Taking input from the user

🎯 By the end of this session, the candidate can

• Read text from the keyboard.

• Convert text input into numbers.

• Structure a program as input → process → output.

• Read a crash message and find the problem line.

1. Programs that listen

So far, every value was written inside the code. Real programs ask the user. In Java, the standard tool for reading the keyboard is called Scanner. Here is the complete pattern; type it exactly, then we'll go through it line by line:

import java.util.Scanner;public class Greet {    public static void main(String[] args) {        Scanner sc = new Scanner(System.in);        System.out.print("What is your name? ");        String name = sc.nextLine();        System.out.println("Hello, " + name + "!");    }}

Output

What is your name? RohitHello, Rohit!

What each new line does:

Promise: we will open this box properly later

Scanner sc = new Scanner(System.in);: for now, "create a keyboard reader named sc." The word new creates an object, and Scanner is a class. Both ideas arrive in Session 14, and then this line will make complete sense. We'll also meet Scanner's other methods (nextInt, nextDouble) there, together with a famous trap they cause.

Promise: we will open this box properly later

import java.util.Scanner;: for now, "tell Java which toolbox the Scanner tool comes from." Why does Scanner need an import while System and Math never do? Session 16 (packages) answers that.

2. Turning text into numbers

nextLine() always gives text. The text "25" is not the number 25, and you can't do maths with it:

import java.util.Scanner;public class AddTwo {    public static void main(String[] args) {        Scanner sc = new Scanner(System.in);        System.out.print("First number: ");        String a = sc.nextLine();        System.out.print("Second number: ");        String b = sc.nextLine();        System.out.println(a + b);   // "10" + "20" = 1020 (text joining!)        int x = Integer.parseInt(a); // convert text to int        int y = Integer.parseInt(b);        System.out.println(x + y);   // 30    }}

Output

First number: 10Second number: 20102030

A shorter style reads and converts in one line. This is the pattern we’ll use for every number from now on:

System.out.print("Your age: ");int age = Integer.parseInt(sc.nextLine());

Integer.parseInt follows the same ClassName.tool(...) pattern as Math.max, so it's covered by promise. We'll learn what Integer itself is (a wrapper class) in Session 23.

📝 Note: Why not sc.nextInt()?

Scanner also has nextInt() and nextDouble(), and you'll see them in many tutorials. Mixing them with nextLine() causes a confusing bug where an input is "skipped". Reading every line with nextLine() and converting with parseInt avoids that bug completely. We'll study the trap properly in Session 14.

3. Input → Process → Output

Almost every program follows this shape. Planning it this way before coding makes problems much easier.

import java.util.Scanner;public class Main {    public static void main(String[] args) {        Scanner sc = new Scanner(System.in);        // INPUT        System.out.print("Total bill: ");        double bill = Double.parseDouble(sc.nextLine());        System.out.print("Number of people: ");        int people = Integer.parseInt(sc.nextLine());        System.out.print("Tip %: ");        int tipPercent = Integer.parseInt(sc.nextLine());        // PROCESS        double tip = bill * tipPercent / 100;        double perPerson = (bill + tip) / people;        // OUTPUT        System.out.printf("Tip: %.2f%n", tip);        System.out.pritf("Each person pays: %.2f%n", perPerson);    }}

4. When the user types something wrong

Run the age program and type twenty instead of a number:

Output

Your age: twentyException in thread "main" java.lang.NumberFormatException: For input string: "twenty"    at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)    at java.base/java.lang.Integer.parseInt(Integer.java:661)    at java.base/java.lang.Integer.parseInt(Integer.java:777)    at Age.main(Age.java:8)

This is a runtime error: the code compiled fine, but crashed while running. The red text looks scary, but reading it is simple:

1. First line: what went wrong: NumberFormatException: For input string: "twenty". The text couldn't be turned into a number.

2. Find the line with your file name: Age.java:8. That's where your code triggered it. Ignore the java.base lines; they're inside Java's own code.

Promise: We will open this box properly later

The crash message: for now, “the program crashed; read the first line and find your file name.“ In session 26 we’ll learn exception handling: how to catch these problems and response politely (“Please enter a number“) instead of crashing.

Interview Questions

Practice

1. easy Ask for a name and birth year and print "Hi Neha, you are about 24 years old" (use 2026 as the current year).

2. easy Ask for the length and width of a room in feet and print the area in square feet and square metres (1 sq ft = 0.0929 sq m).

3. easy Ask for marks in 5 subjects and print the total and the percentage with 2 decimals.

4. medium Ask for a number of days and print it as years, weeks and days (ignore leap years).

import java.util.Scanner;public class DaysBreakdown {    public static void main(String[] args) {        Scanner sc = new Scanner(System.in);        System.out.print("Number of days: ");        int totalDays = Integer.parseInt(sc.nextLine());        int years = totalDays / 365;        int plusDays = totalDays % 365;        int weeks = plusDays / 7;        int days = plusDays % 7;        System.out.println(years + " years, " + weeks + " weeks, " + days + " days");    }}

Number of days: 10002 years, 38 weeks, 4 days