‘Test Class’ Error:Static methods cannot be invoked through an object instance

I am new to writing test classes. Here is the method I have written my test class on

Class:

public with sharing class caseManage{
    public static string IdSplit(Id i){
        string s = i+'';
        return s.substring(0,s.length()-3);
    }
}

Test Class:

public static testmethod void unitTest1() {
    caseTriggerPostHandler postObj = new caseTriggerPostHandler();
    testDataUtil testHandler = new testDataUtil();        
    Id i;
    Contact con = testHandler.createContact();
    i = con.id;
    postObj.IdSplit(i);
    system.assertEquals(15, 'i');        
}

As I said, I am pretty much new to development (admin backrground) so I am sure there are bunch of errors in my test class. Please suggest me corrections to my test class.

Answers:

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Method 1

As the error suggests static class cannot be instantiated

Hence you just need to call the method without creating an instance of the class

public static testmethod void unitTest1() {
//caseTriggerPostHandler postObj = new caseTriggerPostHandler();//Comment this no need to instantiate
testDataUtil testHandler = new testDataUtil();        
Id i;
Contact con = testHandler.createContact();
     i = con.id;
   caseTriggerPostHandler.IdSplit(i);//Call the method directly using class.methodname
  system.assertEquals(15, 'i');        
}

Since you are new to apex and Test code do some Trails on TrailHead.

https://developer.salesforce.com/trailhead/module/apex_database

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