Scanner Class

A billing counter program isn't much use if every price is fixed in the code. Real programs ask: what's your name, how many tickets, which date. The most common way for a beginner Java program to read what the user types is the Scanner class. It reads from the keyboard and turns the text into the types you need: int, double, String.

Your First Scanner

import java.util.Scanner;
 
public class Greeting {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Your name: ");
        String name = sc.next();
        System.out.print("Tickets: ");
        int tickets = sc.nextInt();
        int fare = tickets * 120;
        System.out.printf("%s booked %d tickets, Rs. %d%n",
                name, tickets, fare);
    }
}

Your name: Tickets: Srihitha booked 3 tickets, Rs. 360

When you run it yourself and type Srihitha and 3, the screen looks like this:

Your name: Srihitha
Tickets: 3
Srihitha booked 3 tickets, Rs. 360

How it works

1. Line 1 imports Scanner from the java.util package. Without it, the compiler can't find the class.
2. Line 5 creates a Scanner that reads from System.in, the keyboard.
3. Line 7: next() reads one word. Line 9: nextInt() reads one whole number.
4. print (not println) keeps the cursor next to each prompt, so the user types on the same line.

The output block above shows the prompts and the result on one line because what you type isn't part of the program's output.

Try it yourself: type Srihitha Anamaneni as the name. Why does the program crash or behave strangely?

The Reading Methods

How Scanner Sees Your Input

Scanner treats input as a stream of characters and splits it into tokens: pieces separated by spaces or new lines. The next...() methods take one token each. nextLine() is different: it takes everything up to the end of the current line.

The nextInt-Then-nextLine Trap

This is the most common Scanner bug. Read a number, then a full line:

import java.util.Scanner;
 
public class Trap {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int age = sc.nextInt();
        String name = sc.nextLine();
        System.out.println("[" + name + "] is " + age);
    }
}

[] is 25

nextInt() reads 25 but leaves the Enter you pressed after it waiting in the input. Line 7's nextLine() reads up to that new line, finds nothing before it, and returns an empty string. The name you typed is never read.

The fix: after nextInt() (or nextDouble(), next()), call sc.nextLine(); once to throw away the rest of that line, then read the real line:

import java.util.Scanner;
 
public class Fixed {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int age = sc.nextInt();
        sc.nextLine();
        String name = sc.nextLine();
        System.out.println("[" + name + "] is " + age);
    }
}

[Srihitha Anamaneni] is 25

When the User Types Something Wrong

If nextInt() finds abc instead of a number, it throws an InputMismatchException and the program stops. Check first with hasNextInt():

import java.util.Scanner;
 
public class SafeAge {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        if (sc.hasNextInt()) {
            System.out.println("Age: " + sc.nextInt());
        } else {
            System.out.println("Please type a whole number");
        }
    }
}

Please type a whole number

Common Mistakes

Forgetting the import. Without import java.util.Scanner; you get cannot find symbol: class Scanner.

Using next() for names with spaces. next() stops at the first space. Use nextLine() for full names and addresses.

Reading a line right after a number. Add the extra nextLine(), as shown above.

Interview Corner

Practice

1. Easy: Ask for two whole numbers and print their sum and their product.
2. Harder: Ask for a passenger's age (a number) and then their full name (with spaces). Print Srihitha Anamaneni, 25. Make sure the name isn't empty.

Key Takeaways