Recently, in an interview I was asked, what exactly is a bucket in hashmap? Whether it is an array or a arraylist or what?
I got confused. I know hashmaps are backed by arrays. So can I say that bucket is an array with a capacity of 16 in the start storing hashcodes and to which linked lists have their start pointer, ?
No, a bucket is each element in the array you are referring to. In earlier Java versions, each bucket contained a linked list of Map entries. In new Java versions, each bucket contains either a tree structure of entries or a linked list of entries.
From the implementation notes in Java 8:
/* * Implementation notes. * * This map usually acts as a binned (bucketed) hash table, but * when bins get too large, they are transformed into bins of * TreeNodes, each structured similarly to those in * java.util.TreeMap. Most methods try to use normal bins, but * relay to TreeNode methods when applicable (simply by checking * instanceof a node). Bins of TreeNodes may be traversed and * used like any others, but additionally support faster lookup * when overpopulated. However, since the vast majority of bins in * normal use are not overpopulated, checking for existence of * tree bins may be delayed in the course of table methods. ...