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. We can declare any datatype with the var keyword.
Java
// Java program to explain that// var can used to declare any datatypeclass Demo1 { public static void main(String[] args) { // int var x = 100; // double var y = 1.90; // char var z = 'a'; // string var p = "tanu"; // boolean var q = false; // type inference is used in var keyword in which it // automatically detects the datatype of a variable System.out.println(x); System.out.println(y); System.out.println(z); System.out.println(p); System.out.println(q); }} |
Output
100 1.9 a tanu false
2. var can be used in a local variable declaration.
Java
// Java program to demonstrate that// var can be used to declare a local variableclass Demo2 { public static void main(String[] args) { // local variable var x = 100; // print x to the console System.out.println(x); }} |
Output
100
3. var cannot be used in an instance and global variable declaration.
Java
// Java program to demonstrate that// var cannot be used to declare// instance and global variablesclass Demo3 { // instance variable var x = 50; 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.
Java
// Java program to demonstrate that// var cannot be used as a Generic// typeimport java.util.*;class Demo4 { public static void main(String[] args) { // Generic list using var var<var> al = new ArrayList<>(); // add elements al.add(10); al.add(20); al.add(30); // print the list System.out.println(al); }} |
Output
prog.java:10: error: 'var' is not allowed here
var<var> al = new ArrayList<>();
^
1 error
5. var cannot be used with the generic type.
Java
// Java program to demonstrate that// var cannot be used with Generic typeimport java.util.*;class Demo5 { public static void main(String[] args) { // var used with Generic type var<Integer> al = new ArrayList<Integer>(); // add elements al.add(10); al.add(20); al.add(30); // print the list System.out.println(al); // This is valid since type is decided // based on ArrayList<String> var list = new ArrayList<String>(); }} |
Output
prog.java:9: error: illegal reference to restricted type 'var'
var<Integer> al = new ArrayList<Integer>();
^
1 error
6. var cannot be used without explicit initialization.
Java
// Java program to demonstrate that// var cannot be used without explicit// initializationimport java.io.*;class Demo6 { public static void main(String[] args) { // declaration without // initialization var variable; // This is also not valid var variable = null; }} |
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 Expression.
Java
// Java program to demonstrate that// var cannot be used with Lambda// Expressionimport java.util.*;interface myInt { int add(int a, int b);}class Demo7 { public static void main(String[] args) { // var cannot be used since they // require explicit target type var obj = (a, b) -> (a + b); // calling add method System.out.println(obj.add(2, 3)); }} |
prog.java:13: error: cannot infer type for local variable obj
var obj = (a, b) -> {
^
(lambda expression needs an explicit target-type)
1 error
8. var cannot be used for method parameters and return type.
Java
// Java program to explain that// var cannot be used for a method// parameters and return typeclass Demo8 { // method1 using var // as a return type var method1() { return ("Inside Method1"); } // method2 using var for a // parameter void method2(var a) { System.out.println(a); } public static void main(String[] args) { // create an instance Demo1 obj = new Demo1(); // call method1 var res = obj.method1(); // call method2 obj.method2(); }} |
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
