Scanner Class and StringTokenizer in Java (ISC Class 11 Computer Science)

Understand Streams, Tokens, the Scanner Class and StringTokenizer

Scanner sc = new Scanner(System.in);
int n = sc.nextInt();         // reads the next token as int
double d = sc.nextDouble();
String w = sc.next();         // next token (a word)
String line = sc.nextLine();  // rest of the current line

StringTokenizer st = new StringTokenizer("to be or not");   // default delimiters: whitespace
StringTokenizer st2 = new StringTokenizer("12,45;7", ",;");  // user-defined delimiters

Streams, Tokens, the Scanner Class and StringTokenizer: key facts

Scanner methods

• •nextInt()
• •nextDouble()
• •next()
• •nextLine()
• •hasNext()

Typed reading.

Tokenizer methods

• •countTokens()
• •hasMoreTokens()
• •nextToken()

Splitting a string.

Delimiter rule

Default delimiters: space, tab, newline | Custom delimiters given as a String

Not returned as tokens.

How to answer Streams, Tokens, the Scanner Class and StringTokenizer questions

1. 1Write the input exactly as typed, with line breaks.
2. 2For each Scanner call, consume the token or the rest of the line.
3. 3Remember the leftover newline after nextInt().
4. 4For StringTokenizer, list the tokens after removing delimiters.

Streams, Tokens, the Scanner Class and StringTokenizer 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 Streams, Tokens, the Scanner Class and StringTokenizer

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

• next reads a word, nextLine reads a line

  next() stops at whitespace; nextLine() at the newline.

  Use it when: Scanner.

• Tokens exclude delimiters

  Delimiters vanish.

  Use it when: StringTokenizer.

• Count then loop

  countTokens() before nextToken().

  Use it when: Avoiding exceptions.

Streams, Tokens, the Scanner Class and StringTokenizer: solved examples

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

StringTokenizer st = new StringTokenizer("12,45;7,90", ",;");
int c = st.countTokens();
int sum = 0;
while (st.hasMoreTokens())
    sum += Integer.parseInt(st.nextToken());
System.out.println(c + " " + sum);

Where students lose marks in Streams, Tokens, the Scanner Class and StringTokenizer

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

• ✗Mistake: Using nextLine() directly after nextInt()

  Leftover newline.

  Wrong working

  int a = sc.nextInt(); String s = sc.nextLine();

  Right working

  Add sc.nextLine(); between them

  ✓Do this: Consume the newline.

  Usually costs: 2 marks.

• ✗Mistake: Forgetting the import

  Slipping.

  Wrong working

  Scanner without import java.util.Scanner;

  Right working

  import java.util.*;

  ✓Do this: Write the import.

  Usually costs: 1 mark.

• ✗Mistake: Counting delimiters as tokens

  Misreading.

  Wrong working

  12,45;7,90 has 7 tokens

  Right working

  4 tokens

  ✓Do this: Delimiters are removed.

  Usually costs: 1-2 marks.

Practice questions on Streams, Tokens, the Scanner Class and StringTokenizer

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

   Which class is used to split a string into tokens?
   (a) Scanner (b) StringTokenizer (c) Math (d) Character

   Hint

   Name says it.

   Answer

   (b) StringTokenizer

2. Q2 · 2 marks · Short answerBoard level

   A user types Neha Sharma and presses Enter. What does String x = sc.next(); store, and what would String y = sc.nextLine(); have stored instead? (2 marks)

   Hint

   Whitespace vs newline.

   Answer

   x = "Neha" (stops at the space); y = "Neha Sharma" (the whole line).

3. Q3 · 4 marks · Short answerHarder board level

   Give the output: StringTokenizer s = new StringTokenizer("this is, a test", ", "); System.out.println(s.countTokens()); while (s.hasMoreTokens()) System.out.print(s.nextToken().length() + " "); Also state the count for "10-20+30-40*2" with delimiters "-+*". (4 marks)

   Hint

   Delimiters are comma and space.

   Answer

   Tokens: this, is, a, test: countTokens() = 4. Output: 4 on the first line, then 4 2 1 4 on the next. For "10-20+30-40*2" the tokens are 10, 20, 30, 40, 2: count 5.

4. Q4 · 5 marks · Long answerHarder board level

   Write a program fragment that reads 5 integers separated by spaces in one line using Scanner and prints their sum and average. State the output for the input 10 20 30 40 50. (5 marks)

   Hint

   Loop with nextInt().

   Answer

   int s = 0; for (int i = 0; i < 5; i++) s += sc.nextInt(); System.out.println(s + " " + (double) s / 5); Output: 150 30.0

5. Q5 · 4 marks · Short answerHOTS / competency

   A program uses nextInt() for the number of students, then in a loop uses nextLine() to read each name. It reads the first name as an empty string. Explain the cause, give two ways to fix it and the effect on the number of names read for n = 3. (4 marks)

   Hint

   Leftover newline.

   Answer

   The newline after the number is left in the stream, so the first nextLine() returns "". Fix: call sc.nextLine() once after nextInt(), or read the number with Integer.parseInt(sc.nextLine()). With the bug, 3 calls give 1 empty and 2 real names (the third name is never read).

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.Standard input stream?Show answer

   System.in.

2. 2.Scanner method for an int?Show answer

   nextInt().

3. 3.StringTokenizer count method?Show answer

   countTokens().

4. 4.Default delimiter?Show answer

   Whitespace.

5. 5.Wrong type token throws?Show answer

   InputMismatchException.

Streams, Tokens, the Scanner Class and StringTokenizer: frequently asked questions