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📦 Variables & Types · Beginner

Constants with final in Java

final variables, static final constants and naming.

🧩 The mysteryA variable marked final can never change... yet a final StringBuilder happily changes its contents. Is final lying?

Assign once, then locked

final means a variable can be assigned exactly once. Any later assignment is a compile error.

final int limit = 10;
limit = 20;  // error: cannot assign a
             // value to final variable

Blank finals

You can declare a final variable without a value and assign it later, once. That's called a blank final. It must be definitely assigned before it's read.

final String label;
label = "ready";   // first: ok
// label = "go";   // second: error

Class constants

A fixed value shared by a whole class is declared static final and named in UPPER_SNAKE_CASE. Java has no working const keyword: const is reserved but does nothing.

public static final int MAX_USERS = 100;
static final double TAX_RATE = 0.2;
🔮 Predict it

Your turn

A StringBuilder is a piece of text you can change. It's final here. What does this print?

final StringBuilder sb =
    new StringBuilder("Hi");
sb.append("!");
System.out.println(sb);
  1. Hi
  2. Hi!
  3. Compile error
Show the answer

Hi! final locks the variable, meaning which object it points to. It does not freeze the object. append changes the object, and that's allowed.

What final blocks

✗ Not allowed
final StringBuilder sb =
    new StringBuilder("Hi");
sb = new StringBuilder();

Pointing a final variable at a different object is a compile error.

✓ Allowed
final StringBuilder sb =
    new StringBuilder("Hi");
sb.append("!");

Changing the object it points to is fine: final reference is not the same as an immutable object.

💼 In the real world

In real projects

Magic numbers like 0.2 scattered through code are a maintenance nightmare. A named constant like TAX_RATE explains itself and changes in one place. Many teams also mark locals final to signal they never change.

Key takeaways

  1. final variable: assigned exactly once
  2. Constants: static final int MAX_USERS = 100;
  3. A blank final can be assigned later, but only once
  4. final reference ≠ immutable object
🤯 Did you know?

Constants like static final int MAX = 100; are copied (inlined) into every class that uses them at compile time. Change MAX without recompiling those classes and they keep using the old 100!

Practice questions

What does this print?

final int limit = 10;
limit = 20;
System.out.println(limit);
  1. Compile error
  2. 20
  3. 10
  4. Throws IllegalStateException
Check your answer

Compile error. limit is final and already assigned, so javac reports "cannot assign a value to final variable limit".

Which keyword makes this a constant?

public static ___ int MAX_USERS = 100;
  1. const
  2. final
  3. static
  4. constant
Check your answer

final. Java uses static final for constants. const is a reserved word but isn't used by the language at all.

Next: var lets Java work out the type for you. Does that make Java dynamically typed like JavaScript? Not even close.