What is the best way to filter a Java Collection?

Question

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

Answer 1

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.

Answer 2

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);

Answer 3

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);

Answer 4

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

Answer 5

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".

Answer 6

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