Java 10 - Local Variable Type
Inference
In one of my
previous blog, I have already discussed about Type Inference. If you don’t have
the idea about Type Inference then you can check my blog here.
If we want
to understand Type Inference in one
line then we can say that It is the capability of the compiler to automatically
detect the datatype of a variable at the compiler time.
What is Local Variable type inference?
Java 10
added new feature that allows the developer to skip the type declaration
associated with local variables (those defined inside method definitions,
initialization blocks, for-loops, and other blocks like if-else), and the type
is inferred by the JDK. It will, then, be the job of the compiler to figure out
the datatype of the variable.
Until Java 9, we had to define the type of the
local variable.
E.g.: String message = “Welcome to Waheedtechblog.com”;
The above
statement is right as that’s how things have been since the inception of java but
if you observe, the type of the object at right side is clearing mentioning the
type of data that we have defined at left side which makes the variable
redundant.
So with Java
10, We can declare a local variable
without defining its data type at left side.
Var message = “Welcome to Waheedtechblog.com”;
Over here, we
don’t have to provide the data type of String to local variable message. Compiler
will get the datatype based on right hand side value.
Note:
- This feature is available only for local variables with the
initializer. It cannot be used for member
variables, method parameters, return types, etc as the initializer is
required as without which compiler won’t be able to infer the type.
- To support backward compatibility, var is not a java reserved
keyword. So, we can create variable with name var as it is allowed.
Let’s see where all we can use var
package
com.waheedtechblog.typeinference;
import java.util.function.BiConsumer;
import java.util.function.Consumer;
public class TypeInferenceExample {
static {
// static
block variable
var staticVaribale = "var in static block";
Consumer<String> displayOnConsole = msg -> {
System.out.println(msg);
};
displayOnConsole.accept(staticVaribale);
}
public static void main(String[] args) {
TypeInferenceExample inferenceExample = new TypeInferenceExample();
BiConsumer<String, String> displayOnConsole = (str, msg) -> {
System.out.println(str + msg);
};
// Before
Java 10
String message = "Welcome
to Waheedtechblog.com";
displayOnConsole.accept("Before JDK 10: ", message);
// Using JDK
10, Local Variable
var message10 = "Welcome to
Waheedtechblog.com";
displayOnConsole.accept("With JDK 10: ", message);
// local
variable declaration in enhanced loops
String[] countryName = { "India", "Japan", "UAE", "USA", "UK" };
displayOnConsole.accept("", "List of Country using
enhanced loop");
for (var country : countryName) {
displayOnConsole.accept("", country);
}
// basic for
loop
displayOnConsole.accept("", "List of Country using basic
loop");
for (var i = 0; i < countryName.length; i++) {
displayOnConsole.accept("", countryName[i]);
}
displayOnConsole.accept("Length of string '" + message10 + "' is: ",
String.valueOf(inferenceExample.getLenth(message10)));
// var is not keyword, so you can create variable with name var as well.
String var = "var is not keyword";
displayOnConsole.accept("", var);
}
// can return
var type as well
private int getLenth(String msg) {
var length = msg.length();
return length;
}
}
Output:
var
in static block
Before
JDK 10: Welcome to Waheedtechblog.com
With
JDK 10: Welcome to Waheedtechblog.com
List
of Country using enhanced loop
India
Japan
UAE
USA
UK
List
of Country using basic loop
India
Japan
UAE
USA
UK
Length
of string 'Welcome to Waheedtechblog.com' is: 29
var is not keyword
Illegal use of Var:
1.
Can’t use as a class field
Class Test {
Var
msg; // not allowed as it is local variable inference
}
2.
Local variable without initialization
Private void getOTP(){
Var
otp ; // cant use without initializer
}
3. Not allowed as parameter for any methods
Private void getOTP(var secretKey){
//Error,
cannot use var on method parameter
}
4. Not permitted in method return type
Private var getOTP(String secretKey){
//Error,
Method return type cannot be var
}
5. Not permitted with variable initialized with
‘NULL’
Private String getOTP(String secretKey){
Var msg = null; Error, cant initialize as null
}
You can download the source code from my GitHub repository.
Happy Coding..!!!
