the value type. The manual approach checks for key existence before adding to the list, while the Java 8+ computeIfAbsent method reduces lookups and boilerplate. External library alternatives are also shown: MultiValuedMap from Apache Commons Collections, LinkedMultiValueMap from Spring, and Multimap from Google Guava, all of which abstract away the list management entirely.