HasMap的实现是否将键值对存储在链表中

Question

I read in a book where it was mentioned that when we put elements in HashMap, internally it is stored in bucket. My question is

1. Does hashmap store key-value pair altogether in the form of linked list? or does it store in linked list only when there is a collision?

2. How does it retrieve the object when 2 different objects are stored in the same bucket?

Thanks!

Answer 1

Lots of details at http://en.wikipedia.org/wiki/Hash_table

See also Internal implementation of java.util.HashMap and HashSet

And of course you can use the source , Luke.

Updated: to specifically answer your Q, it stores an Entry, which has a reference to the next item in the bucket (if any). If there is only one item in the bucket then the reference will be null:

static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;

Answer 2

The data type of the bucket Array is Map.Entry . When multiple entries fall in the same bucket they are stored in what is conceptually a unidirectional linked list, by holding references to the next Entry. Just the Entry at the 'head' is inside the array that is the buckets. However, there is never any use a java.util.LinkedList or some actual list class. The entries just form a list in and of themselves by holding references to their bucket-mates.

When there's more than one in a bucket, it starts with the one that's actually in the Map.Entry[] , which is the head of the list, and just starts traversing and checking .equals() until it finds a match or next is null.

Answer 3

Best way to understand is by tracing source

• In eclipse If configured jdk hit ctrl+shift+T type HashMap and AbstractMap
• AbstractMap and HashMap

You only need to do it once !