String 类的 Java hashCode 方法的时间复杂度是多少？

Question

Is Java's hashCode method for String class computed in constant time or linear time? What is the algorithm used?

Answer 1

The documentation tells you the function:

s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

It's computed once using a linear-time pass, and then cached so it's only O(1) to retrieve it in the future.

Answer 2

According to the docs

public int hashCode()

Returns a hash code for this string. The hash code for a String object is computed as

 s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]

As you can see, the time complexity is the O(n) where n is the number of characters in the string. After one pass, it is cached so the time complexity is effectively O(1).

Answer 3

Time Complexity of Object::hashCode method is O(1), because it has no any interaction with the data of your object. It is a native method and written with C language. The integer value which is returned, is probably heap memory address with some modifications (bitwise operations) since every address in memory representing unique value.

For example 4GB memory addresses will be represented in hexadecimal format from 0x00000000 to 0xffffffff range and each of this value is unique. Thus, Java does not require extra computation to ensure uniqueness of these values. Therefore hashCode gives constant time complexity.