Why HashMap is 2 to the power of n

Photo understand the composition of HashMap

hashMap mainly of type Node arrays, linked lists, red-black tree composition. As can be seen from the figure, Node objects stored in the array, each Node object is associated with a list, if the list of elements to use more than eight red-black tree is stored.

About 2 power

Using the array length -1 modulo hash value for determining a location in Node element array.

tab[i = (n - 1) & hash]
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The purpose of the source code to do so

In a word using modulo a power of 2 -1 way it is to allow a uniform distribution of elements in the array, using the array space.

The basic idea

n is the length of the array, the value is an n-th power of 2.

Suppose n = 8, hashcode = 0100010001100

(N-1) & hash is calculated as follows:

 0100010001100    //hashcode的值
&         0111    //(8-1)=7的二进制代码
          0100    //相&之后的值 

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You will find hashcode value 0100010001100, the value after the phase & 0100the same back three values, good to do so is to get hashcode low value.

to sum up

Low values ​​obtained in determining the position of the element in the hash array, hash is to improve the utilization of the array.