1.3 Variables

Learning Objectives

•Declare variables with appropriate types and names.
•Explain the scope and lifetime of local variables.
•Apply Java naming conventions for variables.

Introduction

A variable is a named, typed storage location whose value can change while a program runs. Declaring a variable reserves memory of the right size and shape for its type; every variable must be declared before it is used, and its type cannot change afterward.

Key Concepts

Declaration syntax: `type name;` optionally followed by `= value`.

Local variables live inside a method and only exist while that method executes.

Java naming convention: variables use camelCase (`studentName`, not `StudentName` or `student_name`).

A local variable must be assigned a value before it is read — Java will not compile otherwise.

Java Syntax

Code Examples

`studentName` is declared and initialized in one step. `score` is declared first, then assigned separately — both are valid as long as a value exists before it's read.

Common Mistakes

⚠Reading a local variable before assigning it a value (a compile error in Java, unlike some languages that default to zero/null).
⚠Reusing a variable name already declared in the same scope.
⚠Ignoring camelCase convention, which hurts readability and consistency with the rest of the Java ecosystem.

Practice

Practice 1

Short Answereasy

What happens if you try to print a local `int` variable that was declared but never assigned a value?

Summary

Variables are named, typed storage locations. Local variables are scoped to the method they're declared in and must be initialized before use.

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