Strange looking guava code

Question

I'm having trouble following the below code snippet:

prices = pricesService.getProductsByCategory(category);
List<Double> discountedPrices = 
    Lists.newArrayList(Iterables.transform(prices, new Function<Double, Double>() {
        public Double apply(final Double from) {
            return from *.88;
        }
    }));

I know what the result of the code is and it's correct in unit tests, but I'm not overly familiar with guava or how/why this implementation works. Also currently it doesn't appear to be safe if there is a null value in the list 'prices' either? So what I'm after:

1. A general explanation of how the code works.
2. Is it currently null safe? If not how can it made to be?

Answer 1

It creates a new List of Doubles which are 0.88 * the original.

The constructs are:

Anonymous inner class

This is a way how callbacks / closures are sometimes done in Java. See also Java tutorial on this.

new Function<Double, Double>() {
    public Double apply(final Double from) {
        return from *.88;
    }
}

Callback using the above function

Iterables.transform(prices, *func*)

Converting the result to ArrayList

The result of the above is an Iterable, so it needs to be stored to a list. See also Lists.newArrayList vs new ArrayList

Lists.newArrayList( ... )

Answer 2

1) So Guava has a static util class callled Iterables that has a method called transform that takes a collection and a guava Function instance as variables. In this case the developer used an in line anonymous function that returns a double value by the overridden method "apply".

A more traditional implementation would have been something like this:

List<Double> discountedPrices = Lists.newArrayList();
for(Double price: prices) {
    discountedPrices.add(price * .88);
}

2) Not entirely sure what you mean by null safe? assuming you mean what would happen if the list 'prices' contained a null value? If so guava has another solution for you in Iterables.filter(Collection, Predicate). In your case you'd want to filter out nulls and there is a built in guava Predicate for this purpose. So in your case you could do something like:

 prices = Iterables.filter(prices, Predicates.notNull();
 List<Double> discountedPrices = Lists.newArrayList(
 Iterables.transform(prices, new Function<Double, Double>() {
        public Double apply(final Double from) {
            return from *.88;
        }
    }));

This first line returns the prices collection without nulls in it and the 2nd would behave exactly as before and you could safety assume nulls had been removed already.