Finding all objects that have a given property inside a collection [duplicate]

Answer 1

A very common problem and I have used google collections and here is my code

public class FindByIdPredicate implements Predicate<IDObject> {

private Long entityId;

public FindByIdPredicate(final Long entityId) {
    this.entityId = entityId;

}

@Override
public boolean apply(final IDObject input) {
    return input.getId().equals(this.entityId);
}

/**
     * Pass in the Collection
 * @param Collection
 * @return IdObject if present or null
 */
public IDObject getEntity(final Collection<? extends IDObject> collection) {

    for (IDObject idObject : collection) {
        if (this.apply(idObject)) {
            return idObject;
        }
    }
    return null;

}

/**
 * @param Set
 * @return IdObject if present or null
 */
@SuppressWarnings("unchecked")
public <T> T getEntity(final Set<? extends IDObject> set) {

    for (IDObject idObject : set) {
        if (this.apply(idObject)) {
            return (T) idObject;
        }
    }
    return null;

}

}

Hope this helps

Answer 2

Try the commons collections API:

List<Cat> bigList = ....; // master list

Collection<Cat> smallList = CollectionUtils.select(bigList, new Predicate() {
    public boolean evaluate(Object o) {
        Cat c = (Cat)o;
        return c.getFavoriteFood().equals("Wiskas") 
            && c.getWhateverElse().equals(Something);
    }
});

Of course you don't have to use an anonymous class every time, you could create implementations of the Predicate interface for commonly used searchs.

Answer 3

Sounds a lot like something you would use LINQ for in .NET

While there's no "real" LINQ implementation for java yet, you might want to have a look at Quaere which could do what you describe easily enough.

Answer 4

Solutions based on predicates, filters and custom iterators/comparators are nice, but they don't provide analogue with database index. For instance: I would like to search collection of cats different ways: by sex and age and by age, so it looks like two indexes: 1) [sex, age] 2) [age]. Values, accessed by that indexes could be hashed, to implement fast lookup, without iterating the whole collection. Is anywhere there such solution?

Answer 5

Just FYI there are 3 other answers given to this question that use Guava, but none answer the question. The asker has said he wishes to find all Cats with a matching property, e.g. age of 3. Iterables.find will only match one, if any exist. You would need to use Iterables.filter to achieve this if using Guava, for example:

Iterable<Cat> matches = Iterables.filter(cats, new Predicate<Cat>() {
    @Override
    public boolean apply(Cat input) {
        return input.getAge() == 3;
    }
});

Answer 6

Using Commons Collections:

EqualPredicate nameEqlPredicate = new EqualPredicate(3);
BeanPredicate beanPredicate = new BeanPredicate("age", nameEqlPredicate);
return CollectionUtils.filter(cats, beanPredicate);