Research on hash storage mechanism by analyzing JDK source code

Analysis of its Hash storage mechanism through HASHMAP and HashSet source code

Collections and references

Just like an array of reference types, when we put a Java object into an array, we are not really putting the Java object into an array, but putting the reference of an object into an array, each of which is a reference variable.

In fact, there are many similarities between HashSet and HashMap, for HashSet, the system uses the Hash algorithm to determine the storage location of the set elements, so as to ensure that the collection elements can be saved and fetched quickly. For HashMap, System key-value as a whole Processing, the system always calculates the Key-value storage location according to the Hash algorithm, which can guarantee the key-value pair of the Map to be saved and fetched quickly.

Before introducing the collection store, it is important to point out that while the collection is known to store Java objects, it does not actually put Java objects into the set collection, but rather to keep references to those objects in the Set collection. That is, a Java collection is actually a collection of multiple reference variables that point to the actual Java object.

Storage implementation of HASHMAP

When the program tries to put multiple key-value into the HASHMAP, take the following snippet as an example:

HashMap<String　,　Double>　map　=　new　 HashMap<String　,　Double>();
map.put("语文"　,　80.0);
map.put("数学"　,　89.0);
map.put("英语"　,　78.2);

HashMap uses a so-called "Hash algorithm" to determine where each element is stored.

When the program executes Map.put ("language", 80.0); , the system will invoke the Hashcode () method of "language" to get its hashcode value-each Java object has a hashcode () method that can obtain its hashcode value. After the hashcode value of the object is obtained, the system will determine the storage location of the element based on the hashcode value.

We can see the source code for the put (K key, V value) method of the HashMap class:

public　V　put(K　key,　V　value)
{
//　如果　key　为　null，调用　putForNullKey　方法进行处理
if　(key　==　null)
return　putForNullKey(value);
//　根据　key　的　keyCode　计算　Hash　值
int　hash　=　hash(key.hashCode());
//　搜索指定　hash　值在对应　table　中的索引　
int　i　=　indexFor(hash,　table.length);
//　如果　i　索引处的　Entry　不为　null，通过循环不断遍历　e　元 素的下一个元素
for　(Entry<K,V>　e　=　table[i];　e　!=　null;　e　=　 e.next)
{
Object　k;
//　找到指定　key　与需要放入的　key　相等（hash　值相同
//　通过　equals　比较放回　true）
if　(e.hash　==　hash　&&　((k　=　e.key)　==　key
||　key.equals(k)))
{
V　oldValue　=　e.value;
e.value　=　value;
e.recordAccess(this);
return　oldValue;
}
}
//　如果　i　索引处的　Entry　为　null，表明此处还没有　Entry
modCount++;
//　将　key、value　添加到　i　索引处
addEntry(hash,　key,　value,　i);
return　null;
}