java - Calculating total price -

i have abstract class transaction, calculate total price of each transaction. total price calculated getting price of each product in map , multiply price quantity of each product. don't know how multiply these prices quantities values in map. can me, please? tried , nothing works.

public abstract class transaction {     //attributes     ...      //links      map<product,integer> products;      //constructor      transaction()     {         id = newtrid.incrementandget();         date = new date();         products = new hashmap<>();     }      abstract void addproduct(product aproduct, int aquantity);       bigdecimal calculatetotal()     {         bigdecimal total = new bigdecimal(0);          for(product eachproduct : products.keyset())         {             total.add(eachproduct.getprice());         }         (integer eachproduct : products.values())         {          }          return total;     } } 

bigdecimal immutable , adddoes not change object called. need reassign result of add:

bigdecimal calculatetotal() {   bigdecimal total = new bigdecimal(0);   (map.entry<product, integer> entry : products.entryset()) {     total = total.add(bigdecimal.valueof(entry.getkey().getprice() * entry.getvalue()));   }   return total; }