使用不同散列的密钥也可以映射到HashMap中的相同索引? [英] Do keys with different hashes also get mapped to the same index in HashMap?
问题描述
e.hash == hash
?
public V put(K key,V value){
if(key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash,table.length);
for(Entry e = table [i]; e!= null; e = e.next){
Object k;
if(e.hash == hash &&((k = e.key)== key || key.equals(k))){
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
返回oldValue;
}
}
modCount ++;
addEntry(hash,key,value,i);
返回null;
}
散列码可以是2 ^ 32个值中的一个,散列表有很多存储桶(仅表格需要16GB内存)很少见。所以是的,你可以在地图的同一个桶中拥有不同哈希的对象(AFAIK它是一个简单的模数运算 hachCode%numberOfBuckets
)。
请注意,代码不直接使用 key.hashCode()
,而是 hash(key.hashCode() )
。
Looking at the code specifically line 393, it looks like different hashes have been mapped to same index. I had an understanding that the hashcode is used to determine what bucket in a HashMap is to be used, and the bucket is made up of a linked list of all the entries with the same hashcode. They why have the e.hash == hash
check ?
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
Since a hashcode can be one in 2^32 values, it is rare that the hashmap has so many buckets (just the table would require 16GB of memory). So yes, you can have objects with different hashes in the same buckets of the maps (AFAIK it is a simple modulus operation of hachCode % numberOfBuckets
).
Note that the code does not use directly key.hashCode()
, but hash(key.hashCode())
.
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