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This article introduces how to understand Linq4j in Java. The content is very detailed. Interested friends can use it for reference. I hope it will be helpful to you.

During the development of JAVA for a period of time, a big problem is the collection operation. I am used to the concise syntax of LINQ, but I really don't like the collection operation of JAVA. I can only filter through the loop of C series. Because there is no delayed execution feature, the memory occupation is really unflattering. So linq4j, a portable version of linq for JAVA, was found online.

one。 Installation

The Github address of the project is: https://github.com/julianhyde/linq4j. It's obviously a personal project, paying homage to the author.

It is not deployed in the standard maven library, so it needs to be compiled and generated manually. Use the standard command line:

Git clone git://github.com/julianhyde/linq4j.git linq4j # git Clone to linq4j directory mvn compile # compile mvn test # Test mvn jar:jar # generate jar package

After using maven, the work efficiency has been greatly improved. Of course, there is a similar tool nuget under NET.

two。 Extended functions of Linq4j

Because JAVA currently does not have anonymous functions and extension functions, and the built-in standard iterator interface Iterator is weak. So Linq4j adds a series of generic interfaces and functions:

1. New iterator interface: Enumerable, which extends the functionality of Iterator

two。 A set of functions similar to the "delegate" nature:

(1) return the generic delegate of R: public interface Function {}

(2) receive T and return the generic delegate of R: public interface Function1 {}

(3) receive T1GT2 and return the generic delegate of R, defined as follows:

/ * Function with two parameters. * * @ param result type * @ param type of parameter 1 * @ param type of parameter 2 * / public interface Function2 extends Function {R apply (T1 v1, T2 v2);}

Of course, there are not only these built-in functions, but also a series of non-generic delegates, including the Predicate function that returns Bool. Due to space constraints, we will not introduce them one by one here.

3. A series of Expressions, the specific use of which is described below.

three。 Usage

The library implements most of the functions of LINQ, including filter, sorter, grouper, type conversion and so on. Let's introduce it with an example.

Define an entity first:

Public class Person {public int Age; public String Name; public boolean Sex;}

Our basic task is to pick out all the gender true names in a Person collection and arrange them in the default descending order of string. * you should get the List type.

/ / Linq4j: public void Test (ArrayList persList) {java.util.List nameStrings= Linq4j.asEnumerable (persList) .where (new Predicate1 () {public boolean apply (Person arg0) {return arg0.Sex;}}) .select (new Function1 () {public String apply (Person arg0) {return arg0.Name) }}) .orderByDescending (new Function1 () {public String apply (String arg0) {/ / TODO Auto-generated method stub return arg0;}}) .toList ();}

The style of this code is very similar to that of C #, because the interface Enumerable can be spliced, so it can be implemented through simple Where,Select and orderByDescending. However, because LINQ does not have anonymous functions, it looks like a headache to have to add functions to the functions. In addition, since there is no extension function, you need to use Linq4j's static method before the method.

This function is implemented using standard Linq as follows:

Var userNames = from din persons where d.Sex orderby d.Name descending select d.Name

In .NET, we can use closures, such as in the implementation of filter functions, to access external data. But we can look at the following examples:

The basic logic of this function is to find people whose names are on the blacklist in personList. Set two Linq4j, however, pay attention to the final keyword of the blacklist array, if there is no this key will report an error, JAVA does not have a closure, so the blacklist array should not be modified, whether the advantages of this syntax sugar outweigh the disadvantages, still need to be discussed by readers.

Public List SelectBlackList (ArrayList persList) {final String [] blackList = {"zhang", "wang", "li"}; return Linq4j.asEnumerable (persList) .where (new Predicate1 () {public boolean apply (Person arg0) {return Linq4j.asEnumerable (blackList). Contains (arg0.Name)) }). ToList ();}

This function is implemented using standard Linq as follows:

Public List GetBlacklist (IEnumerable persons) {String [] blackList = {"zhang", "wang", "li"}; var result= from d in persons where blackList.Contains (d.Name) select d; return result.ToList ();}

* discuss collection type conversion, for example, class Worker inheritance implements the Person interface.

Public class Worker: Person {public string Commpay;}

So, a function is defined as void Func (List nodes). And the parameter type I want to pass in is List, and the compiler must report an error! What shall I do?

For .NET, there are inverter and covariant features, or I can do this:

Public void Test3 (Listworkers) {this.Func1 (workers); / / Compiler will report an error this.Func1 (workers.OfType ());} public void Func1 (IEnumerablepersons) {/ / it is just a demonstration and no function is implemented.

For JAVA, the general approach is to add a cast to the outside and convert it to List by creating a new Person collection and foreach iterator using cast. This is too much trouble. With LiNQ4J, we have a similar syntax:

Public void Func2 (List person) {/ / demo, no function} public void Test3 (List workers) / / 1. Through the most simple and rough circular writing, to achieve the function, do not dare to compliment. {/ / Func2 (workers); / / here the compiler will report List persons = new ArrayList (); for (Person person: workers) {persons.add (person);} Func2 (persons) } public void Test4linq (List workers) / / 2.linq4j {List persons = Linq4j.asEnumerable (workers) .ofType (Person.class). ToList (); Func2 (persons);}

In addition to providing this explicit declaration function writing, linq4j also implements the following expression writing, which looks really high-end and classy:

/ / use lambda, this time call whereN ParameterExpression parameterE = Expressions.parameter (Employee.class); ParameterExpression parameterN = Expressions.parameter (Integer.TYPE) Final Queryable nh4 = Linq4j.asEnumerable (emps) .asQueryable (). Where N (Expressions.lambda (Predicate2.class) Expressions.andAlso (Expressions.equal (Expressions.field (parameterE, Employee.class, "deptno") Expressions.constant (10)), Expressions.lessThan (parameterN, Expressions.constant (3)), parameterE, parameterN))

It looks bluffing, but it's not hard to think of it. This feature uses the static methods of the Expressions class to provide a series of off-the-shelf functions to call, further improving the usability to some extent. For details, you can refer to the source code of linq4j, which is not intended to be discussed in depth here.

four。 Summary

Linq4j implements most of the functions of standard Linq, while using the Expression class to simplify the implementation of many simple functions. It's easy to use, but I don't have time to do specific performance tests, so I don't have a say in performance. But in any case, pay homage to the author's skill. If you are free, you can take a look at the source code of linq4j, there will be a lot of gains.

Collection operation is the most common development situation in application development. Unfortunately, JAVA itself has not made much contribution there. It is difficult to say whether linq4j can be used in large-scale projects. If you can enjoy this convenience in the language itself, you should be happy. Net students should be happy. We can only hope that the new features of lamda expressions brought by JAVA8 will better solve this problem, of course, only in 2014.

For the convenience of those students who do not use maven, the attachment adds linq4j's jar package to download. Note that the suffix is changed to jar after download.

On how to understand the Linq4j in Java to share here, I hope that the above content can be of some help to you, can learn more knowledge. If you think the article is good, you can share it for more people to see.

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