What is the equivalent of the C# 'var' key

Question

One use of the var keyword in C# is implicit type declaration. What is the Java equivalent syntax for var?

Answer 1

If you add Lombok to your project you can use its val keyword.

http://projectlombok.org/features/val.html

Answer 2

There is none. Alas, you have to type out the full type name.

Edit: 7 years after being posted, type inference for local variables (with var) was added in Java 10.

Edit: 6 years after being posted, to collect some of the comments from below:

• The reason C# has the var keyword is because it's possible to have Types that have no name in .NET. Eg:

  var myData = new { a = 1, b = "2" };
  

  In this case, it would be impossible to give a proper type to myData. 6 years ago, this was impossible in Java (all Types had names, even if they were extremely verbose and unweildy). I do not know if this has changed in the mean time.

• var is not the same as dynamic. variables are still 100% statically typed. This will not compile:

  var myString = "foo";
  myString = 3;
  

• var is also useful when the type is obvious from context. For example:

  var currentUser = User.GetCurrent();
  

  I can say that in any code that I am responsible for, currentUser has a User or derived class in it. Obviously, if your implementation of User.GetCurrent return an int, then maybe this is a detriment to you.

• This has nothing to do with var, but if you have weird inheritance hierarchies where you shadow methods with other methods (eg new public void DoAThing()), don't forget that non-virtual methods are affected by the Type they are cast as.

  I can't imagine a real world scenario where this is indicative of good design, but this may not work as you expect:

  class Foo {
      public void Non() {}
      public virtual void Virt() {}
  }
  
  class Bar : Foo {
      public new void Non() {}
      public override void Virt() {}
  }
  
  class Baz {
      public static Foo GetFoo() {
          return new Bar();
      }
  }
  
  var foo = Baz.GetFoo();
  foo.Non();  // <- Foo.Non, not Bar.Non
  foo.Virt(); // <- Bar.Virt
  
  var bar = (Bar)foo;
  bar.Non();  // <- Bar.Non, not Foo.Non
  bar.Virt(); // <- Still Bar.Virt
  

  As indicated, virtual methods are not affected by this.

• No, there is no non-clumsy way to initialize a var without an actual variable.

  var foo1 = "bar";        //good
  var foo2;                //bad, what type?
  var foo3 = null;         //bad, null doesn't have a type
  var foo4 = default(var); //what?
  var foo5 = (object)null; //legal, but go home, you're drunk
  

  In this case, just do it the old fashioned way:

  object foo6;
  

Answer 3

It will be supported in JDK 10. It's even possible to see it in action in the early access build.

The JEP 286:

Enhance the Java Language to extend type inference to declarations of local variables with initializers.

So now instead of writing:

List<> list = new ArrayList<String>();
Stream<> stream = myStream();

You write:

var list = new ArrayList<String>();
var stream = myStream();

Notes:

• var is now a reserved type name
• Java is still commitment to static typing!
• It can be only used in local variable declarations

---

If you want to give it a try without installing Java on your local system, I created a Docker image with JDK 10 installed on it:

$ docker run -it marounbassam/ubuntu-java10 bash
root@299d86f1c39a:/# jdk-10/bin/jshell
Mar 30, 2018 9:07:07 PM java.util.prefs.FileSystemPreferences$1 run
INFO: Created user preferences directory.
|  Welcome to JShell -- Version 10
|  For an introduction type: /help intro

jshell> var list = new ArrayList<String>();
list ==> []