模运算
取模可以转换成二进制的位与运算当d为2的指数幂
n % d = n & (d-1)
You can do modulus very efficiently for a power of 2. n % d = n & (d-1) when d is a power of 2, and modulus is used to determine which index an item maps to in the internal array – which means it occurs very often in a Java HashMap.
There’s one more thing about choosing the hash & (n – 1) versus modulo and that is negative hashes. hashcode is of type int, which of course can be negative. modulo on a negative number (in Java) is negative also, while & is not.
HashMap’s null key and null value
In the Javadoc of HashMap, there is one sentence below:
HashMap permits null values and the null key
When we look into the source code, we find the null key’s hashcode is set to 0, and 0%n(the current capicity of hashtable) always equals 0, so the null key’s node always allocated at the 0 index of the hashtable.
the difference between HashMap and HashTable
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.
- after my test demo, hashtable not only doesn’t accept null key but also null value, below is why hashtable can not put null key, null value
- HashTable use synchronized to implement synchronize mechanism.
HashMap 的无序性
HashMap 不能保证Map中元素的插入顺序和遍历顺序一致，也不能保证时间上的顺序一致
