HashMap and hashtable of the set

HashMap、Hashtable The difference between

1.HashTable

• The underlying array + Linked list implementation , No matter what key still value Neither can it be. null, Thread safety , The way to achieve thread safety is to lock the whole when modifying data HashTable, Low efficiency .
• initial size by 11, Capacity expansion ：newsize = olesize*2+1

2.HashMap

hashMap Of jdk7 and jdk8 There's a difference ;

1. Realization way

jdk7 Using arrays + Linked list ,jdk8 The array used + Linked list + Red and black trees

2. When a new node is inserted into the linked list, the insertion order is different

jdk7 Insert in the head （ It is easy to cause the problem of dead loop when multithreading capacity expansion ）,jdk8 Insert at the end

Common characteristics ：

• Can be stored null Key sum null value , Thread unsafe
• initial size by 16, Capacity expansion ：newsize = oldsize*2,size It must be 2 Of n The next power
• The expansion is for the whole Map, At each expansion , The elements in the original array are recalculated in turn , And reinsert