Extraction of list of objects from list of objects in Java

Question

Suppose that you have a class A like follows:

public class A{
  private B b;

  public B getB(){return b;}
}

Then, you have a list of A objects and you want to create another list that contains the related B objects, i.e.

List<A> listOfA=...
List<B> listOfB = extractFrom(listOfA, "getB");

Exists a library that does the magic described by function extractFrom? Maybe it uses reflection, I don't know. Or, maybe the library relies on an Extractor interface that uses generics, e.g.

public interface Extractor<T,V>{

  public T extract(V source);
}

and then one does:

Extractor ex=...;
List<B> listOfB = extractFrom(listOfA, ex);

PS: The title is horrible! Please, edit the title so that it becomes more meaningfull.

Just as a hint: You are looking for the equivalent in Java of Python's map(function, iterable, ...) function, described here: docs.python.org/2/library/functions.html#map — Daniel S.
– Daniel S., Commented Jun 4, 2013 at 14:19
Ok, I will implement it from myself. Usually I don't like to reinvent the well! — mat_boy
– mat_boy, Commented Jun 4, 2013 at 14:19

fge · Accepted Answer · 2013-06-04 14:19:08Z

4

There is a solution. This uses Guava.

You need a Function<A, B>, then apply that function using Lists.transform():

final Function extractB = new Function<A, B>()
{
    @Override
    public B apply(final A input)
    {
        return input.getB();
    }
}

// And then:

final List<B> listOfB = Lists.transform(listOfA, extractB);

Note that Java 8 will have Function, among other niceties "invented" by Guava (Predicate is another example).

answered Jun 4, 2013 at 14:19

fge

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3 Comments

mat_boy Over a year ago

Oh! Great! I was very surprised to discover that none already implemented such a useful utility! Thank you!

fge Over a year ago

Guava is a gold mine. Just Use It(tm) :p

mat_boy Over a year ago

I know, I use it! But I don't know all the utilities. :)

Community · Accepted Answer · 2017-05-23 11:57:23Z

1

This is know as map.

Here in SO have an answer

In addition you can do with the apache commons using transformers

By example:

Collection<B> bs = CollectionUtils.collect(collectionOfA, new Transformer() {
    public Object transform(Object a) {
        return ((A) a).getB();
    }
});

edited May 23, 2017 at 11:57

CommunityBot

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answered Jun 4, 2013 at 14:37

Marcio Junior

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1 Comment

Marcio Junior Over a year ago

Yes. I saw a time ago an implementation of apache commons that use generics, but I don't found the link.

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