Java AbstractList.indexOf() is a method in the Java AbstractList class that returns the index of the first occurrence of a specified element in the list, or -1 if the element is not found. Introduced in Java 1997 with Java 1.2, indexOf() is frequently used in list-based data structures to find the position of an element, which can be useful in search, removal, or modification operations.

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