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📦 Variables & Types · Beginner

Local type inference with var in Java

var infers the type at compile time; still statically typed; where it is not allowed.

🧩 The mysteryvar x = 5; then x = "hello"; In JavaScript that's fine. In Java? Let's find out whether var makes Java dynamic.

The compiler fills in the type

Since Java 10, var tells the compiler: "infer this local variable's type from its initializer." Less typing, same type safety.

var count = 5;        // int
var name = "Ada";     // String
var list = new ArrayList<String>();

Inferred once, fixed forever

The type is decided at compile time and never changes. Java stays statically typed: var x = 5; makes x an int, exactly as if you had written int.

🔮 Predict it

Your turn

What happens?

var x = 5;
x = "hello";
System.out.println(x);
  1. hello
  2. 5
  3. Compile error
Show the answer

Compile error. x was inferred as int on the first line, and a String can never go into an int. var is not dynamic typing.

🔮 Predict it

Your turn

What does this print?

var total = 7;
var each = total / 2;
System.out.println(each);
  1. 3.5
  2. 3
  3. 3.0
Show the answer

total is inferred as int, so total / 2 is integer division and each is the int 3. Want 3.5? Write var total = 7.0;.

⚠️ The trap

var needs something to infer from

Without an initializer there's nothing to infer, so var count; doesn't compile. null alone has no useful type either, so var x = null; fails too ("variable initializer is 'null'").

var count;           // error
var nothing = null;  // error

Where var is allowed

Only in local spots: local variables with an initializer, for-loop variables, and lambda parameters (Java 11). Not fields, method parameters or return types: those are part of a class's API and must be written out.

for (var n : names) { }   // ok
class Shop {
    var total = 0;        // error: field
}
💼 In the real world

In real projects

Teams use var to cut noise like Map<String, List<Order>> map = new HashMap<>();, but avoid it when the type isn't obvious, as in var x = service.process();. Readability wins.

Key takeaways

  1. var x = 5; → x is an int, forever
  2. Needs an initializer; var x = null; doesn't compile
  3. Allowed: locals, for loops, lambda params (Java 11)
  4. Not allowed: fields, method params, return types
🤯 Did you know?

With var you can hold an object whose type has no name: var p = new Object() { int x = 5; }; and then p.x works. With a declared type of Object, it wouldn't.

Practice questions

What does this print?

var price = 10;
var total = price / 4;
System.out.println(total);
  1. 2
  2. 2.5
  3. 2.0
  4. Compile error
Check your answer

2. price is inferred as int, so price / 4 is integer division and total is an int with value 2.

Which line does NOT compile?

  1. var name = "Ada";
  2. var count;
  3. var list = new ArrayList<String>();
  4. var ratio = 0.5;
Check your answer

var count;. var needs an initializer to infer the type from. var count; has nothing to infer, so it's rejected.

Next: the big idea of this world. Some variables hold values, others hold addresses. Mix them up and data changes behind your back.