Local Variable Type Inference or LVTI in Java 10

Class_name variable_name=new Class_name(arguments);

For example:

Or

It looks fine, right? Yeah, because this is how things have been since the inception of Java. But there is one issue: It’s pretty obvious that if the type of the object is clearly mentioned at the right side of the expression, mentioning the same thing before the name of the variable makes it redundant. Plus, in the second example, you can see that it’s obvious that after the ‘=’ sign, it’s clearly a string as nothing except for a string can be enclosed in double inverted commas. Therefore, there arose a need to eliminate this redundancy and make variable declaration shorter, and more convenient.

How to declare local variables using LVTI:

Instead of mentioning the variable datatype on the left-side, before the variable, LVTI allows you to simply put the keyword ‘var’. For example,

Can be re-written as:

Use Cases

Here are the cases where you can declare variables using LVTI:

1. In a static/instance initialization block

   
   

   
   
   

   // Declaration of variables in static/init  // block using LVTI in Java 10 class A {     static     {         var x = "Hi there";         System.out.println(x)'     }     public static void main(String[] ax)     {     } }

       Output:
        Oh hi there
   

2. As a local variable

   
   

   
   
   

   // Declaration of a local variable in java 10 using LVTI class A {     public static void main(String a[])     {         var x = "Hi there";         System.out.println(x)     } }

         Output:
         Hi there
   

3. As iteration variable in enhanced for-loop

   
   

   
   
   

   // Declaring iteration variables in enhanced for loops using LVTI in Java class A {     public static void main(String a[])     {         int[] arr = new int[3];         arr = { 1, 2, 3 };         for (var x : arr)             System.out.println(x + "\n");     } }

   Output:
   1
   2
   3
   

4. As looping index in for-loop

   
   

   
   
   

   // Declaring index variables in for loops using LVTI in Java class A {     public static void main(String a[])     {         int[] arr = new int[3];         arr = { 1, 2, 3 };         for (var x = 0; x < 3; x++)             System.out.println(arr[x] + "\n");     } }

   Output:
   1
   2
   3
   

5. As a return value from another method

   
   

   
   
   

   // Storing the return value of a function in a variable declared with LVTI class A {     int ret()     {         return 1;     }     public static void main(String a[])     {         var x = new A().ret();         System.out.println(x);     }

   Output:
   1
   

6. As a return value in a method

   
   

   
   
   

   // Using a variable declared  //using the keyword 'var' as a return value of a function class A {     int ret()     {         var x = 1;         return x;     }     public static void main(String a[])     {         System.out.println(new A().ret());     } }

   Output:
   1
   

Error cases:
There are cases where declaration of local variables using the keyword ‘var’ produces an error. They’re mentioned below:

1. Not permitted in class fields

   
   

   
   
   

   // Sample java code to demonstrate  //that declaring class variables  //using 'var' is not permitted class A {     var x; /* Error: class variables can't be declared             using 'var'. Datatype needs              to be explicitly mentioned*/ }

2. Not permitted for uninitialized local variables

   
   

   
   
   

   // Sample java code to demonstrate  //that declaring uninitialized  //local variables using 'var' produces an error class A {     public static void main(String a[])     {         var x; /* error: cannot use 'var'                  on variable without initializer*/     } }

3. Not allowed as parameter for any methods

   
   

   
   
   

   // Java code to demonstrate that // var can't be used in case of  //any method parameters class A {     void show(var a) /*Error: can't use 'var'                        on method parameters*/     {     } }

4. Not permitted in method return type

   
   

   
   
   

   // Java code to demonstrate // that a method return type // can't be 'var' class A {     public var show() /* Error: Method return type                                       can't be var*/     {         return 1;     } }

5. Not permitted with variable initialized with ‘NULL’

   
   

   
   
   

   // Java code to demonstrate that local  variables initialized with 'Null'  can't be declared using 'var'*/ class A {     public static void main(String a[])     {         var x = NULL; // Error: variable initializer is 'null'     }