var Keyword (JEP 286)

The var keyword simplifies the declaration of local variables. By using this keyword the variable type is inferred from the initialization expression. This allows more concise variable declarations without usually redundant type declarations.

import java.util.Arrays; public class VarKeyword { public static void main(String[] args) { var world = "World"; var subjects = Arrays.asList(world, "Galaxy", "Universe"); for (var s : subjects) { System.out.println(s); } } }

The var can be used in the following places:

Local variable declaration with initialization
Variables declared in for loops (classic or enhanced)
Variables in try-with-resources blocks

A local variable declared with var might also be explicitly declared final:

final var word = "World";

But since local variables which are not reassigned are nowadays implicitly final the unnecessary final declaration contradicts the original objective for conciseness.

Lambda Parameters

With Java 11 and JEP 323 for consistency it became possible to use var also for lambda parameters, which are implicitly typed:

Predicate<String> toolong = (var s) -> s.length() > 42;

This provides a concise syntax to add annotations to lambda parameters:

Predicate<String> toolong = (@NonNull var s) -> s.length() > 42;

Limitations

The usage of the var keyword is limited to local variable declarations with initializers. It can not be used for other declarations such as:

Uninitialized local variables
Local variables initialized with null
Local variables where the resulting type cannot be inferred (e.g. lambdas and method references)
Fields
Method parameters or return types
catch clauses

Also note that for parameterized types using the diamond operator will fall back to the base type of the respective type parameters. For example

var list = new ArrayList<>();

will result in the type ArrayList<Object> for the list variable.

Be aware that any local variable declared with var adopts the specific type of the initializer expression. If you want to reassign a different value later the type might not match. In this example the local variable should better have the type java.util.List but with the var statement the type java.util.ArrayList is assumed:

import java.util.ArrayList; import java.util.List; public class TooSpecificType { public static void main(String[] args) { var list = new ArrayList<String>(); // will not compile list = List.of("one", "two", "three"); } }

New Use Cases for Local Variables

The var keyword infers the variable type from the initialization expression. This opens access to types which can not be declared with explicit type declarations.

Anonymous Classes

The first example is anonymous inner classes. Using the var keyword the compiler uses the actual anonymous type and it is possible to access the API of the anonymous class:

public class AnonymousType { public static void main(String[] args) { var foo = new Object() { String field = "Hello"; }; System.out.println(foo.field); } }

Intersection Types

With the var statement it is possible to declare a local variable that has the type of multiple mix-in interfaces. This is not possible with local variables using explicit type declarations.

public class IntersectionType { public static void main(String[] args) { var guest = (Welcome & Goodbye) () -> "World"; System.out.println(guest.welcome()); System.out.println(guest.goodbye()); } }

import java.util.function.Supplier; public interface Welcome extends Supplier<String> { default String welcome() { return String.format("Hello %s", get()); } }

import java.util.function.Supplier; public interface Goodbye extends Supplier<String> { default String goodbye() { return String.format("Goodbye %s", get()); } }

Intersection types may also be implicitly determined by the compiler, for example when the compiler needs to find the common type of different objects supplied to a parameterized function with multiple parameters. This can result in quite unexpected inferred types:

import java.util.List; public class FunnyIntersectionType { public static void main(String[] args) { var list = List.of(3.5, "42"); // will not compile, note compiler error: String first = list.get(0); } }