Java ConcurrentSkipListMap.putIfAbsent() is a method in the Java ConcurrentSkipListMap class that inserts a key-value pair only if the specified key is not already present in the map. If the key exists, it does nothing and returns the existing value. Introduced in Java 2014 with Java 8, putIfAbsent() is thread-safe and commonly used for adding entries in a concurrent map without overwriting existing ones.

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ConcurrentSkipListMap.html