var keyword in Java

The var reserved type name (not a Java keyword) was introduced in Java 10. Type inference is used in var keyword in which it detects automatically the datatype of a variable based on the surrounding context. The below examples explain where var is used and also where you can't use it.

1. Using var with Different Data Types

var can be used to declare variables of any data type, provided the initializer is present.

class Demo1 {
    public static void main(String[] args) {

        var x = 100;        // int
        var y = 1.9;        // double
        var z = 'a';        // char
        var p = "tanu";     // String
        var q = false;      // boolean

        System.out.println(x);
        System.out.println(y);
        System.out.println(z);
        System.out.println(p);
        System.out.println(q);
    }
}

100
1.9
a
tanu
false

2. var Can Be Used for Local Variables

var is allowed only inside methods, blocks, or constructors.

class Demo2 {
    public static void main(String[] args) {

        var x = 100;  // local variable
        System.out.println(x);
    }
}

100

3. var Cannot Be Used for Instance or Global Variables

class Demo3 {
    var x = 50; // Not allowed

    public static void main(String[] args) {
        System.out.println(x);
    }
}

Output

prog.java:8: error: 'var' is not allowed here
var x = 50;
^
1 error

4. var Cannot Be Used as a Generic Type

import java.util.*;

class Demo4 {
    public static void main(String[] args) {

        var<var> list = new ArrayList<>();      // invalid
    }
}

Output

prog.java:10: error: 'var' is not allowed here
var<var> al = new ArrayList<>();
^
1 error

5. var Cannot Be Used with Explicit Generic Declarations

import java.util.*;

class Demo5 {
    public static void main(String[] args) {
        var<Integer> list = new ArrayList<Integer>();   // Invalid

                var validList = new ArrayList<String>(); // valid
    }
}

Output

prog.java:9: error: illegal reference to restricted type 'var'
var<Integer> al = new ArrayList<Integer>();
^
1 error

6. var Requires Explicit Initialization

The compiler must infer the type at declaration time.

class Demo6 {
    public static void main(String[] args) {
        var x; //    Invalid
        var y = null;      // Invalid
    }
}

Output

prog.java:13: error: cannot infer type for local variable variable
var variable;
^
(cannot use 'var' on variable without initializer)
prog.java:16: error: cannot infer type for local variable variable
var variable = null;
^
(variable initializer is 'null')
2 errors

7. var Cannot Be Used with Lambda Expressions (Without Target Type)

Lambda expressions require an explicit target type.

interface MyInt {
    int add(int a, int b);
}
class Demo7 {
    public static void main(String[] args) {
        // Invalid
        var obj = (a, b) -> a + b;
    }
}

Output:

prog.java:13: error: cannot infer type for local variable obj
var obj = (a, b) -> {
^
(lambda expression needs an explicit target-type)
1 error

Correct Usage:

MyInt obj = (a, b) -> a + b;

8. var Cannot Be Used for Method Parameters or Return Types

class Demo8 {
    // Invalid return type
    var method1() {
        return "Hello";
    }
    // Invalid parameter
    void method2(var a) {
        System.out.println(a);
    }
}

Output

prog.java:6: error: 'var' is not allowed here
var method1()
^
prog.java:11: error: 'var' is not allowed here
void method2(var a)
^
2 errors