Python-like list comprehension in Java

Solution 1:

In Java 8 you can use method references:

List<String> list = ...;
list.replaceAll(String::toUpperCase);

Or, if you want to create a new list instance:

List<String> upper = list.stream().map(String::toUpperCase).collect(Collectors.toList());

Solution 2:

Basically, you create a Function interface:

public interface Func<In, Out> {
    public Out apply(In in);
}

and then pass in an anonymous subclass to your method.

Your method could either apply the function to each element in-place:

public static <T> void applyToListInPlace(List<T> list, Func<T, T> f) {
    ListIterator<T> itr = list.listIterator();
    while (itr.hasNext()) {
        T output = f.apply(itr.next());
        itr.set(output);
    }
}
// ...
List<String> myList = ...;
applyToListInPlace(myList, new Func<String, String>() {
    public String apply(String in) {
        return in.toLowerCase();
    }
});

or create a new List (basically creating a mapping from the input list to the output list):

public static <In, Out> List<Out> map(List<In> in, Func<In, Out> f) {
    List<Out> out = new ArrayList<Out>(in.size());
    for (In inObj : in) {
        out.add(f.apply(inObj));
    }
    return out;
}
// ...
List<String> myList = ...;
List<String> lowerCased = map(myList, new Func<String, String>() {
    public String apply(String in) {
        return in.toLowerCase();
    }
});

Which one is preferable depends on your use case. If your list is extremely large, the in-place solution may be the only viable one; if you wish to apply many different functions to the same original list to make many derivative lists, you will want the map version.