Variables and Types

After this lesson, you will be able to:

• Declare variables with explicit types (int, double, String, boolean)
• Explain why Java requires type declarations and how the compiler uses them
• Use Scanner to read user input from the console
• Perform arithmetic with integers and doubles, understanding integer division and casting
• Convert between types using casting and String conversion methods

From Python’s Flexibility to Java’s Contracts

From CSCD 110: In Python, x = 5 works without declaring a type — Python figures it out at runtime. In Java, you must declare the type: int x = 5;. This is static typing — the compiler checks types before your program ever runs. More upfront work, but bugs are caught earlier.

In Python, you wrote:

age = 25
name = "Alice"
gpa = 3.7

Python figured out the types for you. In Java, you tell the compiler what type each variable holds:

int age = 25;
String name = "Alice";
double gpa = 3.7;

This is called static typing — every variable has a type that’s fixed at compile time. It’s more typing (literally), but the compiler now knows exactly what operations are valid. Try age + name and the compiler tells you immediately if that makes sense.

Primitive Types

Java has eight primitive types. You’ll use four constantly:

Plus byte, short, long, and float — you’ll encounter these less often.

Key difference from Python: Python has int with unlimited size and float. Java’s int overflows at ~2.1 billion, and double has finite precision. These limits matter in real programs.

Declaring vs. Assigning

Declaration creates the variable with its type. Assignment gives it a value. You can do both at once or separately:

// Declaration + assignment (most common)
int score = 100;

// Declaration first, assignment later
double temperature;
temperature = 98.6;

// Multiple declarations of the same type
int x = 0, y = 0, z = 0;

Common Pitfall: Using a variable before assigning it is a compiler error in Java. Python would give you a runtime NameError, but Java catches this at compile time.

Integer Division and Modulus

This catches every student at least once:

int a = 7;
int b = 2;
System.out.println(a / b);   // Prints 3, not 3.5!
System.out.println(a % b);   // Prints 1 (remainder)

When both operands are int, Java performs integer division — it truncates the decimal part. To get 3.5, at least one operand must be a double:

System.out.println(7.0 / 2);     // 3.5
System.out.println((double) 7 / 2);  // 3.5 — cast forces double division

int result = 5 / 2;
System.out.println(result);

double result = 5 / 2;
System.out.println(result);

Reading Input with Scanner

Java uses the Scanner class to read console input:

import java.util.Scanner;

public class TempConverter {
    public static void main(String[] args) {
        Scanner console = new Scanner(System.in);

        System.out.print("Enter temperature in Fahrenheit: ");
        double fahrenheit = console.nextDouble();

        double celsius = (fahrenheit - 32) * 5.0 / 9;
        System.out.println("Celsius: " + celsius);
    }
}

Common Pitfall: Mixing nextInt() or nextDouble() with nextLine() leaves a stray newline in the buffer. Call console.nextLine() after nextInt() to consume it before reading a full line.

String Basics

String is not a primitive — it’s a class (notice the capital S). But you use it constantly:

String greeting = "Hello";
String name = "World";
String message = greeting + ", " + name + "!";  // Concatenation
System.out.println(message);  // Hello, World!

Useful String methods:

• message.length() — character count
• message.charAt(0) — first character ('H')
• message.toUpperCase() — "HELLO, WORLD!"
• message.equals("Hello, World!") — content comparison (NOT ==)

Critical: Use .equals() to compare strings, never ==. The == operator compares references (memory addresses), not content. Two strings with identical text can be == false.

Summary

Java variables have fixed types declared at compile time. Primitives (int, double, boolean, char) store values directly. String is a class for text. Integer division truncates — cast to double when you need decimals. Use Scanner for input and .equals() to compare strings.