Scanner and Basic I/O — Reading Input, Console, System Streams, and printf Formatting

In this tutorial, you will learn about Scanner and Basic I/O. We cover key concepts, practical examples, and best practices to help you master this topic.

Java's Scanner class provides flexible text Parsing for user input, while Console, System.in/out/err, and printf handle basic I/O operations. Every interactive program — from simple command-line tools to complex CLI applications — uses these facilities to communicate with the user.

What You'll Learn

• Reading input with Scanner
• Using Console for password input
• System.in, System.out, System.err
• Formatting output with printf and String.format

Why It Matters

Reading input correctly is trickier than it seems. Scanner has subtle behaviors around newlines, locales, and resource management. Understanding these prevents the common "nextLine() after nextInt()" bug.

Real-World Use

CLI tools, interactive scripts, and CI pipeline prompts all use basic I/O. Build tools (Maven, Gradle) print formatted output. printf-style formatting is used throughout logging and reporting.

The Scanner Class

Scanner reads and parses text from various input sources:

Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = scanner.nextLine();
System.out.println("Hello, " + name);
scanner.close();

Reading Different Types

Scanner scanner = new Scanner(System.in);

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

System.out.print("Enter salary: ");
double salary = scanner.nextDouble();

System.out.print("Are you employed? (true/false): ");
boolean employed = scanner.nextBoolean();

System.out.print("Enter a word: ");
String word = scanner.next();  // reads one token

scanner.nextLine(); // consume the leftover newline
System.out.print("Enter full address: ");
String address = scanner.nextLine(); // reads entire line

The nextLine() Trap

Scanner scanner = new Scanner(System.in);
System.out.print("Enter age: ");
int age = scanner.nextInt();     // reads int, leaves "\n" in buffer
System.out.print("Enter name: ");
String name = scanner.nextLine(); // reads the leftover "\n", not the name!

Fix: add an extra scanner.nextLine() after nextInt() to consume the newline.

Delimiters

Scanner scanner = new Scanner("apple,banana,cherry");
scanner.useDelimiter(",");
while (scanner.hasNext()) {
    System.out.println(scanner.next());
}

Reading from Files

Scanner scanner = new Scanner(Path.of("data.txt"));
while (scanner.hasNextLine()) {
    String line = scanner.nextLine();
    System.out.println(line);
}

The Console Class

Console is best for password input (disables echoing):

Console console = System.console();
if (console == null) {
    System.out.println("No console available (running in IDE?)");
    return;
}

String username = console.readLine("Username: ");
char[] password = console.readPassword("Password: ");
System.out.println("Welcome, " + username);

// Always clear password arrays
Arrays.fill(password, ' ');

System.console() returns null when the program is run in an IDE (no terminal attached).

System.out, System.in, System.err

System.out.println("Normal output");   // stdout
System.err.println("Error output");    // stderr
int data = System.in.read();           // raw byte input

System.out and System.err are PrintStream objects. System.in is an InputStream.

Redirecting

System.setOut(new PrintStream("output.log"));
System.setErr(new PrintStream("error.log"));

printf and Formatting

Printf Syntax

System.out.printf("Hello, %s! You are %d years old.%n", name, age);

%n is the platform-independent newline.

Common Format Specifiers

Examples

// Decimal precision
System.out.printf("Price: $%.2f%n", 19.99); // $19.99

// Width and alignment
System.out.printf("%-10s %5d%n", "Alice", 95);
System.out.printf("%-10s %5d%n", "Bob", 87);
// Alice         95
// Bob           87

// Comma formatting
System.out.printf("%,d%n", 1000000); // 1,000,000

// Date formatting
LocalDate today = LocalDate.now();
System.out.printf("%1$tB %1$td, %1$tY%n", today); // June 28, 2026

String.format()

String message = String.format("Hello, %s! You have %d new messages.", name, count);

Formatter Class

StringBuilder sb = new StringBuilder();
Formatter formatter = new Formatter(sb);
formatter.format("Value: %.2f", 3.14159);
System.out.println(sb); // Value: 3.14

Common Mistakes

1. Not calling close() on Scanner wrapping System.in. It is technically optional (closing System.in is unusual), but IDEs may warn. Do not close Scanner wrapping System.in if you still need input.
2. Forgetting to consume the newline after nextInt(). Always call nextLine() after reading primitives to consume the leftover newline.
3. Assuming System.console() is never null. In IDEs, System.console() returns null. Always check for null.
4. Using %d for floating-point values. %d is for integers. Use %f for floating point. Wrong specifier throws IllegalFormatConversionException.
5. Locale-specific number formatting. Scanner uses the default locale — in some locales, 3.14 must be written as 3,14. Either set locale to Locale.US or handle the input format.

Practice Questions

1. What is the difference between next() and nextLine() in Scanner?
next() reads the next token (delimited by whitespace). nextLine() reads the entire line until the newline character.

2. Why does nextInt() followed by nextLine() skip the second input?
nextInt() leaves the newline character in the buffer. nextLine() consumes that leftover newline. Add an extra nextLine() after nextInt().

3. What does System.console() return in an IDE?
null. The Console object is only available when the program is run from an interactive terminal.

4. How do you format a number with commas using printf?
System.out.printf("%,d", number) for integers, System.out.printf("%,.2f", number) for decimals.

5. Why is printf preferred over string concatenation for formatted output?
It is more readable, supports locale-aware formatting, and separates format from data.

Challenge Question:
Write a program InteractiveCalculator.java that reads arithmetic expressions from the user (e.g., "3 + 4", "10 * 2.5") and prints the result. Use Scanner with appropriate delimiters. Handle errors: invalid expressions, division by zero, and exit when the user types "quit".

FAQ

{{< faq "Can I use Scanner with a String?" "Yes: new Scanner(\"content\"). This is useful for parsing strings without creating files." >}}

Mini Project

Write a program InteractiveTodo.java that:

1. Displays a menu: (1) Add task, (2) List tasks, (3) Remove task, (4) Exit
2. Reads the user's choice with Scanner
3. Stores tasks in an ArrayList<String>
4. Uses Console for password protection (ask for PIN before showing tasks)
5. Formats the task list with printf (index, task name, created date)
6. Handles edge cases: empty list, invalid menu choice, removing non-existent index
7. Loops until the user chooses Exit

What's Next

Basic I/O handles text, but numbers with precision require special care. Lesson 32 covers Math and BigDecimal — the Math class, BigInteger for arbitrary precision integers, and BigDecimal for financial calculations with precise rounding.