Java User Input with Scanner

Programs become interesting when they react to the user. In Java, the easiest way to read keyboard input is the Scanner class.

Reading input with Scanner#

import java.util.Scanner;

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

        System.out.print("Your name: ");
        String name = in.nextLine();

        System.out.print("Your age: ");
        int age = in.nextInt();

        System.out.println("Hi " + name + ", next year you'll be " + (age + 1));
        in.close();
    }
}

Your name: Asha Kumar
Your age: 21
Hi Asha Kumar, next year you'll be 22

• import java.util.Scanner; brings the class in (it’s not in java.lang).
• new Scanner(System.in) reads from the keyboard.

Method	Reads
nextLine()	the whole line (with spaces)
next()	one word (up to a space)
nextInt()	an int
nextDouble()	a double
nextBoolean()	true/false
hasNextInt()	checks if the next token is an int (doesn’t read)

The nextInt() + nextLine() trap#

System.out.print("Age: ");
int age = in.nextInt();          // user types 21 and presses Enter
System.out.print("City: ");
String city = in.nextLine();     // returns "" immediately!

nextInt() reads 21 but leaves the Enter (newline) behind. The next nextLine() reads that empty leftover. Fix: call in.nextLine(); once after nextInt() to consume it, or read every line with nextLine() and convert:

int age = Integer.parseInt(in.nextLine().trim());

Validating input#

If the user types abc where you expect a number, nextInt() throws InputMismatchException. Check first:

import java.util.Scanner;

public class SafeInput {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Enter marks (0-100): ");
        while (!in.hasNextInt()) {
            System.out.print("Numbers only, try again: ");
            in.next();                      // throw away the bad token
        }
        int marks = in.nextInt();
        System.out.println(marks >= 40 ? "Pass" : "Fail");
    }
}

Printing with printf#

printf formats output neatly, great for tables and bills:

public class Bill {
    public static void main(String[] args) {
        String[] items = {"Notebook", "Pen set", "Backpack"};
        double[] prices = {120, 250.5, 1899};
        double total = 0;
        System.out.printf("%-10s %10s%n", "Item", "Price");
        for (int i = 0; i < items.length; i++) {
            System.out.printf("%-10s %10.2f%n", items[i], prices[i]);
            total += prices[i];
        }
        System.out.printf("%-10s %10.2f%n", "TOTAL", total);
    }
}

Item            Price
Notebook       120.00
Pen set        250.50
Backpack      1899.00
TOTAL         2269.50

Format	Meaning
%d	integer
%f, %.2f	decimal, 2 decimal places
%s	String
%c	char
%n	new line (works on every OS)
%10s	right-align in 10 spaces
%-10s	left-align in 10 spaces
%,d	thousands separator: 1,000,000

A mini program: simple calculator#

import java.util.Scanner;

public class Calculator {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("First number: ");
        double a = Double.parseDouble(in.nextLine());
        System.out.print("Operator (+ - * /): ");
        String op = in.nextLine().trim();
        System.out.print("Second number: ");
        double b = Double.parseDouble(in.nextLine());

        double result = switch (op) {
            case "+" -> a + b;
            case "-" -> a - b;
            case "*" -> a * b;
            case "/" -> a / b;
            default -> Double.NaN;
        };
        System.out.printf("%.2f %s %.2f = %.2f%n", a, op, b, result);
    }
}

(You’ll learn switch properly in the next lesson.)

Other ways to read input#

• BufferedReader with InputStreamReader: faster, used in competitive programming.
• Command-line args: values passed when starting the program.
• Files: covered in the File I/O lesson.

Common mistakes#

• Forgetting import java.util.Scanner;.
• The nextInt() then nextLine() empty-line trap.
• Creating a new Scanner inside a loop: create one and reuse it.
• Closing a Scanner on System.in and then trying to read again: closing it closes the keyboard stream too.

Exercise#

1. Ask for length and width, then print the area and perimeter of a rectangle.
2. Ask for a temperature in Celsius and print Fahrenheit with one decimal place.
3. Ask for 3 subject marks and print the total, the average (2 decimals) and the grade.
4. Print a multiplication table for a number the user enters, nicely aligned with printf.

Hint for 4

System.out.printf("%2d x %2d = %3d%n", n, i, n * i); inside a loop from 1 to 10.

Summary#

• Scanner reads from System.in: nextLine, nextInt, nextDouble, hasNextInt.
• Avoid the nextInt + nextLine trap by reading lines and parsing them.
• printf with %d, %.2f, %s, %-10s and %n makes output neat.

Next: making decisions with if, else and switch.