CODEWITHSUNDEEP
java
mat_boy
mat_boy

Reputation: 13666

Extraction of list of objects from list of objects in Java

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.

Upvotes: 1

Views: 81

Answers (2)

Marcio Junior
Marcio Junior

Reputation: 19128

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();
    }
});

Upvotes: 1

fge
fge

Reputation: 121710

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).

Upvotes: 4

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