Java 10 Type Inference explained with Code Samples

A new feature added by Java 10 is the Local Variable Type Inference. In this article, I will be covering this feature in detail.

Introduction

Before Java 10, while declaring a variable, you explicitly had to specify its type, The following code demonstrates this:

int i = 10;
String s = "Hello";

So this code declares an int variable i and assigns it the value 10. It also declares a String variable s and assigns it the value “Hello“.

Java 10 introduces the Local Variable Type Inference feature. This means that Java can determine the data type of a variable based on the value assigned to the variable. So you do not need to explicitly specify the data type. Java 10 introduces a new type var. You need to specify ‘var‘ instead of the data type. The following code demonstrates this:

var i = 6;
var s = "Hello";

So this code declares the variables i and s. No data type is specified, the compiler infers the data type based on the value assigned to the variables.

You cannot use var for a variable without initializing it. So the following code will cause a compilation error:

var a;

Since a is not initialized, the compiler cannot determine its type. So the above code causes a compilation error.

Valid Uses of Var

It is valid to use var only in specific scenarios. These are elaborated below.

Local Variable

As seen above, you can use var for a local variable

Loop control variable

You can use var as a loop control variable. The following code demonstrates this:

List<Integer> numbers = Arrays.asList(2,4,6,8,10);
for(var i = 0; i < numbers.size();i++) {
  System.out.println(numbers.get(i));
}

Here, the code uses var for the loop control variable i which iterates over the numbers list and prints each number.

As a return value in a method

You can use var as the return value in a method. The following code demonstrates this:

public static int doSomething() {
  var i = 8;
  return i;
}

Here, the code uses var for the variable i which is returned by the method doSomething.

To assign value returned by a method

You can use var to assign the value returned by a method. The following code demonstrates this:

var value = doSomething();

Here, the code uses var for the variable value which is  assigned the value returned by the method doSomething.

Invalid Uses of Var

There are some scenarios where it is invalid to use var. These are elaborated below.

Instance variables

You cannot use var an an instance variable. So the following code is invalid:

public class Circle {
  
  private var radius; //will cause a compilation error

}

Here, the code uses var for the private instance field radius which is invalid.

Method Parameters/Return Type

You cannot use var for method parameters or return type. So the following code snippets are invalid:

public static var doSomething()) {

}

public static void doSomethingElse(var a) {

}

So the method doSomething specifies a return type of var which is invalid. The method doSomethingElse specifies an argument a of type var which is also invalid.

Array Initializer

You cannot use var while initializing an array. So the following code is invalid:

var myArray[] = {2,4,6,8,10};

Here, the code uses var with myArray and initializes it to some values which is invalid.

Conclusion

So in this article, we saw how Java 10 Local Variable Type Inference works. We walked through the various scenarios where using var is valid and invalid.