Hi Guys!

Good day. I am creating a test method for a class i just created from a recycled apex code.
I have a requirement of updating the custom lookup field product__c in our opportunity object.
This field gets its value from the opportunity line item.  

I've created an apex class, got the code somewhere from the internet, i read and tried a lot, fortunately made it working for our requirement. :)
The problem is i been trying to make a test method for it but keeps failing. I am a new admin and do not know much about coding.

Can someone help me to make a test class please. :) TIA guys!

Heres my apex class:

public class UpdateOLIProdMgr {
    public static void updateOppProdMgr(OpportunityLineItem[] oppLineItems)
        {
            List<Opportunity> opps = new List<Opportunity>();
            List<Id> idPriceBookEntry = new List<Id>();
            List<Id> idProducts = new List<Id>();
            List<String> strFamily = new List<String>();
             

            for(OpportunityLineItem oppLineItem:oppLineItems)
            {
                idPriceBookEntry.add(oppLineItem.PricebookEntryId);             
            }   
            
            
            List<PricebookEntry> sObjPriceBookEntries = [Select Product2Id, Product2.Family, Product2.Name from PricebookEntry where Id in :idPriceBookEntry]; 
            for(PricebookEntry objPriceBookEntry:sObjPriceBookEntries)
            {
                idProducts.add(objPriceBookEntry.Product2Id);
                strFamily.add(objPriceBookEntry.Product2.Family);
            }
            
                        
                       
            if (sObjPriceBookEntries.size() > 0)
            {
                                 
                Integer intCounter = 0;
                for(OpportunityLineItem oppLineItem:oppLineItems)
                {
                    Opportunity opp = new Opportunity(id = oppLineItem.OpportunityId,Product__c = idProducts[intCounter]);                                                                                                       
                    opps.add(opp);
                    intCounter++;
                }
             
                Database.update(opps);  
            }
                                
        }
    
    public static void updateOLIMgr(OpportunityLineItem[] oppLineItems)
    {
        List<Id> idOpp = new List<Id>();
               
        for(OpportunityLineItem oppLI: oppLineItems)
        {
            idOpp.add(oppLI.OpportunityId);
        }       
        
         
        
    }   
    
    public static void eraseOppProd(OpportunityLineItem[] oppLineItems)
    {
        List<Opportunity> opps = new List<Opportunity>();
        List<Id> idOpp = new List<Id>();
            
        for(OpportunityLineItem oppLI: oppLineItems)
        {
            idOpp.add(oppLI.OpportunityId);
        }       
        
        List<Opportunity> sObjOpp = [Select Id, Product__c from Opportunity where Id in :idOpp];
        
        for(Opportunity opp:sObjOpp)
            {
                Opportunity oppty = new Opportunity(id=opp.id, Product__c = null, X50__c = null);                                                                                                        
                opps.add(oppty);
            }
        database.update(opps);
    }
  }