What is the best alternative for linq Java?

To get full LINQ java 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 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.