The var keyword in Java

Java's var keyword allows developers to declare local variables without specifying a data type such as int, long, String or char.

A new language feature introduced in the 2018 JDK 10 release, the Java var keyword performs type inference.

Java, var and inferred types

Type inference means Java will guess the data type when the variable is created, and enforce that typing throughout the program.

With Java's var keyword combined with type inference, Java programs remain strongly typed while the developer is relieved of the burden of explicitly stating the type of each variable.

Variable Declarations in Java

Here's how the Java var keyword looks when used in a program

var theMagicNumber = 7;
var theGuess = 5;         

Yes, this is Java.

• It's not JavaScript
• It's not C octothorpe
• It's not Turbo Pascal
• It's not Swift
• It's not Kotlin

This is Java, and this is how you can declare local variables in Java.

Java's new language features

I'm always surprised by how many Java developers are unaware of the fact that that you can declare local variables in Java with the var keyword.

(Actually, var is not a keyword. It's a reserved word. But let's not get into that.)

The var keyword makes Java code look cleaner, but it also has another great benefit. It makes Java easier to learn!

In my article on how to write your first Java program, I used the var keyword extensively. It just makes learning easier and more fun. If you want to learn to program, Java is definitely the way to go! (Video tutorial here)

A good rule of thumb when it comes to the var keyword is that it can be used inside methods to declare local variables.

Where is Java's var not allowed?

However, the var keyword is not allowed in all context. Here are six instances where the Java var keyword is not allowed:

1. Method Parameters
2. Field or instance variables
3. Return types for methods
4. Parameters for constructors
5. Lambda parameters
6. Array declarations

So there are definitely some compiler enforced rules about when to use Java's var keyword.

Java var keyword best practices

There are also some best practcies surrounding the use of Java's var keyword, such as:

• Use Java's var to improve readability
• Don't overuse Java's var and make things unreadable
• Use clear and unambiguous variables names
• Combine final and var when appropriate
• Avoid using var in nested blocks of code. It can get confusing
• Use explicit types if the data type is not obvious, especially with polymorphic components
• Be aware of any team standards that might exist pertaining to Java's var keyword
• Be consistent with the use of var
• Make sure your IDE supports its use (Most IDEs today do)

So there you go! Java just keeps getting better and better, and Java's var keyword is a great example of that!

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