Java: Finding objects in collections.

Question

This problem occurs over and over. I have some complicated object, such as a Cat, which has many properties, such as age, favorite cat food, and so forth.

A bunch of Cats are stored in a Java Collection, and I need to find all the Cats that are aged 3, or those whose favorite cat food is Whiskas. Surely, I can write a custom method that finds those Cats with a specific property, but this gets cumbersome with many properties; is there some generic way of doing this?

Answer 1

You could write a method that takes an instance of an interface which defines a check(Cat) method, where that method can be implemented with whatever property-checking you want.

Better yet, make it generic:

public interface Checker<T> {
    public boolean check(T obj);
}

public class CatChecker implements Checker<Cat> {
    public boolean check(Cat cat) {
        return (cat.age == 3); // or whatever, implement your comparison here
    }
}

// put this in some class
public static <T> Collection<T> findAll(Collection<T> coll, Checker<T> chk) {
    LinkedList<T> l = new LinkedList<T>();
    for (T obj : coll) {
         if (chk.check(obj))
             l.add(obj);
    }
    return l;
}

Of course, like other people are saying, this is what relational databases were made for...

Answer 2

You could store those objects on a database. If you don't want the overhead of a full blown database server you can use an embedded one like HSQLDB. Then you can use Hibernate or BeanKeeper (simpler to use) or other ORM to map objects to tables. You keep using the OO model and get the advanced storage and querying benefits from a database.

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

See here: SO: Java: What is the best way to filter a Collection?

Answer 5

You can use something like JoSQL, and write 'SQL' against your collections: http://josql.sourceforge.net/

Which sounds like what you want, with the added benefit of being able to do more complicated queries.

Answer 6

You could try some of the generic code in the Apache Commons project. The Collections subproject provides code for finding objects that match a particular Predicate, as well as a large number of predicates (equals, null, instanceof, etc). The BeanUtils subproject allows you to make predicates that test properties of beans.

Use the CollectionUtils class to search within a collection. There are a few methods for this, but check out the select() method, in particular. Use the following classes to construct predicates, or write your own: Predicate, PredicateUtils, BeanPredicate.

This is all sometimes a bit cumbersome, but at least it's generic! :-)

Answer 7

Here's an idea for a searcher class you could parameterize with the specific values you want to look for.

You could go further and store the names of the properties as well, probably in a Map with the desired values. In this case you would use reflection on the Cat class to call the appropriate methods given the property names.

public class CatSearcher {

  private Integer ageToFind = null;
  private String  foodToFind = null;

  public CatSearcher( Integer age, String food ) {
    this.ageToFind = age;
    this.foodToFind = food;
  }

  private boolean isMatch(Cat cat) {
    if ( this.ageToFind != null && !cat.getAge().equals(this.ageToFind) ) {
       return false;
    }
    if ( this.foodToFind != null && !cat.getFood().equals(this.foodToFind) {
       return false;
    }
    return true;
  }

  public Collection<Cat> search( Collection<Cat> listToSearch ) {
    // details left to the imagination, but basically iterate over
    // the input list, call isMatch on each element, and if true
    // add it to a local collection which is returned at the end.
  }

}

Answer 8

I suggest using Jxpath, it allows you to do queries on object graphs as if it where xpath like

JXPathContext.newContext(cats).
     getValue("//*[@drinks='milk']")

Answer 9

I have been using Google Collections for this kind of problem. There is a class called Iterables that can take an interface called Predicate as a parameter of a method that is really helpful.

Cat theOne = Iterables.find(cats, new Predicate<Cat>() {
    boolean apply(Cat arg) { return arg.age() == 3; }
});

Check it here!

Answer 10

Try the commons collections API:

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

List<Cat> smallList = CollectionUtils.find(new Predicate() {
    public boolean evaluate(Object o) {
        Cat c = (Cat)o;
        return c.getFavoriteFood()=="Wiskas" 
            && c.getWhateverElse()==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 11

Again with the commons collections API: You get the "checker" type code when you Implement the Predicate Separately:-

public class CatPredicate implements Predicate {

    private int age; 


    public CatPredicate(int age) {
        super();
        this.age = age;
    }


    @Override
    public boolean evaluate(Object o) {
        Cat c (Cat)o;
        return c.getAge()==this.age;
    }

}

which gets used as :-

CollectionUtils.filter(catCollectionToFilter, new CatPredicate(3))

Answer 12

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 13

Write it in scala :)

collection.filter(_.age > 3 && _.sex == 'male') //etc...