Is there something like LINQ for Java?

It's important to note that LINQ are four things:

• Monadic comprehension
• Database integration
• SQL-like syntax
• AST manipulation

People who just have heard of it may think of it simply as database integration. People who have worked a little with it probably think of SQL-like syntax. Those who really dug in will be aware of the monadic comprehension aspect of it, even if they don't know it for what it is.

If one takes Scala, for example, it has monadic comprehension without the other three. There is a library called ScalaQuery which provides database integration through the monadic comprehension (the intrinsic ability to do so being the main reason monads are cool). Another project, called ScalaQL, I think, intends to provide pretty much the same thing, but using a compiler plugin to enhance it. I wasn't aware of Miguel Garcia's work you mentioned, but, having seen other stuff he has accomplished, I'm thrilled by it.

One doesn't need special syntax to do monadic comprehension, however. It just makes it uncluttered by boilerplate. So that aspect of it is instantly available to languages with the right level of generics support.

Two things Scala doesn't do. The first is SQL-like syntax. That much can't be helped: SQL syntax looks out of place in Scala. I think it's safe to say most Scala programmers would prefer to stay with what is familiar to them -- the so-called for comprehensions.

The other thing is the one I haven't discussed yet, AST manipulation. That is the ability to manipulate code that has been parsed by the compiler, but not yet transformed in byte code, granting the ability to alter it before the generation is completed.

I think such a thing would be a boon to Scala -- heck, to any language. But, then again, I have a background as a Forth programmer, where the ability to alter code as it was being compiled was a God-given right. .Net can do it through LINQ, and so can some other languages, such as Ruby.

LINQ would be hard in Java due to the current lack of closures. Assuming Java 7 really does get reasonably compact closure support and extension methods, LINQ in terms of "dot notation" should be feasible even if it doesn't get the equivalent of query expressions.

The Google Collections Library (now at 1.0 - but to be replaced by Guava when that is ready) contain many of the required methods - and I wouldn't be surprised to see 101 LINQ-like APIs spring up as soon as the closure support looks reasonably final.

I can't see (at the moment) Java getting anything like expression trees, however - so I suspect you'll be limited to LINQ to Objects unless you have custom compilation.

Check out Quaere. It's a LINQ-like DSL for Java that you can include as a library. Example:

// Get all StackOverflow users with more than 20,000 reputation points.
Iterable<User> topUsers = from("u").in(entityManager.entity(User.class)).
    where(gt("u.getReputation()", 20000)).
    select("u");

System.out.println("Users with more than 20,000 reputation:");
for (User u : topUsers) {
    System.out.println(u.getName());
}

However, note that Java doesn't have a concept analogous to extension methods. Whatever's in Quaere is pretty much what you're stuck with; if you need to make your own special utilities, they'll probably have to be in separate utility classes (ick).

Additionally, because Java < 7 has no native closures, you're stuck with strings to reference things, and your IDE can't introspect those to show you problems if you mistype something. (A smarter IDE might be able to handle this shortcoming, however, if somebody were to write introspection plugins for Quaere.)

Look at Scala, which is powerful functional programming language, but is similar to Java and runs on Java platform.

In Scala it is possible to use essentially the same code constructs as in LINQ, albeit without special query comprehensions syntax present in C# or VB.

EDIT :

Here's an example of Scala's querying capabilities :

// Get all StackOverflow users with more than 20,000 reputation points.
val topUsers = for{
    u <- users
    if u.reputation > 20000
} yield u;

println ("Users with more than 20,000 reputation:")
for (u <- topUsers) {
    println u.name
}