Java 10 – Local Variable Type Inference

In this article we will see Java10 feature called Local Variable Type Inference proposed as part of JEP 286. From the first version of Java it is strongly typed language where we need to mention each variable data type. We all were feeling Java is verbose language and expecting precise, compact way of writing Java code. Java 8 addressed this concern some what. Java 10 added Local Variable Type Inference with initializer to eliminate verbosity. For example,

jshell> Map<String,String> map = new HashMap<>();
jshell> var map = new HashMap<>(); //This is valid with Java10

Here LHS variable datatype will be determined by RHS statement. For example,

jshell> var i = 3;
i ==> 3 //based on RHS, the LHS datatype is int.
jshell>int i=3,j=4; //Valid Declaration
but,
jshell> var j=4,k=5; //Not a Valid Declaration
| Error:
|'var' is not allowed in a compound declaration
| var j=4,k=5;
|^

You can use this feature for enhanced for loop and for loop as well.

jshell> List names = Arrays.asList("ABC","123","XYZ");
names ==> [ABC, 123, XYZ]
jshell> for(var name : names){
...> System.out.println("Name = "+ name);
...> }

Name = ABC
Name = 123
Name = XYZ

We can use Local Variable Type Inference in the for loop as well.

jshell> int[] arr = {1,2,3,4};
arr ==> int[4] { 1, 2, 3, 4 }

jshell> for (var i=0;i<arr.length;i++){
   ...> System.out.println("Value = "+i);
   ...> }
Value = 0
Value = 1
Value = 2
Value = 3

There are certain scenarios where this feature is not valid to use. For example,

• Not valid for constructor variables
• Not valid for instance variables
• Not valid for method parameters
• Not valid to assign NULL value
• Not valid as return type

Let us see examples for above statements.

jshell> public class Sample {
   ...>    private var name = "xyz";
   ...>    public Sample(var name) {
   ...>     this.name=name;
   ...>    }
   ...>    public void printName(var name){
   ...>      System.out.println(name);
   ...>    }
   ...>    public var add(int a, int b) {
   ...>     return a+b;
   ...>    }
   ...> }
|  Error:
|  'var' is not allowed here
|     private var name = "xyz"; //Instance variable
|             ^-^
|  Error:
|  'var' is not allowed here
|     public Sample(var name) { //Constructor variable
|                   ^-^
|  Error:
|  'var' is not allowed here
|     public void printName(var name){ //Method parameter
|                           ^-^
|  Error:
|  'var' is not allowed here
|     public var add(int a, int b) { //Method return type
|            ^-^

jshell> public class Sample {
   ...>    
   ...>    public static void main(String[] args) {
   ...>     var s = null;
   ...>    }
   ...> }
|  Error:
|  cannot infer type for local variable s
|    (variable initializer is 'null')
|      var s = null;
|      ^-----------^

When we migrate the code from lower versions to Java10, we no need to worry about the Local Variable Type Inference as this has the backward compatibility.

In the coming post we will learn another topic. Till then stay tuned!

About Siva Prasad Rao Janapati

Siva Janapati is an Architect with experience in building Cloud Native Microservices architectures, Reactive Systems, Large scale distributed systems, and Serverless Systems. Siva has hands-on in architecture, design, and implementation of scalable systems using Cloud, Java, Go lang, Apache Kafka, Apache Solr, Spring, Spring Boot, Lightbend reactive tech stack, APIGEE edge & on-premise and other open-source, proprietary technologies. Expertise working with and building RESTful, GraphQL APIs. He has successfully delivered multiple applications in retail, telco, and financial services domains. He manages the GitHub(https://github.com/2013techsmarts) where he put the source code of his work related to his blog posts.