The var Keyword

Reading Time: 6 Minutes Difficulty: Beginner

---

Topic Summary

Introduced in Java 10, the var keyword allows the compiler to automatically infer (figure out) the type of a local variable from its assigned value. Instead of writing ArrayList<String> list = new ArrayList<>(), you write var list = new ArrayList<String>(). The code becomes shorter, but the type safety remains exactly the same - var is NOT a dynamic type like JavaScript's var.

---

What You'll Learn

• What var is and how it works
• The strict rules for using var
• Where var can and cannot be used
• When to use var and when to avoid it

---

Prerequisites

• Basic Java variable declarations
• Understanding of data types in Java

---

Explanation

What is var?

var is a reserved type name (not a keyword, technically) that tells the Java compiler: "figure out the type for me based on what I'm assigning."

// Without var
ArrayList<String> names = new ArrayList<String>();
Map<String, List<Integer>> map = new HashMap<String, List<Integer>>();

// With var - much shorter
var names = new ArrayList<String>();
var map = new HashMap<String, List<Integer>>();

The type is inferred at compile time - this is NOT dynamic typing. The variable names is still ArrayList<String> after the compiler processes it. At runtime, there is zero difference.

---

How the Compiler Infers the Type

The compiler looks at the right-hand side of the assignment:

var x = 42;           // inferred as int
var pi = 3.14;        // inferred as double
var name = "Alice";   // inferred as String
var list = new ArrayList<String>();  // inferred as ArrayList<String>

The type is locked at the point of declaration - you can't reassign a var variable to a different type:

var count = 10;
count = 20;       //  OK - still int
count = "hello";  //  Compile error - can't assign String to int

---

Rules for Using var

1. Only for local variables - inside methods, constructors, or initialization blocks:

var x = 5; //  inside method

2. Must be initialized at declaration - the compiler needs the right-hand side to infer type:

var x;       //  Cannot infer type - no initializer
var x = null; //  Cannot infer type from null alone

3. Cannot be null without a cast:

var x = (String) null; //  inferred as String (rare use case)

4. Cannot be used for method parameters:

void process(var data) { } //  NOT allowed

5. Cannot be used for fields (class-level variables):

class MyClass {
    var count = 0; //  NOT allowed for fields
}

6. Cannot be used for return types:

var getCount() { return 5; } //  NOT allowed

7. Cannot be used in lambda parameter types (Java 11 allows it in certain cases):

Consumer<String> c = (var s) -> System.out.println(s); //  Java 11+

---

Where var IS Allowed

Context	Example
Local variable in method	var list = new ArrayList<>();
For-each loop variable	for (var item : list)
Traditional for loop variable	for (var i = 0; i < 10; i++)
Try-with-resources	try (var conn = getConnection())

---

var with Generics

var works great with complex generic types:

// Old - verbose
Map<String, List<Integer>> studentScores = new HashMap<String, List<Integer>>();

// With var - cleaner right side
var studentScores = new HashMap<String, List<Integer>>();

Be careful: if you use the diamond operator <> with var, the compiler may infer Object instead of the correct generic type:

var list = new ArrayList<>(); // inferred as ArrayList<Object>! Not ideal
var list = new ArrayList<String>(); //  inferred as ArrayList<String>

---

When to Use var

Use var when:

• The type is obvious from the right-hand side: var name = "Alice"
• The type is very long and repetitive: var map = new HashMap<String, List<String>>()
• In for-each loops: for (var student : students)
• When using Builder pattern with long chains

Avoid var when:

• The type isn't obvious: var result = getValue() - what does getValue() return?
• It reduces readability: var x = process(y, z) - what type is x?
• The code is part of a public API (though var can't be used in public method signatures anyway)
• For beginners learning type systems - explicit types are educational

---

Real-World Analogy

var is like saying "you know what I mean" to the compiler. Instead of saying "Give me a red, 2-door, manual transmission Honda Civic" every time, you point to one and say "give me one of those." The compiler knows exactly what you mean by looking at what you're pointing at - and it locks in that understanding permanently.

---

Code Example

Example 1: Basic var Usage

import java.util.*;

public class VarDemo {
    public static void main(String[] args) {
        // Basic types
        var age = 25;                   // int
        var salary = 75000.50;          // double
        var name = "Alice";             // String
        var isActive = true;            // boolean

        System.out.println(name + " | Age: " + age + " | Salary: " + salary);

        // Collections
        var list = new ArrayList<String>();
        list.add("Java");
        list.add("Python");
        list.add("Go");

        // For-each with var
        for (var language : list) {
            System.out.println("Language: " + language);
        }

        // Map with var
        var scores = new HashMap<String, Integer>();
        scores.put("Alice", 95);
        scores.put("Bob", 82);
        scores.put("Charlie", 78);

        for (var entry : scores.entrySet()) {
            System.out.println(entry.getKey() + " → " + entry.getValue());
        }
    }
}

Output

Alice | Age: 25 | Salary: 75000.5
Language: Java
Language: Python
Language: Go
Alice → 95
Bob → 82
Charlie → 78

---

Example 2: var with Complex Types

import java.util.*;
import java.util.stream.*;

public class VarWithStreams {
    public static void main(String[] args) {
        var numbers = List.of(5, 3, 8, 1, 9, 2, 7, 4, 6);

        // Without var:
        // List<Integer> evenNumbers = numbers.stream()
        //     .filter(n -> n % 2 == 0)
        //     .collect(Collectors.toList());

        // With var:
        var evenNumbers = numbers.stream()
                .filter(n -> n % 2 == 0)
                .collect(Collectors.toList());

        System.out.println("Even numbers: " + evenNumbers);

        // var in traditional for loop
        for (var i = 0; i < 5; i++) {
            System.out.print(i + " ");
        }
        System.out.println();

        // var with try-with-resources
        try (var scanner = new Scanner(System.in)) {
            // scanner is inferred as Scanner
            System.out.println("Scanner ready");
        }
    }
}

Output

Even numbers: [8, 2, 4, 6]
0 1 2 3 4
Scanner ready

---

Example 3: What NOT to Do

public class VarMistakes {
    // var count = 0; //  NOT allowed as class field

    public static void main(String[] args) {
        //  Good - type is obvious
        var message = "Hello World";

        //  Questionable - what type is result?
        var result = compute(); // hard to tell without checking compute()

        //  This gives ArrayList<Object> - not what you probably want
        var list = new ArrayList<>(); // use ArrayList<String>() instead

        //  Better
        var typedList = new ArrayList<String>();
        typedList.add("Java");

        System.out.println(message);
        System.out.println(typedList);
    }

    static String compute() {
        return "done";
    }
}

Output

Hello World
[Java]

---

Common Mistakes

• Mistake: Using var without initialization: var x; → Fix: Always provide a value: var x = 5;
• Mistake: Using var x = null; expecting type inference → Fix: Compiler can't infer type from null alone - use explicit type or cast: var x = (String) null
• Mistake: Using var in method parameters or return types → Fix: var is only for local variables - not parameters, fields, or return types
• Mistake: Using var list = new ArrayList<>() without specifying generic type → Fix: Use var list = new ArrayList<String>() so the inferred type is ArrayList<String> not ArrayList<Object>
• Mistake: Overusing var and making code unreadable → Fix: If the type isn't obvious from the right-hand side, use an explicit type

---

Best Practices

• Use var when the type is obvious and the right-hand side clearly shows what it is
• Avoid var when the method/constructor name doesn't make the type obvious
• Use var freely in for-each loops - it almost always improves readability
• Don't use var for primitive types like int, double in simple cases - explicit types are fine and clear
• Code reviews: if a teammate can't tell the type of a var variable in 2 seconds, use an explicit type

---

Interview Questions

Q: What is var in Java 10? A: var is a reserved type name in Java 10 that enables local variable type inference. The compiler infers the variable's type from its initializer expression. It is not dynamic typing - the type is fixed at compile time.

Q: Where can var NOT be used? A: var cannot be used for: class fields, method parameters, method return types, or variables without an initializer. It's only for local variables inside methods, constructors, and initializer blocks.

Q: What is the difference between Java's var and JavaScript's var? A: They are completely different. Java's var uses compile-time type inference - the type is determined by the compiler and is fixed. Once a var variable is declared as int, it stays int. JavaScript's var is truly dynamic - the same variable can hold different types at runtime.

Q: Can you use var with generics? A: Yes, but be careful. var list = new ArrayList<String>() correctly infers ArrayList<String>. But var list = new ArrayList<>() infers ArrayList<Object> because the diamond operator alone doesn't specify the type.

---

Quick Revision

var = local variable type inference - introduced in Java 10 Compiler figures out type from right-hand side at compile time Type is locked - var is NOT dynamic typing Rules: only local variables, must be initialized, cannot be null alone Cannot use on fields, parameters, or return types Use when type is obvious; avoid when type is unclear from context

---

Related Topics

• Java Generics
• Type System in Java
• Records (Java 16)
• Java Streams with var

---

Next Lesson

Lesson 9 - Records