1.4 Constants

Learning Objectives

•Declare constants using the `final` keyword.
•Explain why constants improve maintainability over "magic numbers".
•Follow Java's naming convention for constants.

Introduction

A constant is a named value that cannot change after it is first assigned. Java has no dedicated `const` keyword for local values; instead, the `final` modifier marks a variable as write-once, and by convention constant names are written in SCREAMING_SNAKE_CASE.

Key Concepts

`final` prevents reassignment after the first assignment — attempting to reassign is a compile error.

Constants are conventionally named in uppercase with underscores, e.g. `MAX_SCORE`.

Class-level constants are usually also `static` so there's one shared copy, not one per object.

Java Syntax

Code Examples

`PI` is declared `static final`, so it is shared across the class and cannot be reassigned. Using the named constant instead of writing `3.14159` directly makes the formula self-documenting.

Common Mistakes

⚠Trying to reassign a `final` variable after its initial assignment (compile error).
⚠Forgetting `static` on a class-level constant, which then wastefully duplicates the value per object instance.
⚠Using lowercase or camelCase for constants, breaking convention and hurting scannability.

Practice

Practice 1

True / Falseeasy

A `final` variable's value can be changed later in the program as long as it stays the same type.

Summary

`final` creates a write-once variable, and by convention such constants are named in uppercase — this replaces hard-to-maintain "magic numbers" scattered through code with a single, clearly-named source of truth.

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