Java-hashmap Source Code Analysis

Java improvement (two or three)-----HASHMAP

HashMap is also the implementation of a hash table-based MAP interface that we use very much collection, which exists in the form of Key-value. In HashMap, Key-value is always treated as a whole, and the system calculates the storage location of Key-value based on the hash algorithm, and we can always save and fetch value quickly by key. The following is an analysis of hashmap access.

First, the definition

HASHMAP implements the map interface and inherits the Abstractmap. Where the map interface defines the key mapping to the value of the rules, and the Abstractmap class provides the backbone of the map interface implementation to minimize the implementation of this interface required work, in fact, the Abstractmap class has implemented a map, here the map LZ think it should be more clear it!

Second, the structure function

The HashMap provides three constructors:

HashMap (): Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).

HashMap (int initialcapacity): Constructs an empty HashMap with the specified initial capacity and default load factor (0.75).

HashMap (int initialcapacity, float loadfactor): Constructs an empty HashMap with the specified initial capacity and load factor.

Two parameters are mentioned here: initial capacity, load factor. These two parameters are important parameters that affect the performance of the HashMap, where capacity represents the number of buckets in the hash table, the initial capacity is the capacity to create a hashtable, and the load factor is the size of the hash table before its capacity is automatically increased by a scale, which measures how much space is used for a hash list, The larger the load factor, the higher the filling of the hash table, and the smaller the inverse. For a hash table using the list method, the average time to find an element is O (1+a), so if the load factor is larger, the use of space is more adequate, but the result is a reduction of the search efficiency, if the load factor is too small, then the hash table data will be too sparse, the space caused a serious waste. The system default load factor is 0.75, and we don't need to modify it in general.

HashMap is a data structure that supports fast access, and it is important to understand its data structure in order to understand its performance.

Third, data structure

We know that the two most commonly used structures in Java are arrays and analog pointers (references), and almost all data structures can be combined using both, HashMap. In fact, HashMap is a "chain table hash", which is the following data structure:

From what we can see is the HashMap bottom implementation or an array, except that each item of the array is a chain. Where the parameter initialcapacity represents the length of the array. The following is the source code for the HashMap constructor:

Java-hashmap Source Code Analysis