Module 03 — Java I/O

1. Scanner in Depth

You already know how to create a Scanner and read a single integer. This section covers every reading method you will use regularly.

import java.util.Scanner;

Scanner input = new Scanner(System.in);

Scanner Reading Methods

import java.util.Scanner;

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

        System.out.print("Enter your age: ");
        int age = input.nextInt();

        System.out.print("Enter your GPA: ");
        double gpa = input.nextDouble();

        System.out.print("Enter your first name: ");
        String firstName = input.next();  // one word only

        System.out.println("Age: " + age);
        System.out.println("GPA: " + gpa);
        System.out.println("Name: " + firstName);
    }
}

Enter your age: 20
Enter your GPA: 3.75
Enter your first name: Alex
Age: 20
GPA: 3.75
Name: Alex

2. The nextLine() Problem

This is the most common Scanner bug beginners write. It catches almost everyone the first time.

What Goes Wrong

When a user types a number and presses Enter, the buffer holds two things: the number, and the newline character (\n) from the Enter key. nextInt() reads the number and stops — it leaves that \n sitting in the buffer. The next nextLine() immediately reads that leftover newline and returns an empty string.

import java.util.Scanner;

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

        System.out.print("Enter your age: ");
        int age = input.nextInt();       // reads 20, leaves '\n' in buffer

        System.out.print("Enter your name: ");
        String name = input.nextLine();  // reads '\n' -- user never types!

        System.out.println("Age: " + age);
        System.out.println("Name: [" + name + "]");  // prints empty brackets
    }
}

Enter your age: 20
Enter your name:
Age: 20
Name: []

The Fix

Add one extra nextLine() call immediately after the numeric read. Its only job is to consume — throw away — that leftover newline.

import java.util.Scanner;

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

        System.out.print("Enter your age: ");
        int age = input.nextInt();
        input.nextLine();   // THE FIX: consume the leftover newline

        System.out.print("Enter your name: ");
        String name = input.nextLine(); // now this works correctly

        System.out.println("Age: " + age);
        System.out.println("Name: " + name);
    }
}

Enter your age: 20
Enter your name: Randy Michak
Age: 20
Name: Randy Michak

3. Dialog Boxes: JOptionPane

Not every Java program runs in a terminal. JOptionPane gives you pop-up dialog boxes for desktop use. It is part of javax.swing, which is bundled with Java — no extra libraries needed.

import javax.swing.JOptionPane;

Showing a Message

showMessageDialog() pops up a window with text and an OK button. Pass null as the first argument to center it on the screen.

JOptionPane.showMessageDialog(null, "Hello, World!");

Getting Input

showInputDialog() shows a text field and returns whatever the user typed as a String.

String name = JOptionPane.showInputDialog("What is your name?");
JOptionPane.showMessageDialog(null, "Hello, " + name + "!");

Converting Input to a Number

Because showInputDialog() always returns a String, you need to convert it when you expect a number.

import javax.swing.JOptionPane;

public class DialogDemo {
    public static void main(String[] args) {
        String ageStr = JOptionPane.showInputDialog("Enter your age:");
        int age = Integer.parseInt(ageStr);     // String to int

        String gpaStr = JOptionPane.showInputDialog("Enter your GPA:");
        double gpa = Double.parseDouble(gpaStr); // String to double

        JOptionPane.showMessageDialog(null,
            "Age: " + age + "\nGPA: " + gpa);
    }
}

4. Formatted Output: printf()

System.out.printf() gives you precise control over how output looks. You define a format pattern and plug in values.

System.out.printf(formatString, value1, value2, ...);

Format Specifiers

public class PrintfDemo {
    public static void main(String[] args) {
        String name = "Alice";
        int age = 21;
        double gpa = 3.876;

        System.out.printf("Name: %s%n", name);
        System.out.printf("Age:  %d%n", age);
        System.out.printf("GPA:  %.2f%n", gpa);  // rounds to 2 decimal places
    }
}

Name: Alice
Age:  21
GPA:  3.88

Column Alignment

System.out.printf("%-15s %5d %8.2f%n", "Alice",   21, 3.876);
System.out.printf("%-15s %5d %8.2f%n", "Bob",     19, 2.950);
System.out.printf("%-15s %5d %8.2f%n", "Charlie", 22, 3.100);

Alice              21     3.88
Bob                19     2.95
Charlie            22     3.10

5. String.format()

String.format() uses the exact same format specifiers as printf(), but instead of printing it returns a formatted String you can store or use anywhere.

String message = String.format("Name: %-10s  GPA: %.2f", "Alice", 3.876);
System.out.println(message);

Name: Alice       GPA: 3.88

This is especially useful with JOptionPane, where you are building a string for a dialog box:

import javax.swing.JOptionPane;

public class FormatDialog {
    public static void main(String[] args) {
        String name = "Randy";
        double balance = 1234.5;

        String msg = String.format("Account: %s%nBalance: $%,.2f", name, balance);
        JOptionPane.showMessageDialog(null, msg);
    }
}

Account: Randy
Balance: $1,234.50

6. Type Conversion (Casting)

Java is strongly typed. You cannot freely mix types, but you can convert between them. Two kinds: widening (automatic) and narrowing (manual).

Widening — Automatic

Going from a smaller type to a larger one is always safe — no data is lost. Java handles it automatically.

int x = 42;
double d = x;       // int to double, no cast needed
System.out.println(d);  // 42.0

long big = 100L;
double bigD = big;  // long to double, still automatic

Narrowing — Manual Cast Required

Going from a larger type to a smaller one can lose data. Java forces you to be explicit with a cast operator: the target type in parentheses.

double price = 9.99;
int dollars = (int) price;    // truncates -- drops the decimal
System.out.println(dollars);  // 9, NOT 10 -- casting does NOT round

double gpa = 3.876;
int truncated = (int) gpa;
System.out.println(truncated); // 3

Widening Order

// byte  short  int  long  float  double
//
// Left to right = widening (automatic)
// Right to left = requires a cast

Integer Division Gotcha

Dividing two integers gives you integer division — the decimal is discarded. This surprises beginners constantly.

int a = 7;
int b = 2;

System.out.println(a / b);          // 3  -- decimal dropped!
System.out.println((double) a / b); // 3.5 -- cast one operand first

7. The Math Class

The Math class is built into Java — no import needed. Every method is static, so you call them directly: Math.methodName().

Core Math Methods

public class MathDemo {
    public static void main(String[] args) {
        System.out.println(Math.pow(3, 4));    // 81.0
        System.out.println(Math.sqrt(144));    // 12.0
        System.out.println(Math.abs(-25));     // 25
        System.out.println(Math.round(4.5));   // 5
        System.out.println(Math.round(4.4));   // 4
    }
}

Random Numbers in a Range

Math.random() gives you a decimal between 0.0 (inclusive) and 1.0 (exclusive). To get a random integer in a specific range, use this formula:

// Random int from min to max (inclusive):
int num = (int)(Math.random() * (max - min + 1)) + min;

// Example: random number from 1 to 6 (simulating a die roll):
int die = (int)(Math.random() * 6) + 1;
System.out.println("You rolled: " + die);

You rolled: 4

Summary

Vocabulary Review

Knowledge Check

Circle the letter of the best answer for each question.

✅ Answer Key