How to filter a Java Collection (based on predicate)?

Question

I want to filter a java.util.Collection based on a predicate.

Evadne

Answer 1

Java 8 (2014) solves this problem using streams and lambdas in one line of code:

List<Person> beerDrinkers = persons.stream()
    .filter(p -> p.getAge() > 16).collect(Collectors.toList());

Here's a tutorial.

Use Collection#removeIf to modify the collection in place. (Notice: In this case, the predicate will remove objects who satisfy the predicate):

persons.removeIf(p -> p.getAge() <= 16);

---

lambdaj allows filtering collections without writing loops or inner classes:

List<Person> beerDrinkers = select(persons, having(on(Person.class).getAge(),
    greaterThan(16)));

Can you imagine something more readable?

Disclaimer: I am a contributor on lambdaj

Caducous

Nice but the static imports obfuscate whats going on. For reference, select/having/on are static imports on ch.lambdaj.Lambda, greaterThan is org.hamcrest.Matchers – Leucocratic

LambdaJ is really sexy, but it worths noting that it implies a significant overhead (2.6 average): code.google.com/p/lambdaj/wiki/PerformanceAnalysis. – Gala

Apparently doesn't work on Android: groups.google.com/forum/#!msg/lambdaj/km7uFgvSd3k/grJhgl3ik5sJ – Sayers

Really like this example of LamdaJ...similar to .NET built-in Lambda functions. And where can a person drink at age 16? We should consider adding a localization constraint. :P – Coruscate

God! I hate that leak with Lambdaj, now that I'm using Java 8 I'm searching for removing every single use of Lambdaj – Phoenicia

I tried to incorporated Lambdaj but it was too difficult and it was not readable to people that did not know Lambdaj. That is just my experience though. – Flesher

Too bad it can't use enums. I switched to Apache Commons. – Melodic

This answer would be even better if it also mentioned the simplest method (since Java 8): Collection.removeIf – Personalize

I've edited your answer to include Collection#removeIf. Hope you don't mind! – Kollwitz

I wonder why the top half was not given as a new answer. If this went through the review edits pipeline today, the edit would probably be rejected. How many upvotes are for the original question and how many are for the edit? – Cornflakes

removeIf example should be persons.removeIf(p -> p.getAge() <= 16); – Vevina

Caution: if you use lambdas inside an Eclipselink (<2.7) entity class, it will give strange Exceptions – Malaysia

Java (8) finally caught up to dotNet Linq. Yay! #circa2007 en.wikipedia.org/wiki/Language_Integrated_Query – Stefa

Can you imagine something more readable? -- have you checked out Kotlin? ;) persons.filter { it.age >= 16 } – Trophy

In C# this is just persons.Where(p => p.getAge() > 16).ToList(). Any idea why java requires boilerplate like stream, Collectors.toList etc? – Deloresdeloria

Call requires API level 24 (current min is 19): `java.util.Collection#removeIf – Runic

it saved my time. – Loyola

Answer 2

Assuming that you are using Java 1.5, and that you cannot add Google Collections, I would do something very similar to what the Google guys did. This is a slight variation on Jon's comments.

First add this interface to your codebase.

public interface IPredicate<T> { boolean apply(T type); }

Its implementers can answer when a certain predicate is true of a certain type. E.g. If T were User and AuthorizedUserPredicate<User> implements IPredicate<T>, then AuthorizedUserPredicate#apply returns whether the passed in User is authorized.

Then in some utility class, you could say

public static <T> Collection<T> filter(Collection<T> target, IPredicate<T> predicate) {
    Collection<T> result = new ArrayList<T>();
    for (T element: target) {
        if (predicate.apply(element)) {
            result.add(element);
        }
    }
    return result;
}

So, assuming that you have the use of the above might be

Predicate<User> isAuthorized = new Predicate<User>() {
    public boolean apply(User user) {
        // binds a boolean method in User to a reference
        return user.isAuthorized();
    }
};
// allUsers is a Collection<User>
Collection<User> authorizedUsers = filter(allUsers, isAuthorized);

If performance on the linear check is of concern, then I might want to have a domain object that has the target collection. The domain object that has the target collection would have filtering logic for the methods that initialize, add and set the target collection.

UPDATE:

In the utility class (let's say Predicate), I have added a select method with an option for default value when the predicate doesn't return the expected value, and also a static property for params to be used inside the new IPredicate.

public class Predicate {
    public static Object predicateParams;

    public static <T> Collection<T> filter(Collection<T> target, IPredicate<T> predicate) {
        Collection<T> result = new ArrayList<T>();
        for (T element : target) {
            if (predicate.apply(element)) {
                result.add(element);
            }
        }
        return result;
    }

    public static <T> T select(Collection<T> target, IPredicate<T> predicate) {
        T result = null;
        for (T element : target) {
            if (!predicate.apply(element))
                continue;
            result = element;
            break;
        }
        return result;
    }

    public static <T> T select(Collection<T> target, IPredicate<T> predicate, T defaultValue) {
        T result = defaultValue;
        for (T element : target) {
            if (!predicate.apply(element))
                continue;
            result = element;
            break;
        }
        return result;
    }
}

The following example looks for missing objects between collections:

List<MyTypeA> missingObjects = (List<MyTypeA>) Predicate.filter(myCollectionOfA,
    new IPredicate<MyTypeA>() {
        public boolean apply(MyTypeA objectOfA) {
            Predicate.predicateParams = objectOfA.getName();
            return Predicate.select(myCollectionB, new IPredicate<MyTypeB>() {
                public boolean apply(MyTypeB objectOfB) {
                    return objectOfB.getName().equals(Predicate.predicateParams.toString());
                }
            }) == null;
        }
    });

The following example, looks for an instance in a collection, and returns the first element of the collection as default value when the instance is not found:

MyType myObject = Predicate.select(collectionOfMyType, new IPredicate<MyType>() {
public boolean apply(MyType objectOfMyType) {
    return objectOfMyType.isDefault();
}}, collectionOfMyType.get(0));

UPDATE (after Java 8 release):

It's been several years since I (Alan) first posted this answer, and I still cannot believe I am collecting SO points for this answer. At any rate, now that Java 8 has introduced closures to the language, my answer would now be considerably different, and simpler. With Java 8, there is no need for a distinct static utility class. So if you want to find the 1st element that matches your predicate.

final UserService userService = ... // perhaps injected IoC
final Optional<UserModel> userOption = userCollection.stream().filter(u -> {
    boolean isAuthorized = userService.isAuthorized(u);
    return isAuthorized;
}).findFirst();

The JDK 8 API for optionals has the ability to get(), isPresent(), orElse(defaultUser), orElseGet(userSupplier) and orElseThrow(exceptionSupplier), as well as other 'monadic' functions such as map, flatMap and filter.

If you want to simply collect all the users which match the predicate, then use the Collectors to terminate the stream in the desired collection.

final UserService userService = ... // perhaps injected IoC
final List<UserModel> userOption = userCollection.stream().filter(u -> {
    boolean isAuthorized = userService.isAuthorized(u);
    return isAuthorized;
}).collect(Collectors.toList());

See here for more examples on how Java 8 streams work.

Youthen

Yeah, but I hate to reinvent the wheel, again, repeatedly. I'd rather find some utility library that does when I want. – Evadne

This isn't the best way in case you don't want the new collection. Use the filter iterator metaphor, which may input into a new collection, or it may be all that you a need. – Spidery

@Nestor: in a Scala comprehension, filtering would be much simpler: val authorized = for (user <- users if user.isAuthorized) yield user – Youthen

Does this modify the original collection or create a brand new one? I tried using this method and logged both my collections(the original and the one returned from the method), they are the same. @Youthen – Gmt

Any idea if this method is meant to modify the original collection. I tried using this method, it ended up filtering my original collection too. – Gmt

@Rohan, this is not meant to mutate the original collection. Note that the result collection above is newly constructed, and the filter method adds to the result collection only if the predicate applies. – Youthen

Answer 3

Use CollectionUtils.filter(Collection,Predicate), from Apache Commons.

Evadne

this is okay, but it's no generic, and modifies the collection in place (not nice) – Evadne

There are other filter methods in CollectionUtils that do not modify the original collection. – Conium

In particular, the method that does not modify the collection in place is org.apache.commons.collections.CollectionUtils#select(Collection,Predicate) – Camber

In Commons Collections v4 this now uses Generics. – Weiler

This method should be used with caution as it relies (at least in implementation of commons-collections-3.2.1) on iterator.remove() method which is optional for the collections, so instead of filtering, say, an array, you might get an UnsupportedOperationException. – Illmannered

Answer 4

"Best" way is too wide a request. Is it "shortest"? "Fastest"? "Readable"? Filter in place or into another collection?

Simplest (but not most readable) way is to iterate it and use Iterator.remove() method:

Iterator<Foo> it = col.iterator();
while( it.hasNext() ) {
  Foo foo = it.next();
  if( !condition(foo) ) it.remove();
}

Now, to make it more readable, you can wrap it into a utility method. Then invent a IPredicate interface, create an anonymous implementation of that interface and do something like:

CollectionUtils.filterInPlace(col,
  new IPredicate<Foo>(){
    public boolean keepIt(Foo foo) {
      return foo.isBar();
    }
  });

where filterInPlace() iterate the collection and calls Predicate.keepIt() to learn if the instance to be kept in the collection.

I don't really see a justification for bringing in a third-party library just for this task.

Nucleolar

My vote goes for this one: it just works, with no external libraries. I never realized instantiating an Iterator could actually be useful compared to using the for-each syntax, or that you could remove items from a list without a ConcurrentModificationException or something like that. :) – Chemotherapy

I think this is the best way using standard Java lib without copying. For 1.8 there would be the stream() feature, but not everyone gets to play with the newest toys :P – Davisdavison

Question title edited to not ask for subjective "best" – Borrell

Answer 5

Consider Google Collections for an updated Collections framework that supports generics.

UPDATE: The google collections library is now deprecated. You should use the latest release of Guava instead. It still has all the same extensions to the collections framework including a mechanism for filtering based on a predicate.

Bole

ya, I knew about the Google collections lib. The version I was using didn't have Collections2 in it. I added a new answer to this question that lists the specific method. – Evadne

Kevin, Iterables.filter() and Iterators.filter() have been there from the beginning, and are usually all you need. – Unlisted

Answer 6

Wait for Java 8:

List<Person> olderThan30 = 
  //Create a Stream from the personList
  personList.stream().
  //filter the element to select only those with age >= 30
  filter(p -> p.age >= 30).
  //put those filtered elements into a new List.
  collect(Collectors.toList());

Laritalariviere

Ugh...it's so verbose. Why couldn't they just do: List<Person> result = personList.filter(p -> p.age > 30); – Evadne

@KevinWong I actually get with example from official tutorial so don't know about another syntax... – Laritalariviere

I don't doubt that this is the official syntax. I'm just saying that it kinda sucks. – Evadne

To use filter directly on Collection you need use removeIf call: download.java.net/jdk8/docs/api/java/util/… – Laritalariviere

@KevinWong "verbose" pretty much describes the whole language I would think. At least they're consistent? – Porett

Why not use Collectors.toList() in the last part? – Phoenicia

Here is a link gavenkoa provided that doesn't 404. personList.removeIf(p -> p.age < 30); Less verbose. Also, I've heard talk about starting to implement apis that accept and return Streams rather than Collections because Streams are very useful and fast but going to/from them is slow. – Broughton

Answer 7

Since the early release of Java 8, you could try something like:

Collection<T> collection = ...;
Stream<T> stream = collection.stream().filter(...);

For example, if you had a list of integers and you wanted to filter the numbers that are > 10 and then print out those numbers to the console, you could do something like:

List<Integer> numbers = Arrays.asList(12, 74, 5, 8, 16);
numbers.stream().filter(n -> n > 10).forEach(System.out::println);

Halvorson

Answer 8

I'll throw RxJava in the ring, which is also available on Android. RxJava might not always be the best option, but it will give you more flexibility if you wish add more transformations on your collection or handle errors while filtering.

Observable.from(Arrays.asList(1, 2, 3, 4, 5))
    .filter(new Func1<Integer, Boolean>() {
        public Boolean call(Integer i) {
            return i % 2 != 0;
        }
    })
    .subscribe(new Action1<Integer>() {
        public void call(Integer i) {
            System.out.println(i);
        }
    });

Output:

1
3
5

More details on RxJava's filter can be found here.

Drucilladrucy

Answer 9

Since java 9 Collectors.filtering is enabled:

public static <T, A, R>
    Collector<T, ?, R> filtering(Predicate<? super T> predicate,
                                 Collector<? super T, A, R> downstream)

Thus filtering should be:

collection.stream().collect(Collectors.filtering(predicate, collector))

Example:

List<Integer> oddNumbers = List.of(1, 19, 15, 10, -10).stream()
            .collect(Collectors.filtering(i -> i % 2 == 1, Collectors.toList()));

Resting

Answer 10

Are you sure you want to filter the Collection itself, rather than an iterator?

see org.apache.commons.collections.iterators.FilterIterator

or using version 4 of apache commons org.apache.commons.collections4.iterators.FilterIterator

Elliotelliott

Answer 11

The setup:

public interface Predicate<T> {
  public boolean filter(T t);
}

void filterCollection(Collection<T> col, Predicate<T> predicate) {
  for (Iterator i = col.iterator(); i.hasNext();) {
    T obj = i.next();
    if (predicate.filter(obj)) {
      i.remove();
    }
  }
}

The usage:

List<MyObject> myList = ...;
filterCollection(myList, new Predicate<MyObject>() {
  public boolean filter(MyObject obj) {
    return obj.shouldFilter();
  }
});

Emirate

Fine, but I prefer Alan implementation because you get a copy of the collection instead of altering it. Moreover, Alan's code is thread safe while yours is not. – Cloy

Answer 12

Let’s look at how to filter a built-in JDK List and a MutableList using Eclipse Collections.

List<Integer> jdkList = Arrays.asList(1, 2, 3, 4, 5);
MutableList<Integer> ecList = Lists.mutable.with(1, 2, 3, 4, 5);

If you wanted to filter the numbers less than 3, you would expect the following outputs.

List<Integer> selected = Lists.mutable.with(1, 2);
List<Integer> rejected = Lists.mutable.with(3, 4, 5);

Here’s how you can filter using a Java 8 lambda as the Predicate.

Assert.assertEquals(selected, Iterate.select(jdkList, each -> each < 3));
Assert.assertEquals(rejected, Iterate.reject(jdkList, each -> each < 3));

Assert.assertEquals(selected, ecList.select(each -> each < 3));
Assert.assertEquals(rejected, ecList.reject(each -> each < 3));

Here’s how you can filter using an anonymous inner class as the Predicate.

Predicate<Integer> lessThan3 = new Predicate<Integer>()
{
    public boolean accept(Integer each)
    {
        return each < 3;
    }
};

Assert.assertEquals(selected, Iterate.select(jdkList, lessThan3));
Assert.assertEquals(selected, ecList.select(lessThan3));

Here are some alternatives to filtering JDK lists and Eclipse Collections MutableLists using the Predicates factory.

Assert.assertEquals(selected, Iterate.select(jdkList, Predicates.lessThan(3)));
Assert.assertEquals(selected, ecList.select(Predicates.lessThan(3)));

Here is a version that doesn't allocate an object for the predicate, by using the Predicates2 factory instead with the selectWith method that takes a Predicate2.

Assert.assertEquals(
    selected, ecList.selectWith(Predicates2.<Integer>lessThan(), 3));

Sometimes you want to filter on a negative condition. There is a special method in Eclipse Collections for that called reject.

Assert.assertEquals(rejected, Iterate.reject(jdkList, lessThan3));
Assert.assertEquals(rejected, ecList.reject(lessThan3));

The method partition will return two collections, containing the elements selected by and rejected by the Predicate.

PartitionIterable<Integer> jdkPartitioned = Iterate.partition(jdkList, lessThan3);
Assert.assertEquals(selected, jdkPartitioned.getSelected());
Assert.assertEquals(rejected, jdkPartitioned.getRejected());

PartitionList<Integer> ecPartitioned = gscList.partition(lessThan3);
Assert.assertEquals(selected, ecPartitioned.getSelected());
Assert.assertEquals(rejected, ecPartitioned.getRejected());

Note: I am a committer for Eclipse Collections.

Trogon

How would you do a removeIf on a list or set for primitives? – Barr

The API for removeIf was added to primitive collections in EC 9.1. eclipse.org/collections/javadoc/9.1.0/org/eclipse/collections/… – Trogon

Answer 13

How about some plain and straighforward Java

 List<Customer> list ...;
 List<Customer> newList = new ArrayList<>();
 for (Customer c : list){
    if (c.getName().equals("dd")) newList.add(c);
 }

Simple, readable and easy (and works in Android!) But if you're using Java 8 you can do it in a sweet one line:

List<Customer> newList = list.stream().filter(c -> c.getName().equals("dd")).collect(toList());

Note that toList() is statically imported

Phoenicia

Answer 14

The Collections2.filter(Collection,Predicate) method in Google's Guava library does just what you're looking for.

Evadne

Answer 15

With the ForEach DSL you may write

import static ch.akuhn.util.query.Query.select;
import static ch.akuhn.util.query.Query.$result;
import ch.akuhn.util.query.Select;

Collection<String> collection = ...

for (Select<String> each : select(collection)) {
    each.yield = each.value.length() > 3;
}

Collection<String> result = $result();

Given a collection of [The, quick, brown, fox, jumps, over, the, lazy, dog] this results in [quick, brown, jumps, over, lazy], ie all strings longer than three characters.

All iteration styles supported by the ForEach DSL are

• AllSatisfy
• AnySatisfy
• Collect
• Counnt
• CutPieces
• Detect
• GroupedBy
• IndexOf
• InjectInto
• Reject
• Select

For more details, please refer to https://www.iam.unibe.ch/scg/svn_repos/Sources/ForEach

Speck

That's pretty clever! A lot of work to implement a nice Ruby-ish syntax though! The negative is that your filter is not a first-class function and hence cannot be re-used. Roll on closures... – Absorbed

Good point. One way to reuse the loop body is by refactoring the loop into a method that takes the selection query as parameter. That is however by far not as handy and powerful as real closures, for sure. – Speck

Answer 16

This, combined with the lack of real closures, is my biggest gripe for Java. Honestly, most of the methods mentioned above are pretty easy to read and REALLY efficient; however, after spending time with .Net, Erlang, etc... list comprehension integrated at the language level makes everything so much cleaner. Without additions at the language level, Java just cant be as clean as many other languages in this area.

If performance is a huge concern, Google collections is the way to go (or write your own simple predicate utility). Lambdaj syntax is more readable for some people, but it is not quite as efficient.

And then there is a library I wrote. I will ignore any questions in regard to its efficiency (yea, its that bad)...... Yes, i know its clearly reflection based, and no I don't actually use it, but it does work:

LinkedList<Person> list = ......
LinkedList<Person> filtered = 
           Query.from(list).where(Condition.ensure("age", Op.GTE, 21));

OR

LinkedList<Person> list = ....
LinkedList<Person> filtered = Query.from(list).where("x => x.age >= 21");

Dalmatian

Link? Even if your library is inefficient or otherwise unusable it might be interesting to look at if the source is available. – Birgit

Made the repo public (net-machine.com/indefero/p/jdclib/source/tree/master). You are interested in the expression package. The test package has a tester with example usage. I never really did much work on the string query interface referenced above (didnt feel like writing a real parser), so the explicit query interface in the tester is the way to go. – Dalmatian

Answer 17

Using java 8, specifically lambda expression, you can do it simply like the below example:

myProducts.stream().filter(prod -> prod.price>10).collect(Collectors.toList())

where for each product inside myProducts collection, if prod.price>10, then add this product to the new filtered list.

Baryon

Answer 18

In Java 8, You can directly use this filter method and then do that.

 List<String> lines = Arrays.asList("java", "pramod", "example");

 List<String> result = lines.stream()              
         .filter(line -> !"pramod".equals(line))     
         .collect(Collectors.toList());              

 result.forEach(System.out::println); 

Gamesome

Answer 19

I wrote an extended Iterable class that support applying functional algorithms without copying the collection content.

Usage:

List<Integer> myList = new ArrayList<Integer>(){ 1, 2, 3, 4, 5 }

Iterable<Integer> filtered = Iterable.wrap(myList).select(new Predicate1<Integer>()
{
    public Boolean call(Integer n) throws FunctionalException
    {
        return n % 2 == 0;
    }
})

for( int n : filtered )
{
    System.out.println(n);
}

The code above will actually execute

for( int n : myList )
{
    if( n % 2 == 0 ) 
    {
        System.out.println(n);
    }
}

Laverne

Answer 20

JFilter http://code.google.com/p/jfilter/ is best suited for your requirement.

JFilter is a simple and high performance open source library to query collection of Java beans.

Key features

• Support of collection (java.util.Collection, java.util.Map and Array) properties.
• Support of collection inside collection of any depth.
• Support of inner queries.
• Support of parameterized queries.
• Can filter 1 million records in few 100 ms.
• Filter ( query) is given in simple json format, it is like Mangodb queries. Following are some examples.
• { "id":{"$le":"10"}
  • where object id property is less than equals to 10.
• { "id": {"$in":["0", "100"]}}
  • where object id property is 0 or 100.
• {"lineItems":{"lineAmount":"1"}}
  • where lineItems collection property of parameterized type has lineAmount equals to 1.
• { "$and":[{"id": "0"}, {"billingAddress":{"city":"DEL"}}]}
  • where id property is 0 and billingAddress.city property is DEL.
• {"lineItems":{"taxes":{ "key":{"code":"GST"}, "value":{"$gt": "1.01"}}}}
  • where lineItems collection property of parameterized type which has taxes map type property of parameteriszed type has code equals to GST value greater than 1.01.
• {'$or':[{'code':'10'},{'skus': {'$and':[{'price':{'$in':['20', '40']}}, {'code':'RedApple'}]}}]}
  • Select all products where product code is 10 or sku price in 20 and 40 and sku code is "RedApple".

Greenes

You should disclaim that you are the author (as I think it is the case). – Bridal

Yes, I am the author of this library. – Greenes

Answer 21

Use Collection Query Engine (CQEngine). It is by far the fastest way to do this.

See also: How do you query object collections in Java (Criteria/SQL-like)?

Sprage

Answer 22

Some really great great answers here. Me, I'd like to keep thins as simple and readable as possible:

public abstract class AbstractFilter<T> {

    /**
     * Method that returns whether an item is to be included or not.
     * @param item an item from the given collection.
     * @return true if this item is to be included in the collection, false in case it has to be removed.
     */
    protected abstract boolean excludeItem(T item);

    public void filter(Collection<T> collection) {
        if (CollectionUtils.isNotEmpty(collection)) {
            Iterator<T> iterator = collection.iterator();
            while (iterator.hasNext()) {
                if (excludeItem(iterator.next())) {
                    iterator.remove();
                }
            }
        }
    }
}

Ligulate

Just implement the proper excludeItem per filter. You'll end up having separate filters exactly as you have sorters on Collections... – Ligulate

Answer 23

The simple pre-Java8 solution:

ArrayList<Item> filtered = new ArrayList<Item>(); 
for (Item item : items) if (condition(item)) filtered.add(item);

Unfortunately this solution isn't fully generic, outputting a list rather than the type of the given collection. Also, bringing in libraries or writing functions that wrap this code seems like overkill to me unless the condition is complex, but then you can write a function for the condition.

Villainous

Answer 24

https://code.google.com/p/joquery/

Supports different possibilities,

Given collection,

Collection<Dto> testList = new ArrayList<>();

of type,

class Dto
{
    private int id;
    private String text;

    public int getId()
    {
        return id;
    }

    public int getText()
    {
        return text;
    }
}

Filter

Java 7

Filter<Dto> query = CQ.<Dto>filter(testList)
    .where()
    .property("id").eq().value(1);
Collection<Dto> filtered = query.list();

Java 8

Filter<Dto> query = CQ.<Dto>filter(testList)
    .where()
    .property(Dto::getId)
    .eq().value(1);
Collection<Dto> filtered = query.list();

Also,

Filter<Dto> query = CQ.<Dto>filter()
        .from(testList)
        .where()
        .property(Dto::getId).between().value(1).value(2)
        .and()
        .property(Dto::grtText).in().value(new string[]{"a","b"});

Sorting (also available for the Java 7)

Filter<Dto> query = CQ.<Dto>filter(testList)
        .orderBy()
        .property(Dto::getId)
        .property(Dto::getName)
    Collection<Dto> sorted = query.list();

Grouping (also available for the Java 7)

GroupQuery<Integer,Dto> query = CQ.<Dto,Dto>query(testList)
        .group()
        .groupBy(Dto::getId)
    Collection<Grouping<Integer,Dto>> grouped = query.list();

Joins (also available for the Java 7)

Given,

class LeftDto
{
    private int id;
    private String text;

    public int getId()
    {
        return id;
    }

    public int getText()
    {
        return text;
    }
}

class RightDto
{
    private int id;
    private int leftId;
    private String text;

    public int getId()
    {
        return id;
    }

    public int getLeftId()
        {
            return leftId;
        }

    public int getText()
    {
        return text;
    }
}

class JoinedDto
{
    private int leftId;
    private int rightId;
    private String text;

    public JoinedDto(int leftId,int rightId,String text)
    {
        this.leftId = leftId;
        this.rightId = rightId;
        this.text = text;
    }

    public int getLeftId()
    {
        return leftId;
    }

    public int getRightId()
        {
            return rightId;
        }

    public int getText()
    {
        return text;
    }
}

Collection<LeftDto> leftList = new ArrayList<>();

Collection<RightDto> rightList = new ArrayList<>();

Can be Joined like,

Collection<JoinedDto> results = CQ.<LeftDto, LeftDto>query().from(leftList)
                .<RightDto, JoinedDto>innerJoin(CQ.<RightDto, RightDto>query().from(rightList))
                .on(LeftFyo::getId, RightDto::getLeftId)
                .transformDirect(selection ->  new JoinedDto(selection.getLeft().getText()
                                                     , selection.getLeft().getId()
                                                     , selection.getRight().getId())
                                 )
                .list();

Expressions

Filter<Dto> query = CQ.<Dto>filter()
    .from(testList)
    .where()
    .exec(s -> s.getId() + 1).eq().value(2);

Eleonoreeleoptene

Answer 25

My answer builds on that from Kevin Wong, here as a one-liner using CollectionUtils from spring and a Java 8 lambda expression.

CollectionUtils.filter(list, p -> ((Person) p).getAge() > 16);

This is as concise and readable as any alternative I have seen (without using aspect-based libraries)

Spring CollectionUtils is available from spring version 4.0.2.RELEASE, and remember you need JDK 1.8 and language level 8+.

Viole

Answer 26

I needed to filter a list depending on the values already present in the list. For example, remove all values following that is less than the current value. {2 5 3 4 7 5} -> {2 5 7}. Or for example to remove all duplicates {3 5 4 2 3 5 6} -> {3 5 4 2 6}.

public class Filter {
    public static <T> void List(List<T> list, Chooser<T> chooser) {
        List<Integer> toBeRemoved = new ArrayList<>();
        leftloop:
        for (int right = 1; right < list.size(); ++right) {
            for (int left = 0; left < right; ++left) {
                if (toBeRemoved.contains(left)) {
                    continue;
                }
                Keep keep = chooser.choose(list.get(left), list.get(right));
                switch (keep) {
                    case LEFT:
                        toBeRemoved.add(right);
                        continue leftloop;
                    case RIGHT:
                        toBeRemoved.add(left);
                        break;
                    case NONE:
                        toBeRemoved.add(left);
                        toBeRemoved.add(right);
                        continue leftloop;
                }
            }
        }

        Collections.sort(toBeRemoved, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o2 - o1;
            }
        });

        for (int i : toBeRemoved) {
            if (i >= 0 && i < list.size()) {
                list.remove(i);
            }
        }
    }

    public static <T> void List(List<T> list, Keeper<T> keeper) {
        Iterator<T> iterator = list.iterator();
        while (iterator.hasNext()) {
            if (!keeper.keep(iterator.next())) {
                iterator.remove();
            }
        }
    }

    public interface Keeper<E> {
        boolean keep(E obj);
    }

    public interface Chooser<E> {
        Keep choose(E left, E right);
    }

    public enum Keep {
        LEFT, RIGHT, BOTH, NONE;
    }
}

This will bee used like this.

List<String> names = new ArrayList<>();
names.add("Anders");
names.add("Stefan");
names.add("Anders");
Filter.List(names, new Filter.Chooser<String>() {
    @Override
    public Filter.Keep choose(String left, String right) {
        return left.equals(right) ? Filter.Keep.LEFT : Filter.Keep.BOTH;
    }
});

Shipping

Answer 27

In my case, I was looking for list with specific field null excluded. This could be done with for loop and fill the temporary list of objects who have no null addresses. but Thanks to Java 8 Streams

List<Person> personsList = persons.stream()
.filter(p -> p.getAdrress() != null).collect(Collectors.toList());

#java #collection #collections #java8 #streams

Pismire

Answer 28

With Guava:

Collection<Integer> collection = Lists.newArrayList(1, 2, 3, 4, 5);

Iterators.removeIf(collection.iterator(), new Predicate<Integer>() {
    @Override
    public boolean apply(Integer i) {
        return i % 2 == 0;
    }
});

System.out.println(collection); // Prints 1, 3, 5

Managerial

Answer 29

An alternative (more lightweight) alternative to Java collection streams is the Ocl.java library, which uses vanilla collections and lambdas: https://github.com/eclipse/agileuml/blob/master/Ocl.java

For example, a simple filter and sum on an ArrayList words could be:

ArrayList<Word> sel = Ocl.selectSequence(words, 
                             w -> w.pos.equals("NN")); 
int total = Ocl.sumint(Ocl.collectSequence(sel,
                             w -> w.text.length())); 

Where Word has String pos; String text; attributes. Efficiency seems similar to the streams option, eg, 10000 words are processed in about 50ms in both versions.

There are equivalent OCL libraries for Python, Swift, etc. Basically Java collection streams has re-invented the OCL operations ->select, ->collect, etc, which existed in OCL since 1998.

According

Answer 30

I prefer this way:

public static void main(String[] args) {
    
    //Create predicate
    Predicate<Integer> FILTER_ODD_ONES =
        number -> number % 2 != 0;
    
    //Apply predicate
    Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
        .stream()
        .filter(FILTER_ODD_ONES)
        .collect(Collectors.toList());
  }

Laverty

This is the same as the accepted answer. – Hostetter

Answer 31

Elaborating a little bit on the answer that was given,

List<Person> beerDrinkers = persons.stream().filter(p -> p.getAge() > 16).collect(Collectors.toList());

persons.stream(): converts the List<Person> called persons into a stream of Person objects.

.filter(p -> p.getAge() > 16): is the filtering operation. It applies the provided predicate (lambda expression) to each Person object in the stream. In this case, it filters out any Person objects whose age is not greater than 16.

.collect(Collectors.toList()): This collects the filtered Person objects into a new List<Person>.

So, beerDrinkers will contain only those Person objects whose age is greater than 16.

Eringo