Input and Output with Scanner

Most practice programs – and almost every coding round – read input and print results. In Java the standard tool for reading from the keyboard is the Scanner class. It is easy to use, with one famous trap that catches nearly every beginner; you'll learn to avoid it here.

Reading input

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Name: ");
        String name = sc.nextLine();
        System.out.print("Age: ");
        int age = sc.nextInt();
        System.out.print("CGPA: ");
        double cgpa = sc.nextDouble();
        System.out.println(name + " (" + age + ") has CGPA " + cgpa);
    }
}

Name: Meera Nair
Age: 21
CGPA: 8.7
Meera Nair (21) has CGPA 8.7

The nextInt() + nextLine() trap

nextInt() reads the number but leaves the Enter key (the newline) in the input. The next nextLine() immediately reads that empty remainder and returns "".

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Roll no: ");
        int roll = sc.nextInt();
        System.out.print("Full name: ");
        String name = sc.nextLine();          // reads the leftover newline!
        System.out.println("[" + roll + "] [" + name + "]");
    }
}

Roll no: 42
Full name: [42] []

Fix it by consuming the leftover newline, or read every line as text and convert it yourself:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Roll no: ");
        int roll = Integer.parseInt(sc.nextLine().trim());   // read the whole line
        System.out.print("Full name: ");
        String name = sc.nextLine();
        System.out.println("[" + roll + "] [" + name + "]");
    }
}

Roll no: 42
Full name: Kabir Singh
[42] [Kabir Singh]

Validating input

If the user types letters where a number is expected, nextInt() throws InputMismatchException. Check first with hasNextInt():

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int age;
        while (true) {
            System.out.print("Age: ");
            if (sc.hasNextInt()) {
                age = sc.nextInt();
                if (age > 0 && age < 120) break;
                System.out.println("Enter a realistic age.");
            } else {
                System.out.println("Digits only, please.");
                sc.next();                 // discard the bad word
            }
        }
        System.out.println("Saved: " + age);
    }
}

Age: twenty
Digits only, please.
Age: 250
Enter a realistic age.
Age: 21
Saved: 21

Reading many values

Coding-test inputs usually start with a count, followed by that many numbers:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        long sum = 0;
        int max = Integer.MIN_VALUE;
        for (int i = 0; i < n; i++) {
            int x = sc.nextInt();
            sum += x;
            max = Math.max(max, x);
        }
        System.out.println("Sum = " + sum + ", Max = " + max);
    }
}

Sum = 59, Max = 23

Formatted output

public class Main {
    public static void main(String[] args) {
        String[] items = {"Pen", "Notebook", "Bag"};
        int[] qty = {2, 5, 1};
        double[] price = {10, 45.5, 899};

        System.out.printf("%-10s %5s %10s%n", "Item", "Qty", "Amount");
        double total = 0;
        for (int i = 0; i < items.length; i++) {
            double amount = qty[i] * price[i];
            total += amount;
            System.out.printf("%-10s %5d %10.2f%n", items[i], qty[i], amount);
        }
        System.out.printf("%-10s %5s %10.2f%n", "TOTAL", "", total);
        String line = String.format("Paid %,.2f on %s", 1234567.891, "07-11-2026");
        System.out.println(line);
    }
}

Item         Qty     Amount
Pen            2      20.00
Notebook       5     227.50
Bag            1     899.00
TOTAL               1146.50
Paid 1,234,567.89 on 07-11-2026

Practice

Summary

Interview practice: Java basics interview questions.