From
java10 onwards you no need to explicitly declare the type of local variables.
You can
write using ‘var’ like below.
var a =
10;
Type of
local variable is inferred by the compiler.
public
static void demoFunction(){
var a = 10;
System.out.println("a : " + a);
}
For
example, in the above function, I defined a variable ‘a’ using var
keyword. Type of the variable is
inferred by the compiler from the right hand side expression.
var is
not a keyword in Java?
‘var’ is
not a keyword, but 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).
In Java,
keywords are not used as identifiers or variable names. Since ‘var’ is not a
keyword, you can use it as variable name too.
public
static void demoFunction(){
var var = 10;
System.out.println("var : " +
var);
}
Since var
is not a keyword, you can use ‘var’ as a method name, parameters name, package name
etc.,
Type
inference is only for local variables
You can’t
use ‘var’ on instance properties, return types, method parameters etc.,
Main.java
package com.sample.app; public class Main { public static void demoFunction(){ var a = 10; var var = 11; // You can use var as a variable name System.out.println("a : " + a); System.out.println("var : " + var); //You can use var in a for loop. for(var i = 0; i < 10; i++){ System.out.print(i + " "); } System.out.println(); //You can use var while defining array var primes = new int[] {2, 3, 5, 7}; for(var i : primes){ System.out.print(i + " "); } System.out.println(); } public static void main(String args[]) { demoFunction(); } }
Output
a : 10
var : 11
0 1 2 3 4
5 6 7 8 9
2 3 5 7
You can
get the complete working module at below location.
Use below
commands to compile and run the application.
javac
--module-source-path src -d out src/printdata/com/sample/app/Main.java
java
--module-path out -m printdata/com.sample.app.Main
Does
var turns Java into a dynamic language like JavaScript or Python?
No. When
you compile a java program, down the line it infers the types and places it in
the byte code.
Main.java
package com.sample.app; public class Main { public static void demoFunction() { var a = 10; a = "Hello World"; System.out.println("a : " + a); } public static void main(String args[]) { demoFunction(); } }
As you see
above example, I assigned a with value 10 and I again reassigned the string "Hello
World" to the variable a.
When I try
to compile Main.java application, I end up in below compiler error.
$javac --module-source-path src -d out src/printdata/com/sample/app/Main.java
src/printdata/com/sample/app/Main.java:8: error: incompatible types: String cannot be converted to int
a = "Hello World";
^
1 error
Limitations
of var
a. Java
compiler can’t infer a type for null
If you
declare below statement in your application, compiler can’t infer the type.
var a =
null;
Main.java
package com.sample.app; public class Main { public static void main(String args[]) { var a = null; } }
When you
try to compile Main.java, you will end up in below error.
$javac --module-source-path src -d out src/printdata/com/sample/app/Main.java
src/printdata/com/sample/app/Main.java:8: error: cannot infer type for local variable a
var a = null;
^
(variable initializer is 'null')
1 error
b. When
you use ‘var’ for a generic collection, it infers to Object.
var
hobbies = new ArrayList<> ();
In the
above example, new ArrayList<>() would infer the type as
ArrayList<Object>
var map1 =
new HashMap(); // Inferred as HashMap
var map2 =
new HashMap<>(); // Inferred as HashMap<Object, Object>
But if you
explicitly specify the type in <>, then java infers the type.
var list =
new ArrayList<String>(); // infers
ArrayList<String>
c. You
can’t use var on variables without initializer
If you
declare a variable like below, java compile can’t infer the type.
var a;
Main.java
package com.sample.app; public class Main { public static void main(String args[]) { var i; } }
When you
try to compile above program, you will end up in below error.
$javac --module-source-path src -d out src/printdata/com/sample/app/Main.java
src/printdata/com/sample/app/Main.java:7: error: cannot infer type for local variable i
var i;
^
(cannot use 'var' on variable without initializer)
1 error
d. You
can’t use ‘var’ to define multiple variables.
var i =
10, j = 20;
Main.java
package com.sample.app; public class Main { public static void main(String args[]) { var i = 10, j = 20; } }
When you
try to compile above program, you will end up in below error.
$javac --module-source-path src -d out src/printdata/com/sample/app/Main.java
src/printdata/com/sample/app/Main.java:7: error: cannot infer type for local variable i
var i;
^
(cannot use 'var' on variable without initializer)
1 error
e.
lambda expression needs an explicit target-type
Main.java:10:
error: cannot infer type for local
variable f
var f = () -> { };
^
(lambda expression needs an explicit
target-type)
f.
Array initializer needs an explicit target-type
Main.java:10:
error: cannot infer type for local variable k
var k = { 1 , 2 };
^
(array initializer needs an explicit
target-type)
Reference
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