CODEWITHSUNDEEP
javacollections
membersound
membersound

Reputation: 86697

How to extract all elements of a specific property in a list?

how can I best get all "name" string elements of the following structure:

class Foo {
    List<Bar> bars;
}

class Bar {
    private String name;
    private int age;
}

My approach would be:

List<String> names = new ArrayList<String>();

for (Bar bar : foo.getBars()) {
    names.add(bar.getName());
}

This might work, but isn't there something in java.Collections where I just can write like this: Collections.getAll(foo.getBars(), "name");?

Upvotes: 5

Views: 2759

Answers (4)

Craig P. Motlin
Craig P. Motlin

Reputation: 26740

If you use Eclipse Collections and change getBars() to return a MutableList or something similar, you can write:

MutableList<String> names = foo.getBars().collect(new Function<Bar, String>()
{
    public String valueOf(Bar bar)
    {
        return bar.getName();
    }
});

If you extract the function as a constant on Bar, it shortens to:

MutableList<String> names = foo.getBars().collect(Bar.TO_NAME);

With Java 8 lambdas, you don't need the Function at all.

MutableList<String> names = foo.getBars().collect(Bar::getName);

If you can't change the return type of getBars(), you can wrap it in a ListAdapter.

MutableList<String> names = ListAdapter.adapt(foo.getBars()).collect(Bar.TO_NAME);

Note: I am a committer for Eclipse collections.

Upvotes: 2

Andy Turner
Andy Turner

Reputation: 140319

Using Java 8:

List<String> names =        
    foo.getBars().stream().map(Bar::getName).collect(Collectors.toList());

Upvotes: 1

David
David

Reputation: 20063

Is there any reason why you couldn't use the MultiMap supplied by Googles collection framework?

MultiMap

This will allow a map with multiple key values.

So you could have a duplicate key (unlike say a hashmap) and return all the values for that key

Then just call get on the associated Map, which would be the same implementation as your expected example of "getAll"

get

Collection get(@Nullable K key) Returns a collection view of all values associated with a key. If no mappings in the multimap have the provided key, an empty collection is returned. Changes to the returned collection will update the underlying multimap, and vice versa.

Parameters: key - key to search for in multimap Returns: the collection of values that the key maps to

Upvotes: -1

Alexey Sviridov
Alexey Sviridov

Reputation: 3490

Somewhat provides Google guava

List<String> names = new ArrayList(Collections2.transform(foo.getBars(), new Function<Bar,String>() {
    String apply(Bar bar) {
        return bar.getName()
    }
});

Upvotes: 1

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