Variables, Data Types & Input - Reference Handout

What You'll Learn Today

What variables are and why we use them
Different data types in Java
How to get input from the user
How to build a simple calculator

Variables in Java

Variable: A container for storing data
Declaration: Telling Java what type of data you want to store
Initialization: Giving your variable a value

Example:

int age = 15; // declares an integer variable called age
String name = "Alex"; // declares a String variable called name

Common Data Types

int: Whole numbers (e.g., 5, -3, 100)
double: Decimal numbers (e.g., 3.14, -0.5)
boolean: true or false
char: Single character (e.g., 'A', 'z')
String: Text (e.g., "Hello")

Getting User Input

Use the Scanner class to read input from the keyboard

Example:

import java.util.Scanner;

public class InputExample {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter your name: ");
        String name = scanner.nextLine();
        System.out.println("Hello, " + name + "!");
    }
}

Type Conversion (Casting)

Sometimes you need to change one type to another
Example: turning a double into an int

double price = 9.99;
int rounded = (int) price; // rounded is 9

Mini-Exercises

Ask the user for their favorite number and print it
Ask for their age and print a message
Try changing variable types and see what happens

Challenge: Build a Simple Calculator

Ask the user for two numbers
Ask for an operation (+, -, *, /)
Print the result

Example (Starter):

import java.util.Scanner;

public class Calculator {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter first number: ");
        int num1 = scanner.nextInt();
        System.out.print("Enter second number: ");
        int num2 = scanner.nextInt();
        System.out.print("Enter operation (+, -, *, /): ");
        String op = scanner.next();
        int result = 0;
        if (op.equals("+")) {
            result = num1 + num2;
        } else if (op.equals("-")) {
            result = num1 - num2;
        } else if (op.equals("*")) {
            result = num1 * num2;
        } else if (op.equals("/")) {
            result = num1 / num2;
        }
        System.out.println("Result: " + result);
    }
}

Troubleshooting Tips

InputMismatchException: Make sure you enter the right type (e.g., a number for int)
Variable not found: Check your spelling
Result is wrong: Double-check your operations

End of Day Checklist

Homework (Optional)

Research Topic: "How do if/else statements work in Java?"

Resource: - https://www.w3schools.com/java/java_conditions.asp

Tomorrow's Preview: Conditionals

Make your programs smarter with decisions
Learn about if/else and switch
Try building a grade calculator