AP CSA Lesson 1.2 – Variables and Data Types

Learning Objectives

  • Understand what variables are and how they store data in memory.
  • Learn the main primitive data types in Java (int, double, boolean, char).
  • Differentiate between primitive types and reference types (like String).
  • Practice declaring, assigning, and using variables.
  • Gain hands-on experience with code demos, popcorn hacks, and homework challenges.

Key Vocabulary

| Term | Definition | Example | |——|————|———| | Variable | Named storage location in memory for data. | int age = 16; | | Declaration | Creating a variable with a type. | double price; | | Initialization | Giving the variable a value at creation. | double price = 4.99; | | Assignment | Giving a value later (after declaration). | price = 5.99; | | Primitive type | Basic data types built into Java. | int, double, boolean, char | | Reference type | Points to objects (more complex). | String, Scanner |

  • Primitive types don’t have methods
  • Reference types are objects and have methods associated with them

Concept Overview

What is a Variable?

Think of a variable as a labeled container that stores data.

  • You can store numbers, text, or true/false values.
  • Java requires you to specify the type (what kind of data goes in the box).

Java Data Types (AP CSA Focus)

Type Meaning Example Notes
int Whole numbers int year = 2025; Range: about -2 billion → 2 billion
double Decimal numbers double price = 4.99; More precise than float
boolean True/False boolean done = false; Used for conditions
char Single character char grade = 'A'; Always single quotes
String Text (sequence of chars) String name = "Paul"; Not primitive (it’s a class)

Declaration vs Initialization vs Assignment

int number;           // declaration (creates variable)
number = 10;          // assignment (stores 10 later)
int age = 16;         // initialization (declaration + assignment)


// Simple Variable Examples
public class VariableBasics {
    public static void main(String[] args) {
        // Step 1: Declare variables (create empty boxes)
        int age;
        double price;
        
        // Step 2: Put values in the boxes
        age = 16;
        price = 4.99;
        
        // Step 3: Create and fill boxes at the same time
        String name = "Alex";
        boolean isStudent = true;
        
        // Print the values
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
        System.out.println("Price: $" + price);
        System.out.println("Is student: " + isStudent);
    }
}

Final Variables (They Never Change!)

Sometimes you want a variable that never changes. Use the word final.

Rules:

  • Use final at the beginning
  • Give it a value
// Final Variables - Values That Never Change
public class FinalVariables {
    public static void main(String[] args) {
        // These values will NEVER change
        final int MAX_STUDENTS = 30;
        final double TAX_RATE = 0.08;
        final String SCHOOL = "My High School";
        
        System.out.println("Maximum students: " + MAX_STUDENTS);
        System.out.println("Tax rate: " + TAX_RATE);
        System.out.println("School: " + SCHOOL);
        
        MAX_STUDENTS = 25; // ERROR! Can't change final variables
    }
}

How to Pick the Right Type

It’s like choosing the right size box for different things:

For Numbers:

  • Whole numbers (1, 2, 100) → use int
  • Decimal numbers (1.5, 3.14) → use double

For Words and Letters:

  • One letter (‘A’, ‘B’) → use char
  • Words or sentences → use String

For Yes/No Questions:

  • True or False → use boolean
// Picking the Right Data Type
public class DataTypeExamples {
    public static void main(String[] args) {
        // Student info - pick the best type for each
        String studentName = "Sarah";        // Words → String
        int grade = 10;                     // Whole number → int
        double gpa = 3.8;                   // Decimal → double
        char team = 'A';                    // One letter → char
        boolean hasPhone = true;            // Yes/No → boolean
        
        // More examples
        int numberOfPets = 2;               // Counting → int
        double temperature = 72.5;          // Decimal → double
        boolean isRaining = false;          // True/False → boolean
        
        System.out.println(studentName + " is in grade " + grade);
        System.out.println("GPA: " + gpa + ", Team: " + team);
        System.out.println("Has phone: " + hasPhone);
        System.out.println("Pets: " + numberOfPets);
        System.out.println("Temperature: " + temperature + "°F");
    }
}

Two Types of Variables

Primitive Types (Simple boxes):

  • Store the actual value
  • Basic types: int, double, boolean, char
  • Can’t be null (empty)

Reference Types (Fancy boxes):

  • Point to more complex things
  • Examples: String
  • Can be null (by default)
  • Have special actions (methods) you can use
// Simple vs Fancy Variable Types
public class TypeComparison {
    public static void main(String[] args) {
        // Simple types (primitives) - store the actual value
        int number1 = 5;
        int number2 = 5;
        System.out.println("Same number? " + (number1 == number2)); // true
        
        // Fancy types (reference) - more complex
        String word1 = "Hello";
        String word2 = "Hello";
        System.out.println("Same word? " + word1.equals(word2)); // true
        
        // String can be empty (null), but int cannot
        String emptyText = null;
        System.out.println("Empty text: " + emptyText);
        
        // This would cause an error:
        // int emptyNumber = null; // NOT ALLOWED!
    }
}

🍿 [Popcorn Hack] Practice #1: Your Favorite Things

Make variables for your favorite things:

  1. Your favorite food (text)
  2. Your age (whole number)
  3. Your height in feet (decimal)
  4. Do you like pizza? (True/False)
  5. First letter of your favorite color
  6. Your birth year (this never changes… what type of variable will you make it??)

Answer is below:

// Practice #1 - Try it yourself first!
public class MyFavoriteThings {
    public static void main(String[] args) {
        // Try writing your own code here first!
        
        // Sample answer:
        String favoriteFood = "Pizza";
        int myAge = 16;
        double heightInFeet = 5.5;
        boolean likesPizza = true;
        char colorFirstLetter = 'B';  // Blue
        final int BIRTH_YEAR = 2008;  // Never changes
        
        System.out.println("Favorite food: " + favoriteFood);
        System.out.println("Age: " + myAge);
        System.out.println("Height: " + heightInFeet + " feet");
        System.out.println("Likes pizza: " + likesPizza);
        System.out.println("Favorite color starts with: " + colorFirstLetter);
        System.out.println("Born in: " + BIRTH_YEAR);
    }
}

🍿 [Popcorn Hack] Practice #2: Pick the Best Type

What type should you use for each of these?

  1. Number of siblings
  2. Your first name
  3. Are you hungry?
  4. Your favorite letter
  5. Your height in inches
  6. Number of days in a year (never changes)

Think about it, then check below!

// Practice #2 - What type for each?
public class PickTheType {
    public static void main(String[] args) {
        // 1. Number of siblings → int (counting, whole number)
        int siblings = 2;
        
        // 2. Your first name → String (text/words)
        String firstName = "Alex";
        
        // 3. Are you hungry? → boolean (yes/no, true/false)
        boolean isHungry = true;
        
        // 4. Your favorite letter → char (single letter)
        char favoriteLetter = 'A';
        
        // 5. Your height in inches → double (might be decimal like 65.5)
        // (Can also be int if your height is a whole number)
        double heightInches = 65.5;
        
        // 6. Days in a year → final int (never changes, whole number)
        final int DAYS_IN_YEAR = 365;
        
        System.out.println("Siblings: " + siblings);
        System.out.println("Name: " + firstName);
        System.out.println("Hungry: " + isHungry);
        System.out.println("Favorite letter: " + favoriteLetter);
        System.out.println("Height: " + heightInches + " inches");
        System.out.println("Days per year: " + DAYS_IN_YEAR);
    }
}

🏠 Homework Hack: Simple Grade Calculator

Make a program that calculates a student’s grade:

What you need to store:

  1. Student’s name (make this final)
  2. Three test scores (whole numbers 0-100)
  3. The class name (make this final)

What to calculate:

  • Average of the three test scores
  • Show what letter grade they earned

Summary

Making Variables:

int age = 16;           // Whole number
double price = 4.99;    // Decimal number  
boolean isReady = true; // True or false
char grade = 'A';       // One letter
String name = "Alex";   // Words/text

Final Variables (Never Change):

final int MAX_SCORE = 100;
final String SCHOOL = "My School";

Picking Types:

  • Counting things → int
  • Money, measurements → double
  • Yes/No questions → boolean
  • One letter → char
  • Names, sentences → String

Remember:

  • Use good names for your variables (age not a)
  • Final variables use ALL_CAPS
  • Strings need double quotes: "hello"
  • Chars need single quotes: 'A'

🎯 Now you know how to store information in Java! Complete the HW to test your skills.

Submission form: https://forms.gle/EJaB8hb5kRYhrYd79