Taking input with Scanner

The skipped question

What is Scanner?

Words you'll meet in this lesson

Scanner methods at a glance

Ways to read input in Java

Scanner can read from more than the keyboard

Reading input, one step at a time

1. Import Scanner

   import java.util.Scanner;

   Without this line javac reports “cannot find symbol: class Scanner”. The import only tells javac the full name; no code is copied.

2. Create one Scanner on System.in

   Scanner sc = new Scanner(System.in);

   A Scanner object is created on the heap. It wraps the standard input stream and keeps its own buffer of characters that haven't been used yet.

3. Prompt, then read

   System.out.print("Cups of chai: ");
   int cups = sc.nextInt();

   Use print (not println) so the cursor stays on the prompt line. nextInt() blocks: the program waits until a line is entered, then skips whitespace, reads digits and converts them to an int.

4. Clear the newline before reading a line

   sc.nextLine();                 // throw away the leftover newline
   System.out.print("Name: ");
   String name = sc.nextLine();

   nextInt stops right after the digits. The newline stays in the buffer. The extra nextLine() consumes it, so the next one waits for real input.

5. Validate, and close when done

   while (!sc.hasNextInt()) {
       System.out.print("Please enter a number: ");
       sc.next();                 // discard the bad token
   }
   int age = sc.nextInt();
   sc.close();

   hasNextInt() looks at the next token without removing it. Closing a Scanner on System.in also closes System.in itself, so do it only once, at the very end.

The input buffer: both views

Left: the console. Right: the characters waiting in Scanner's buffer, and what each method takes.

Reading input in real software

Production systems rarely read the keyboard, but they constantly read and validate input. The lessons from Scanner carry over directly.

Watch it with an example: the nextInt() then nextLine() trap

A kiosk asks for age, then name. The user types 21 Enter Riya Enter, but the name comes out empty. Watch Scanner's input buffer to see why.

Examples are simplified. Production systems use frameworks for input, but the parsing ideas are the same.

Riya's kiosk, fixed

In the live editor, type the input into the STDIN / input box before pressing Run. Each challenge's hint shows sample input.

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Cups of chai: ");
        int cups = sc.nextInt();
        sc.nextLine();                          // clear the leftover newline
        System.out.print("Name: ");
        String name = sc.nextLine();
        System.out.print("Price per cup: ");
        double price = sc.nextDouble();
        System.out.println(name + ", your bill is Rs " + (cups * price));
        sc.close();
    }
}

The console shows your typed input (3, Riya Sharma, 20) next to each prompt.

• import java.util.Scanner;Scanner lives in the java.util package.
• new Scanner(System.in)One Scanner reading from the keyboard. Create it once and reuse it.
• System.out.print("…")print keeps the cursor on the same line, so the answer appears next to the question.
• sc.nextLine();The fix for the trap: consumes the newline left by nextInt().
• sc.nextDouble()Reads a decimal number. Typing 20 is fine; it becomes 20.0.
• sc.close();Closes the Scanner and System.in. Do it once, at the end.

Watch the buffer drain

The input for this run is 3⏎Riya Sharma⏎20⏎. The note on each step shows what is left in Scanner's buffer.

Worked examples: real problems, solved and traced in memory

Real programs read input. Each program below reads from the keyboard with Scanner; the trace and the runner feed it the sample input shown. Watch each next…() call take the next piece of input and store it in a variable.

Basmati Rice
3
92.50

Basmati Rice x3 = Rs 277.5

Ravi 78 85 91

Ravi: total 254, average 84.7

40021876
Asha Kumari Verma

Account: 40021876
Leftover was []
Holder: Asha Kumari Verma

Check yourself

Quick revision

Interview and viva questions

Practice problems

Read input with Scanner and compute something with it. Each problem's sample input is used when you run it. The last three are business tasks.

Meena

Hello, Meena!

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String name = sc.nextLine();
        System.out.println("Hello, " + name + "!");
    }
}

25 17

42

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt(), b = sc.nextInt();
        System.out.println(a + b);
    }
}

2.5

6.25

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double x = sc.nextDouble();
        System.out.println(x * x);
    }
}

12 7

Area 84, perimeter 38

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int l = sc.nextInt(), w = sc.nextInt();
        System.out.println("Area " + l * w + ", perimeter " + 2 * (l + w));
    }
}

17
Kiran Rao

17: Kiran Rao

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int roll = sc.nextInt();
        sc.nextLine();
        String name = sc.nextLine();
        System.out.println(roll + ": " + name);
    }
}

1250

GST: Rs 225.0, total: Rs 1475.0

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double amount = sc.nextDouble();
        double gst = amount * 0.18;
        System.out.println("GST: Rs " + gst + ", total: Rs " + (amount + gst));
    }
}

23.5 104.7

Pay Rs 2460

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        double litres = sc.nextDouble(), rate = sc.nextDouble();
        System.out.println("Pay Rs " + Math.round(litres * rate));
    }
}

Asha Verma
29 Pune

Asha Verma (29), Pune

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String name = sc.nextLine();
        int age = sc.nextInt();
        String city = sc.next();
        System.out.println(name + " (" + age + "), " + city);
    }
}

Real-world scenario questions (asked in company interviews)

Input handling is where programs meet the messy real world. Decide your answer first, then open the model answer.