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Are there any libraries in Java that come close to providing the functionality of Linq?

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I presume you mean "the syntax of LINQ" rather than "the functionality of LINQ". There are many libraries that provide the same functionality :-) –  mikera Sep 12 '12 at 5:07
    
Java equivalent is most likely Groovy. At the expense of loosing type safety though –  gnat Jul 16 '13 at 18:07
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closed as primarily opinion-based by gnat, GlenH7, MichaelT, Kilian Foth, ChrisF Jul 17 '13 at 9:00

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question.

7 Answers

up vote 19 down vote accepted

To get full LINQ query syntax, you would need an update to the language spec and compilers supporting the new keywords and syntax structure. This is far beyond most readers here.

In order to get Linq-like method-chain syntax in Java that looks and works like what's in C#, you need two things Java doesn't support; extension methods (static methods that can be applied to an instance as if it were an instance method), and lambda expressions (anonymous delegates). However, those two can be spoofed with existing Java constructs, if you're willing to incur a little extra coding verbosity. One thing I find particularly amazing about Java is the ability to create anonymous interface implementations; applied to an interface with a single pure function, that basically allows for simple anonymous delegates, provided you have an interface for each method signature you'll need.

So, the IEnumerable (Iterable for the Javaheads) side of Linq, as a method chain, should be possible to approximate in Java; you'd basically implement it as a fluent interface that controls a monad. You'd need a class like a LinqIterable, implementing Iterable, that:

  • can convert any Iterable input into a LinqIterable instance
  • has methods for basic list processing, such as filtering, grouping, concatenation, intersection, ordering, etc that accept single-function interfaces and produce new LinqIterables, allowing chaining of these methods, hopefully in a "lazy" fashion.
  • has methods to transform the LinqIterable back into "concrete" objects such as ToList, ToArray, ToMap, etc

So, you could probably define a "fluent" library that would allow a statement such as:

Map<String, Integer> results = Linq.FromIterable(sourceList)
                           .Where(new Predicate{public bool Op(SomeStruct in){return in.SomeString == "criteria";}})
                           .OrderBy(new Func{public String Op(SomeStruct in){return in.SomeOtherString;}})
                           .Select(new Func{public SomeOtherStruct Op(SomeStruct in){return new SomeOtherStruct(in.SomeOtherString, in.SomeInt);}})
                           .ToMap(new Func{public String Op(SomeOtherStruct in){return in.StringField;}},
                                  new Func{public Integer Op(SomeOtherStruct in){return in.IntField;}});

Creation of the actual library is an exercise far beyond the scope of an answer on an internet Q&A. I just said it was possible, not easy.

Expression trees are not rocket science, but it would be a significant project to implement what .NET has in this area; not only do you need the ability to build an expression tree, you need to be able to "compile" this tree on the fly using emitted bytecodes; I don't know if Java's reflection is quite that powerful.

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Funnily enough extension methods and lambda expressions are both in Java 8. Perhaps LINQ is on the way? –  lxs Jul 16 '12 at 15:55
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Well if you have extension methods and lambdas, you have the "Enumerable" side of .NET's Linq without a huge effort. It will still, as I said, require a change to the Java spec to allow the keywords that make a Linq query, but honestly in day-to-day Linq use I've not found too many queries that aren't just as easily expressed as a method chain, and the compiler pretty much converts Linq syntax to method chains (VS plugins exist that can convert 1:1 between most queries and chains). The next real stepping stone will be expression trees (Queryable Linq). –  KeithS Jul 16 '12 at 17:25
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See:

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would you mind expanding a bit on what each of these resources have and why do you recommend these as answering the question asked? "Link-only answers" are not quite welcome at Stack Exchange –  gnat Jul 17 '13 at 1:32
    
Hmm, I guess it's a case that this Q had already been answered - should be modded as a duplicate? –  Martijn Verburg Jul 17 '13 at 7:43
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@MartijnVerburg - we can't mark questions as duplicates across sites. Answers really do need to be able to stand alone. –  ChrisF Jul 17 '13 at 9:00
    
Edited, hopefully it's a little more helpful now. –  Martijn Verburg Jul 17 '13 at 9:16
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No. Linq requires lots of langage features that Java just dosent have

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Java does have generics. –  Jonas Oct 18 '11 at 16:53
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@Jonas: That's true, but whether they're actually strong enough to support LINQ is another matter. –  DeadMG Oct 18 '11 at 17:06
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@DeadMG still, Java does have generics. –  Mahmoud Hossam Oct 18 '11 at 20:53
    
Edited to remove the reference to generics –  Tom Squires Oct 18 '11 at 21:49
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Use Scala, for the reasons explained here:

use the .NET port of Scala. It gives you full native access to everything in .NET, which obviously includes LINQ.

If you can't, you can try to use libs like functionjava or lambdaj, but you won't come really close to LINQ.

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Scala doesn't include anything like Linq without libraries. –  Jonas Oct 18 '11 at 18:46
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@Jonas: So? Java doesn't include anything like Linq with libraries. –  back2dos Oct 18 '11 at 19:02
    
@back2dos: Yes, thats why you have to answer with a library. –  Jonas Oct 18 '11 at 19:25
    
@Jonas: Scala's standard library is like LINQ. Or the other way round: LINQ is just a set of functional programming idioms for C#. More on that here. –  back2dos Oct 18 '11 at 21:31
    
@back2dos: Thanks for the great link, I took the freedom to include it. –  Landei Oct 19 '11 at 7:45
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I'd add guava to the list of libraries that may help you achieve some of the things LINQ does.

But as others have pointed out, there is no equivalent in pure Java.

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would you mind explaining more on what it does and why do you recommend it as answering the question asked? "Link-only answers" are not quite welcome at Stack Exchange –  gnat Jul 17 '13 at 1:32
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You could use Korma, a DSL for SQL queries written in Clojure.

Syntax is pretty neat:

(select user
  (with address)
  (fields :firstName :lastName :address.state)
  (where {:email "korma@sqlkorma.com"}))

You can call this pretty easily from Java by importing the relevant Clojure libraries and just using the normal Clojure/Java interop.

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So, this will emulate Linq over SQL, not LINQ over objects, right? –  Job Sep 24 '12 at 2:00
    
@Job: yes, it is designed for SQL. Though I don't see any reason that it couldn't be extended / adapted for objects as well. –  mikera Sep 24 '12 at 2:15
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There isn't a native feature in java that mimics linq. However, there are several APIs that provide the same functionality.

For example, this blog post has a custom jar to replicate linq.
http://javaworldwide.blogspot.in/2012/09/linq-in-java.html

And this is an example usage of the API from the post:

Suppose we have an Animal object containing name and ID which is represented with a list of animals. If we want to get the all the Animal names which contains 'o' from the list animals, we can write the following query

from(animals).where("getName", contains("o")).all();

There are two other APIs referenced in the blog posting: lambdaJ and jLinq

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I'm afraid that losing both static type checking and checking of the very existence of a class member is a bit steep a price to call for a call like where("getName", ...). –  9000 Sep 24 '12 at 4:26
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