The following lines of code show how local variables (and all other variables) were defined prior to Java 10:

String name = "Java Everywhere"; 
LocalDateTime dateTime = new LocalDateTime.now();

Starting with Java 10, by using var, you can drop the mandatory explicit type in the declaration of local variables, as follows:

var name = "Java Everywhere";            // variable 'name' inferred as 
                                         // String 
var dateTime = new LocalDateTime.now();  // var 'dateTime' inferred as 
                                         // LocalDateTime 

Does it look like the preceding code doesn't offer a lot of benefits? Imagine you could take the following code:

HashMap<Integer, String> map = new HashMap<Integer, String>(); 

And replace it with this code, instead:

var map = new HashMap<Integer, String>();  

By replacing HashMap<Integer, String> with var, the preceding line of code is much shorter.

When you move away from explicitly stating the data type of the variables, the compiler takes over to determine, or infer, the variable type. Type inference is the compiler's ability to evaluate the information that is already present in the code, like the literal values, operations, and method invocations or their declarations, to determine the variable type. It follows a set of rules to infer the variable type. As a developer, when you choose type inference with var, you should be aware of the compiler's inference algorithm, so that you don't get unexpected results.

With every new feature, you should adhere to a few rules and restrictions and try to follow the best practices to benefit from that feature. Let's start with the compulsory initialization of the variables that are defined using var.