HashMap inside
HashMap maintains an array of buckets. Each bucket is a linkedlist of key value pairs encapsulated as Entry objects.This array of buckets is called table. Each node of the linked list is an instance of a private class called Entry.An entry is a private static class inside HashMap which implements Map.Entry
private static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
final int hash;
V value;
Entry<K,V> next;
}
code: openjdk/8u40-b25/java.util.HashMap line 282 class name is Node<K,V> in open JDK
Each Entry object has a field called next which points to the next Entry thus behaving like a singly linked list. The hash field stores the hashed value of the key.
capacity
HashMap provides overloaded constructors with parameters for initial capacity and load factor but typically no args constructor is the one most frequently used. Default values of these fields are :
initial capacity : 1 « 4 (ie 16)
load factor : 0.75
Whenever the element count of the hashmap reaches the load factor fraction of capacity, the map is resized and capacity is doubled. If capacity provided by client is a power of 2, then real capacity will be same as capacity else real capacity = nearest power of 2 > provided capacity. Maximum capacity is 1«30 (ie 2 ^30) if capacity provided is greater than that, then real capacity = 2^30.
Note that capacity indicates the size of the table array (the array of buckets) and not the number of key-value pairs the HashMap can support
from: Kaustubh Saha’s answer@Quora
Jiayang, Sun 15 April 2016