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How to use Java's var keyword

By Cameron McKenzie

Java’s var keyword

Packaged as part of the 2018 version 10 release, the Java var reserved word introduced type inference to the strongly typed Java programming language.

Type inference in Java happens when the JDK’s compiler, rather than the developer, assigns a data-type to a variable.

As you can see in the following var examples, the left-hand side of the assignment does not reference a Java types such as long, double or ArrayList. Instead, it uses the Java var keyword, which allows the JDK’s compiler to pick the appropriate type instead.

var x = 5.5;                        // double is inferred
var y = System.currentTimeMillis(); // long is inferred
var z = new ArrayList<Double>();    // ArrayList is inferred

Benefits of using var in Java

Developers and programmers have widely adopted the use of var in Java code because it provides a variety of practical benefits,:

• It simplifies syntax within method blocks by removing the need to repeat long or complex type declarations. This makes the code more concise and easier to read, especially when working with generics.
• For developers coming from other languages like JavaScript, Go, Rust, or Pascal, the var keyword feels familiar. This familiarity helps ease the transition into Java and makes it more approachable for multi-language programmers.
• When writing local code inside methods, var streamlines development and makes common operations feel lighter and more fluid. It is particularly useful in loops or temporary operations where the type is already obvious from the context.
• Using var is also helpful when working with generic types or APIs that return long, complex type names. In these cases, the inferred type keeps the code clean and avoids unnecessary repetition.
• The use of var in Java reduces the cognitive load when dealing with short-lived or intermediate variables. Developers can focus more on what the code is doing rather than getting bogged down in type syntax.

Inferred typing with var examples

When you use a var to declare a variable, the JVM assigns a type to the variable based on what it sees on the right-hand side of the assignment operation. That’s why the var reserved word is said to support inferred typing. The Java compiler looks at the manner in which the variable is initialized, and logically infers the type from it.

In the following var examples, it’s not difficult for the reader to logically infer the type of each variable.

Furthermore, the contrast between the explicit type declaration and the use of the var reserved word demonstrates how you can use the var reserved word to create simplified and less verbose code.

String mister = "dressup";
var bean = "lima"; // var infers String type

int x = 10;
var y = 20; // var infers int type

LocalDate today = LocalDate.now();
var rightNow = LocalDate.now(); // var infers LocalDate type

Strong typing and the var keyword

Java’s var keyword reduces the verbosity and ceremony surrounding the declaration of local variables, but it does not sidestep any rules about the platform’s strong typing requirements. All rules about strong typing in Java remain in place.

For example, if Java infers a type to be a String, it will not assigned that type to an int or a float later on in the code.

int x = 10;
x = "ten"; // Error: violation of strong typing in Java

When not to use var in Java

The var keyword cannot be used in place of every variable declaration in a Java program. Use of the var keyword is limited to a confined set of situations and scenarios. Limits and restrictions to the use of var in Java include the following:

• var cannot be used in a method signature.
• var cannot be used to declare instance variables.
• var cannot be used to declare static variables.
• var cannot be used if a variable is initially set to null.

The use of var in Java can make code easier to read and write.

The ongoing adoption of var

The introduction of the Java var reserved word was not a revolutionary change that impacted how the JVM behaved. Instead, it was more of a friendly change that makes Java methods easier to write, easier to read and more approachable for programmers who are new to the language.

Expect to see var appear more frequently in Java codebases in the near future. As organizations move away from Java 8 implementations and adopt Java 21, var will invariably become the norm.

Cameron McKenzie is an AWS Certified AI Practitioner, Machine Learning Engineer, Solutions Architect and author of many popular books in the software development and Cloud Computing space. His growing YouTube channel training devs in Java, Spring, AI and ML has well over 30,000 subscribers.