Use String hashCode() Method? [closed]

问题

From link :

http://www.tutorialspoint.com/java/java_string_hashcode.htm

Relationship between hashCode and equals method in Java

Good hashCode() Implementation

But i cant understand about the hashcode .

Here's an example:

public class StringDemo {
    public static void main(String args[]){
        String strob1="first string";
        System.out.println(strob1.hashCode());
    }

    }

This simple program give me output:-5468287

Can anyone tell me : How it give me output:-5468287 ?

回答1:

String's hash code is computed as:

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

using int arithmetic, where s[i] is the i-th character of the string, n is the length of the string, and ^ indicates exponentiation. (The hash value of the empty string is zero.)

Hence, the overflow of this integer computation can easily occur, resulting in negative according to Java Language specification-15.8.2:

If an integer addition overflows, then the result is the low-order bits of the mathematical sum as represented in some sufficiently large two's-complement format. If overflow occurs, then the sign of the result is not the same as the sign of the mathematical sum of the two operand values.

回答2:

In essence, you won't call it yourself unless you are coding a data structure (which you most probably don't have to).

However, you'll often implement one. Nowadays almost all IDE provide auto-generation of equals & hashCode. I'd advise to just use that for the moment; whenever you implement equals, also generate a hashCode implementation.

回答3:

Why? The method is defined in java.lang.Object. Any class you define inherits it.

Refer enter link description here.

What? I suggest you study more about what is hashing. Wiki hash functions.

回答4:

Hashcode is used to allow objects to be stored in a Map object (or with other data structures that use hashes). The goal of a hashcode is to provide a unique value for objects with different values, but produce the same hashcode if the object is the same. This is essentially a unique index for an object to be looked up by.

A HashMap, which implements the Map interface, depends on a good implementation of hashcode() to evenly distribute among differing objects for optimal performance. With this it can provide O(1) average case speed to operations such as get().

So if you ever intend on using a custom object that you make as a key for a HashMap you should provide an implementation of hashcode() which most IDE's will assist you with.

edit: in your example output of -5468287 that is its hashcode value. If you look at the hashcode of "first strings" which only differs by one character it should be a vastly different number, which is a good thing because it helps evenly distribute objects inside your Map.

来源：https://stackoverflow.com/questions/19985360/use-string-hashcode-method

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java

string

java-7

hashcode

java-8