java - Is hashcode() a good way to compute an unique token from a list of informations? -


i have 

hashmap<string,anobject>

and i'd give string key value infos anobject value contains. suppose anobject made way:

public class anobject(){      public string name;      public string surname; }

is correct assign key to:

string.valueof(o.name.hashcode()+o.surname.hashcode());

? or there better way compute string hash code value list?

no, absolutely not. hashcode() not guaranteed unique.

the rules of hash code simple:

  • two equal values must have same hash code
  • two non-equal values ideally have different hash codes, can have same hash code. in particular, there 232 possible values return hashcode(), more 232 possible strings, making uniqueness impossible.
  • the hash code of object should not change unless equality-sensitive aspect of changes. indeed, it's generally idea make types implementing value equality immutable, @ least in equality-sensitive aspects. otherwise can find can't entry using exact same object reference used key!

hash codes optimization technique make quick find "probably small" set of candidate values equal target, iterate through rigorous equality check find whether of them actually equal target. that's lets key in hash-based collection. key isn't hash itself.

if need create key 2 strings, you're going have make from 2 strings (with sort of delimiter can tell difference between {"a", "bc"} , {"ab", "c"} - understanding delimiter might appear in values if you're not careful).

see eric lippert's blog post on topic more information; that's based on .net rather java, apply. it's worth understanding semantics of hashcode aren't same of cryptographic hash. in particular, it's fine result of hashcode() change if start new jvm create object same fields - no-one should persisting results of hashcode. that's not case sha-256, should permanently stable particular set of data.


Comments

Post a Comment

Popular posts from this blog

commonjs - How to write a typescript definition file for a node module that exports a function? -

consider, toml node module can use: // toml.d.ts declare module toml { export function parse(value:string):any; } declare module "toml" { export = toml; } then: /// <reference path="../../../../../defs/toml/toml.d.ts"/> import toml = require('toml'); toml.parse(...); however, node module export single function, such 'glob' ( https://github.com/isaacs/node-glob ). the node usage of module be: var glob = require("glob") glob("*.js", {}, callback(err, files) { ... }); you'd naively expect this: // glob.d.ts declare function globs(paths:string, options:any, callback:{(err:any, files:string[]):void; ...but because typescripts 'import' semantics bit strange, seems can use 'import .. = require()' statement alias modules . trying call: /// <reference path="../../../../../defs/glob/glob.d.ts"/> import blog = require('glob'); results in: error ts2072: mod...
Read more

openid - Okta: Failed to get authorization code through API call -

i'm integrating okta own idp server using okta's api. i'm implementing authorization code flow following steps below: in own server, use /api/v1/authn endpoint sessiontoken. use sessiontoken obtain authorization calling endpoint: /oauth2/v1/authorize?client_id=" + clientid + "&sessiontoken=" + sessiontoken + "&response_type=code&response_mode=query&scope=openid&redirect_uri=" + redirecturl + "&state=evanyang&nonce=" it's supposed return response status code 302 , location header containing redirect url code value. however, keep getting response status code 200 , without location header, html body saying "you using unsupported browser." , "javascript disabled on browser." according api documentation: http://developer.okta.com/docs/api/resources/oidc.html#authentication-request , sessiontoken parameter sufficient this: an okta one-time sessiontoken. allows api-based ...
Read more

ios - Change Storyboard View using Seague -

i have following code checks connection server, , depending on want change views if no connection found. nsstring *connect = [nsstring stringwithcontentsofurl:[nsurl urlwithstring:@"http://myserver.com"] encoding:nsutf8stringencoding error:nil]; if (connect == null) { [self performseguewithidentifier:@"network_failure" sender:self]; } else { // other code here } my problem not change views. have added storyboard segue between 2 views identifier of network_failure . not have navigation controller or that, problem? thanks help! no not problem. first of all, in objectivec exists nil object oriented null . must use that. so: if(connect == nil) or better: if(!connect) so due fact nil not same of null , probably code does't enter in if scope . you have been able discover breakpoint. anyway: be sure set trigger segue (line) 2 view controller , , name network_failure (that not best name. should instead view1toview2 ) as, m...
Read more