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

2019-01-10 06:06发布

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问题:

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

回答1:

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;

回答2:

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

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

回答3:

JEP - JDK Enhancement-Proposal

http://openjdk.java.net/jeps/286

JEP 286: Local-Variable Type Inference

Author Brian Goetz

// Goals:
var list = new ArrayList<String>();  // infers ArrayList<String>
var stream = list.stream();          // infers Stream<String>

回答4:

I have cooked up a plugin for IntelliJ that – in a way – gives you var in Java. It's a hack, so the usual disclaimers apply, but if you use IntelliJ for your Java development and want to try it out, it's at https://bitbucket.org/balpha/varsity.

回答5:

With the release of JDK 10 on March 20, Java now includes a var reserved type name (not a keyword—see below) as specified in JEP 286. For local variables, the following is now valid in Java 10 or higher:

var map = new HashMap<String, Integer>();

The var reserved type name in Java is nearly identical to the var keyword in C# in that both allow for implicit typing (see below for important differences). var in Java can only be used for implicit type inference in the following contexts (as enumerated in JEP 286: Goals):

local variables with initializers
indexes in the enhanced for-loop
locals declared in a traditional for-loop

Therefore var cannot be used for fields, return types, class names, or interface names. Its rationale is to remove the need for including long type names when declaring and defining local variables, as stated in JEP 286 (authored by Brian Goetz):

We seek to improve the developer experience by reducing the ceremony associated with writing Java code, while maintaining Java's commitment to static type safety, by allowing developers to elide the often-unnecessary manifest declaration of local variable types.

var Scoping in Java

It should be noted that var is not a keyword in Java, but rather a reserved type name. As quoted from JEP 286:

The identifier var is not a keyword; instead it is a reserved type name. This means that code that uses var as a variable, method, or package name will not be affected; code that uses var as a class or interface name will be affected (but these names are rare in practice, since they violate usual naming conventions).

Note that since var is a reserved type name and not a keyword, it can still be used for package names, method names, and variable names (along with its new type-interference role). For example, the following are all examples of valid uses of var in Java:

var i = 0;
var var = 1;
for (var i = 0; i < 10; i++) { /* ... */ }
public int var() { return 0; }
package var;

As quoted from JEP 286:

This treatment would be restricted to local variables with initializers, indexes in the enhanced for-loop, and locals declared in a traditional for-loop; it would not be available for method formals, constructor formals, method return types, fields, catch formals, or any other kind of variable declaration.

Differences Between var in Java & C#

This is one notable difference between var in C# and Java include the following: var can be used as a type name in C# but cannot be used as a class name or interface name in Java. According to the C# documentation (Implicitly Typed Local Variables):

If a type named var is in scope, then the var keyword will resolve to that type name and will not be treated as part of an implicitly typed local variable declaration.

The ability to use var as a type name in C# creates some complexity and introduces some intricate resolution rules, which are avoided by var in Java by disallowing var as a class or interface name. For information on the complexities of var type names in C#, see Restrictions apply to implicitly-typed variable declarations. For more information on the rationale behind the scoping decision for `var in Java, see JEP 286: Scoping Choices.

回答6:

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 ==> []

回答7:

A simple solution (assuming you're using a decent IDE) is to just type 'int' everywhere and then get it to set the type for you.

I actually just added a class called 'var' so I don't have to type something different.

The code is still too verbose, but at least you don't have to type it!

回答8:

You can take a look to Kotlin by JetBrains, but it's val. not var.

回答9:

Java 10 did get local variable type inference, so now it has var which is pretty much equivalent to the C# one (so far as I am aware).

It can also infer non-denotable types (types which couldn't be named in that place by the programmer; though which types are non-denotable is different, e.g. Java doesn't have an equivalent to C# anonymous types).

The one difference I could find is that in C#,

If a type named var is in scope, then the var keyword will resolve to that type name and will not be treated as part of an implicitly typed local variable declaration.

In Java 10 var is not a legal type name.

回答10:

I know this is older but why not create a var class and create constructors with different types and depending on what constructors gets invoked you get var with different type. You could even build in methods to convert one type to another.

回答11:

Lombok supports var but it's still classified as experimental:

import lombok.experimental.var;

var number = 1; // Inferred type: int
number = 2; // Legal reassign since var is not final
number = "Hi"; // Compilation error since a string cannot be assigned to an int variable
System.out.println(number);

Here is a pitfall to avoid when trying to use it in IntelliJ IDEA. It appears to work as expected though including auto completion and everything. Until there is a "non-hacky" solution (e.g. due to JEP 286: Local-Variable Type Inference), this might be your best bet right now.

Note that val is support by Lombok as well without modifying or creating a lombok.config.

回答12:

As of Java 10, the equivalent is ... var.

回答13:

You can, in Java 10, but only for Local variables, meaning,

You can,

var anum = 10; var aString = "Var";

But can't,

var anull = null; // Since the type can't be inferred in this case

Check out the spec for more info.

回答14:

In general you can use Object class for any type, but you have do type casting later!

eg:-

Object object = 12;
    Object object1 = "Aditya";
    Object object2 = 12.12;

    System.out.println(Integer.parseInt(object.toString()) + 2);

    System.out.println(object1.toString() + " Kumar");
    System.out.println(Double.parseDouble(object2.toString()) + 2.12);