Is there a Java equivalent for Python's map function?

Question

I would like to easily transform a collection (list) of objects of class A to a collection of objects of class B, just as Python's map function does it. Is there any "well-known" implementation of this (some kind of library)? I've already searched for it in Apache's commons-lang but with no luck.

Answer 1

Starting from Java 8, it can be done thanks to the Stream API using an appopriate mapper Function that we will use to convert our instances of class A to instances of class B.

The pseudo code would be:

List<A> input = // a given list of instances of class A
Function<A, B> function = // a given function that converts an instance 
                          // of A to an instance of B
// Call the mapper function for each element of the list input
// and collect the final result as a list
List<B> output = input.stream().map(function).collect(Collectors.toList());

Here is a concrete example that will convert a List of String to a List of Integer using Integer.valueOf(String) as mapper function:

List<String> input = Arrays.asList("1", "2", "3");
List<Integer> output = input.stream().map(Integer::valueOf).collect(Collectors.toList());
System.out.println(output);

Output:

[1, 2, 3]

---

For previous versions of Java, you can still use FluentIterable from Google Guava to replace the Stream and use com.google.common.base.Function instead of java.util.function.Function as mapper function.

The previous example would then be rewritten as next:

List<Integer> output = FluentIterable.from(input)
    .transform(
        new Function<String, Integer>() {
            public Integer apply(final String value) {
                return Integer.valueOf(value);
            }
        }
    ).toList();

Output:

[1, 2, 3]