//Our function that implements the update logic. 
//You can also use PureFunction.OnValue or PureFunction.OnKey.
PureFunction.OnKeyValue<String, Double, Return.Map<Double>> discount =  
  (String product, Double price, Optional<Deadline> deadline) -> {
    //if there are less than 2 days to expiry then apply discount.  
    if (deadline.isPresent() && deadline.get().timeLeft().minusDays(2).isNegative()) {  
      double discountedPrice = price * 0.50; //50% discount.
      if (discountedPrice <= 10) { //If the price is below $10
        //return Return.expire(Duration.ZERO); //expire it
        return Return.remove(); //or remove the product
      } else {  
        return Return.update(discountedPrice); //else update with the discounted price
      }  
    } else {
      return Return.nothing(); //else do nothing.
    }
  };  
 
//create our map with functions enabled.    
Map<String, Double, PureFunction<String, Double, Return.Map<Double>>> products =  
  MapConfig.functionsOn(stringSerializer(), doubleSerializer())  
    .registerFunction(discount) //register the discount function
    .get();  
 
//insert two products that expire after a day. 
products.put("MacBook Pro", 2799.00, Duration.ofDays(1));  
products.put("Tesla", 69275.0, Duration.ofDays(1));  
 
//apply the discount function.
products.applyFunction("MacBook Pro", "Tesla", discount);  

products.stream().forEach(System.out::println);