What is the equivalent of the C var keyword in Java

Question

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

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As of Java 10  the equivalent is     var

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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

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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         lt - Foo Non  not Bar Non foo Virt        lt - Bar Virt  var bar    Bar foo  bar Non         lt - Bar Non  not Foo Non bar Virt        lt - 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

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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

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I have cooked up a plugin for IntelliJ that  ndash  in a way  ndash  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

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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 lt  gt  list   new ArrayList lt String gt     Stream lt  gt  stream   myStream      You write   var list   new ArrayList lt String gt     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 gt  var list   new ArrayList lt String gt     list    gt

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

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JEP - JDK Enhancement-Proposal  http   openjdk java net jeps 286  JEP 286  Local-Variable Type Inference  Author  Brian Goetz     Goals  var list   new ArrayList lt String gt         infers ArrayList lt String gt  var stream   list stream                infers Stream lt String gt

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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

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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 lt String  Integer gt       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  lt  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  amp  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

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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   See e g  Tricks with var and anonymous classes  that you should never use at work    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

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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

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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

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This feature is now available in Java SE 10  The static  type-safe var has finally made it into the java world     source  https   www oracle com corporate pressrelease Java-10-032018 html

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If you add Lombok to your project you can use its val keyword   http   projectlombok org features val html

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