Sets vs. Lists in Apex Code in Salesforce.com

Example.cls

// original

public List<selectOption> getSubCategoryEstimate(){

    List<selectOption> options = new List<selectOption>();  

     if(selectedCategory!=null){
     List<Material_Grouping_Estimate__c> listSub  = new List<Material_Grouping_Estimate__c>();  
     listSub = [select Category__c , Sub_Category__c from Material_Grouping_Estimate__c WHERE Category__c like :selectedCategory ORDER BY Sub_Category__c];
     
     if(listSub.get(0).Category__c !=null) {
     options.add(new SelectOption(listSub.get(0).Sub_Category__c, listSub.get(0).Sub_Category__c));
     }
     
     for (Integer i = 1; i < listSub.size(); i++) {
        if(listSub.get(i).Sub_Category__c !=null)
          if (listSub.get(i).Sub_Category__c != listSub.get(i-1).Sub_Category__c)
        {
            options.add(new SelectOption(listSub.get(i).Sub_Category__c, listSub.get(i).Sub_Category__c));
            }
      }
    }
    return options;
}

// This is wasteful because it is reimplementing items which already exist and are very easy to use. 20 lines.

public List<selectOption> getSubCategoryEstimate(String selected_category){
    // You should pass a value instead of just depending on state; state can change without you understanding why; passing a value to a function is always easier to debug in SFDC.
    Set<SelectOption> options = new Set<SelectOption>();
    try{
        for(Material_Grouping_Estimate__c mge: [select Sub_Category__c from Material_Grouping_Estimate__c WHERE Category__c =: selected_category ORDER BY Sub_Category__c]){
            options.add(new SelectOption(mge.Sub_Category__c, mge.Sub_Category__c));
        }
    }catch(System.QueryException e){
        // There are no such items.
    }
    return new List<SelectOption>(options); 
}

// This is far easier to understand and has the same outcome.