A common question in backend developer interview questions and answers, don't miss this one. This is one of the important Java backend developer interview questions. In Java, HashMap works on a number of hashing principles. It leverages the static inner liner class in order to store the entries in the map. The HashMap also makes use of different buckets, with each of the buckets pointing towards a Singly Linked List. It allows for a number of null values and one null key.