Variables and Data Types

AP Computer Science A · AP (Advanced Placement) · 4 min read · Updated: Sep 7, 2026

Welcome to the World of Variables!

In computer science, we need a way to store information so we can use it later. Whether it is your high score in a game, the price of a taco, or whether a light switch is on or off, we use variables. Think of a variable as a labeled storage box: you give the box a name, and you put a specific piece of information inside it.

In this chapter, we will learn how to create these boxes and the three specific types of data that the AP Computer Science A exam expects you to know. Don't worry if you've never coded before—we will take this step by step!

1. The Three Essential Data Types

Java is a "strongly typed" language. This means you must tell the computer exactly what kind of data you want to store in your variable. For the AP exam, you only need to master three primitive data types:

A. $int$ (Integers)

The $int$ type is used for whole numbers. This includes positive numbers, negative numbers, and zero. There are no decimal points allowed here!

Real-world examples: Your age, the number of students in a class, or the year.

Quick Note: While you don't need to memorize the exact numbers, remember that $int$ variables have a limit. They range from $Integer.MIN\_VALUE$ to $Integer.MAX\_VALUE$. You will learn more about these specific limits in Chapter 1.5.

B. $double$ (Decimal Numbers)

The $double$ type is used for numbers that require decimal points. The name "double" comes from "double-precision," meaning it can hold very precise values.

Real-world examples: The price of a soda ($\$1.50$), your GPA ($3.85$), or the value of $\pi$ ($3.14159$).

C. $boolean$ (True or False)

A $boolean$ is the simplest data type. It can only hold one of two values: $true$ or $false$. Computers love booleans because they represent the "on/off" logic of computer chips.

Real-world examples: Is the game over? (true/false), Is the user logged in? (true/false).

Key Takeaway: If it's a whole number, use $int$. If it has a decimal, use $double$. If it's a yes/no choice, use $boolean$. You do not need to learn other types like float, char, or long for this exam!

2. Creating Variables: Declaration and Initialization

Creating a variable is usually a two-step process, though we often do both on one line.

Step 1: Declaration

Declaration is like telling the computer, "Hey, I'm going to need a box for an integer, and I'm going to call it $score$." You state the type and the name.

$int$ $score;$

Step 2: Initialization

Initialization is when you put a value into that box for the first time using the assignment operator ($=$).

$score = 0;$

Doing it all at once

Most of the time, we declare and initialize on the same line to keep our code clean:

$int$ $score = 0;$
$double$ $price = 19.99;$
$boolean$ $isGameOver = false;$

Did you know? The semicolon ($;$) is like a period at the end of a sentence in Java. It tells the computer that your instruction is finished.

3. Naming Your Variables

In Java, we can't just name variables anything we want. There are rules we must follow and conventions that good programmers follow.

The Mandatory Rules:

1. Names can contain letters, numbers, and underscores.
2. Names cannot start with a number.
3. Names are case-sensitive ($score$ and $Score$ are two different boxes!).
4. You cannot use Java "keywords" (you can't name a variable $int$ or $boolean$).

The Java Style Convention: Lower Camel Case

To make code easier to read, Java programmers use lower camel case. You start with a lowercase letter, and every new word after that starts with a capital letter.

Examples:
$playerScore$
$daysUntilGraduation$
$isLightOn$

Key Takeaway: Always pick names that describe what is inside the box. Use $currentHealth$ instead of just $h$. It makes your life much easier when you read your code later!

4. Common Mistakes to Avoid

Don't worry if you run into these; even professional developers make these mistakes!

Type Mismatch: Trying to put a decimal into an $int$ box. For example, $int$ $x = 5.5;$ will cause an error because an $int$ cannot hold decimals.
Forgetful Initialization: Trying to use a variable before you have given it a value. You can't print a score if the computer doesn't know what the score is yet!
Starting with a Capital: In Java, variable names usually start with a lowercase letter. Class names (which we will learn about later) start with uppercase letters. Keeping them distinct helps you stay organized.

Quick Review Box

1. Primitives: Only $int$, $double$, and $boolean$ are on the exam.
2. Declaration: Specifying the type and name ($int$ $myAge;$).
3. Initialization: Giving the variable its first value ($myAge = 16;$).
4. Assignment: The $=$ sign moves the value on the right into the variable on the left.
5. Naming: Use $lowerCamelCase$ for variable names to be a "pro" Java coder.

Next Topic Preview: Now that you have your boxes and data, how do you do math with them? In Chapter 1.3, we'll look at Expressions and Output!

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is a variable in computer science?

A variable is a labeled storage box that holds a specific piece of information for later use.

What does it mean that Java is a "strongly typed" language?

It means you must explicitly declare the specific type of data that a variable will hold.

What is the purpose of the $int$ data type in Java?

The $int$ type is used to store whole numbers, including positive, negative, and zero values, without decimal points.

When should you use a $double$ variable instead of an $int$?

Use a $double$ variable when you need to store numbers that include decimal points.

What are the only two possible values for a $boolean$ variable?

A $boolean$ variable can only be either $true$ or $false$.

What is the difference between variable declaration and initialization?

Declaration defines the type and name of the variable, while initialization is the act of assigning it a value for the first time.

What is the purpose of the semicolon ($;$) in Java code?

The semicolon acts like a period at the end of a sentence, telling the computer that an instruction is finished.

What are the mandatory rules for naming variables in Java?

Names can include letters, numbers, and underscores, but cannot start with a number, cannot be a Java keyword, and are case-sensitive.

What is the "lower camel case" convention in Java?

It is a naming style where the first word is lowercase and every subsequent word starts with a capital letter.

What occurs during a "Type Mismatch" error?

A Type Mismatch occurs when you try to store a value of one data type into a variable of an incompatible type.

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