避免使用循环来操作List

     Learning from functional languages, looping isn’t the best way to work on collections. Suppose we want to filter a list of persons to those who can drink beer. The loop versions looks like:

List<Person> beerDrinkers = new ArrayList<Person>();
for (Person p: persons) {
if (p.getAge() > 16) {
beerDrinkers.add(p);
}
}

This can - even in Java - be rewritten to a more a functional programming style. For example using Google collections filter:

Predicate<HasAge> canDrinkBeer = new Predicate<HasAge>() {
public boolean apply(HasAge hasAge) {
return hasAge.getAge() > 16;
}
};

List<Person> beerDrinkers = filter(persons, canDrinkBeer);

As remarked by Dave Jarvis, I should have dropped the getter, and he’s right ;-)

Predicate canDrinkBeer = new Predicate() {
public boolean apply(HasAge hasAge) {
return hasAge.isOlderThan( 16 );
}
};

which would lead to better code down the road:

Predicate canDrinkBeer = new Predicate() {
public boolean apply( HasAge hasAge, HasAge otherAge ) {
return hasAge.isOlderThan( otherAge );
}
}

The predicate version is slightly larger, but consists of two parts. Each one is easier to understand. While the loop version gets unreadable fast with new requirements, the functional version can easily be combined,

List beerDrinkers = filter(filter(persons, canDrinkBeer), isMale);