Reading Input with the Scanner Class (ICSE Class 10 Java)

Understand Scanner Methods for Reading Values

Scanner Methods for Reading Values: key facts

Setup

import java.util.Scanner; | Scanner in = new Scanner(System.in);

Both lines are needed; the import comes before the class.

Methods by type

int	nextInt()
long	nextLong()
short	nextShort()
float	nextFloat()
double	nextDouble()
word	next()
line	nextLine()

The method decides the type returned.

Character input

• •char c = in.next().charAt(0);
• •next() reads a word
• •charAt(0) takes its first character

Takes the first character of the word typed.

Three data entry ways

Initialization	before execution
Parameters	at the time of method call
Input stream	during execution

A frequent 3-mark question.

How to answer Scanner Methods for Reading Values questions

1. 1Write the typed input as a list of tokens separated by white space, noting the line breaks.
2. 2Take each Scanner call in order and match it to the next token (next, nextInt, nextDouble) or to the rest of the line (nextLine).
3. 3Check that each token fits the type; if not, state that the program stops with a run-time exception.
4. 4Remember the leftover line break after a number when nextLine() follows.
5. 5Then evaluate the output statements using the stored values.

Scanner Methods for Reading Values in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Scanner Methods for Reading Values

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Name the type

  next + Int, Double, Long...: the method name is "next" plus the type.

  Use it when: Choosing the method.

• char has no method

  There is no nextChar(): use next().charAt(0).

  Use it when: Character input.

• Enter is left behind

  A number leaves the Enter key waiting, so nextLine() after it reads nothing.

  Use it when: The skipped-input trap.

Scanner Methods for Reading Values: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

int a = in.nextInt();
double d = in.nextDouble();
String f = in.next();
String l = in.next();
System.out.println(l + ", " + f + " " + (a * d));

int n = in.nextInt();
String s = in.nextLine();
String t = in.nextLine();
System.out.println("[" + s + "]");
System.out.println("[" + t + "]");

import java.util.Scanner;
public class Rect {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.print("Length: ");
        int l = in.nextInt();
        System.out.print("Breadth: ");
        double w = in.nextDouble();
        System.out.println("Area = " + l * w);
        System.out.println("Perimeter = " + 2 * (l + w));
    }
}

char c = in.next().charAt(0);
System.out.println(c);
System.out.println((int) c);
System.out.println(c == 'Y');

Where students lose marks in Scanner Methods for Reading Values

The slips examiners see most often, and what to write instead.

• ✗Mistake: Forgetting the import or the Scanner object

  Without them the compiler cannot find Scanner or in.

  Wrong working

  int n = in.nextInt(); (no import, no Scanner in = ...)

  Right working

  import java.util.Scanner; ... Scanner in = new Scanner(System.in); int n = in.nextInt();

  ✓Do this: Write both lines first in every program.

  Usually costs: 1 mark

• ✗Mistake: Using nextChar()

  The method does not exist in Scanner.

  Wrong working

  char c = in.nextChar();

  Right working

  char c = in.next().charAt(0);

  ✓Do this: Use next().charAt(0).

  Usually costs: 1 mark

• ✗Mistake: Expecting nextLine() right after nextInt() to read the next line

  The leftover line break is read first, giving an empty string.

  Wrong working

  After 5 and Hello World: s = "Hello World"

  Right working

  s = "" ; a second nextLine() gives Hello World

  ✓Do this: Add one extra in.nextLine(); after the number.

  Usually costs: 1 to 2 marks

Practice questions on Scanner Methods for Reading Values

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   In which package is the Scanner class present?
   (a) java.lang (b) java.io (c) java.util (d) java.awt

   Hint

   The import line starts java.

   Answer

   (c) java.util

2. Q2 · 1 mark · Very short answerBoard level

   Name the Scanner method that reads an entire line of text including spaces.

   Hint

   next + Line.

   Answer

   nextLine()

3. Q3 · 2 marks · Short answerHarder board level

   The input is: 3 4
   Write the output:

   int a = in.nextInt(), b = in.nextInt();
   System.out.println(a + b);
   System.out.println(a + "" + b);

   Hint

   One line adds, the other joins.

   Answer

   7
   34

4. Q4 · 4 marks · Long answerHarder board level

   Write a complete program that inputs two integers and displays their average as a real number.

   Hint

   Divide by 2.0 to keep the fraction.

   Answer

   import java.util.Scanner;
   public class Avg {
   public static void main(String[] args) {
   Scanner in = new Scanner(System.in);
   int a = in.nextInt();
   int b = in.nextInt();
   double avg = (a + b) / 2.0;
   System.out.println("Average = " + avg);
   }
   }
   For input 7 and 10 it prints Average = 8.5.

5. Q5 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): The statement char c = in.next().charAt(0); stores only the first character of the word typed.
   Reason (R): next() reads one word and charAt(0) returns the character at index 0 of that word.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   What does index 0 mean?

   Answer

   (a) Both are true and R explains A.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which line creates the Scanner object?Show answer

   Scanner in = new Scanner(System.in);

2. 2.How do you read one character?Show answer

   in.next().charAt(0)

3. 3.What does next() stop at?Show answer

   The first white space (space, tab or line break).

4. 4.Which method reads a double?Show answer

   nextDouble()

5. 5.What is the import statement for Scanner?Show answer

   import java.util.Scanner;

6. 6.What is System.in?Show answer

   The standard input stream, the keyboard.

Scanner Methods for Reading Values: frequently asked questions