public class OpportunityHelpers {
	public static void recalculateOpportunitiesAmount(List<Opportunity> oppList){

		Map<Id, Decimal> totalAmountForOppty = new Map<Id, Decimal>();
		for(OpportunityLineItem opptylineItem : [SELECT Id, OpportunityId, TotalPrice  FROM OpportunityLineItem WHERE Opportunityid IN :oppList]){
			if(!totalAmountForOppty.containsKey(opptylineItem.OpportunityId)){
				totalAmountForOppty.put(opptylineItem.OpportunityId, opptylineItem.TotalPrice);
			}
			else{
				 Decimal amount = totalAmountForOppty.get(opptylineItem.OpportunityId);
				 amount += opptylineItem.TotalPrice;
				 totalAmountForOppty.put(opptylineItem.OpportunityId, amount);
				}
		}

		for(Opportunity opp : oppList){
			opp.Amount = totalAmountForOppty.get(opp.Id);
		}

		update oppList;
	}
}