Java char, Unicode and Table-lookup Programs (ISC Class 11 Computer Science)

Understand Characters in Java: char, Unicode, Character Arithmetic and Table Lookup

char c = 'A';
c += 2;                              // compound assignment keeps type: c is 'C'
System.out.println(c);               // C
System.out.println(c + 1);           // 68  (int)
System.out.println((char) (c + 1));  // D
System.out.println('a' + 'b');       // 195 (int)
System.out.println("" + 'a' + 'b');  // ab

String w[] = {"zero","one","two","three","four","five","six","seven","eight","nine"};

Characters in Java: char, Unicode, Character Arithmetic and Table Lookup: key facts

char facts

• •char = 16 bits, 0 to 65,535
• •Promoted to int in expressions
• •(char) restores a character

Unicode value.

Digit to value

value = ch - '0' | ch = (char) (value + '0')

Digit characters.

Caesar shift

(char) ((ch - 'a' + k) % 26 + 'a') | Wraps around z to a

Small letters.

How to answer Characters in Java: char, Unicode, Character Arithmetic and Table Lookup questions

1. 1Write the codes of the characters involved.
2. 2Do the arithmetic as int.
3. 3Cast to char only when a character is wanted.
4. 4For lookup, compute the index and read the table.

Characters in Java: char, Unicode, Character Arithmetic and Table Lookup 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 Characters in Java: char, Unicode, Character Arithmetic and Table Lookup

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

• char + int = int

  Cast back with (char).

  Use it when: Arithmetic.

• ch minus zero char

  ch - '0' gives the digit value.

  Use it when: Digits.

• u plus four hex

  \u0915 is a Unicode escape.

  Use it when: Escapes.

Characters in Java: char, Unicode, Character Arithmetic and Table Lookup: solved examples

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

char c = 'A';
c += 2;
System.out.println(c);
System.out.println(c + 1);
System.out.println((char) (c + 1));
System.out.println('a' + 'b');
System.out.println("" + 'a' + 'b');

String s = "xyz", r = "";
for (int i = 0; i < s.length(); i++) {
    char ch = s.charAt(i);
    r = r + (char) ((ch - 'a' + 3) % 26 + 'a');
}
System.out.println(r);

String s = "" + 407;
for (int i = 0; i < s.length(); i++)
    System.out.print(w[s.charAt(i) - '0'] + " ");

Where students lose marks in Characters in Java: char, Unicode, Character Arithmetic and Table Lookup

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

• ✗Mistake: Printing a char sum as a character

  Missing the cast.

  Wrong working

  System.out.println('a' + 'b') gives ab

  Right working

  195

  ✓Do this: char + char is int.

  Usually costs: 2 marks.

• ✗Mistake: Forgetting the wrap-around in a shift

  Direct addition.

  Wrong working

  'z' + 3 = '}'

  Right working

  (25 + 3) % 26 = 2 gives c

  ✓Do this: Use % 26.

  Usually costs: 2 marks.

• ✗Mistake: Using the char as an index directly

  Not subtracting '0'.

  Wrong working

  w[ch]

  Right working

  w[ch - '0']

  ✓Do this: Convert the digit first.

  Usually costs: 2 marks.

Practice questions on Characters in Java: char, Unicode, Character Arithmetic and Table Lookup

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

   The value of (int) 'B' is:
   (a) 66 (b) 65 (c) 98 (d) 2

   Hint

   A is 65.

   Answer

   (a) 66

2. Q2 · 3 marks · Short answerBoard level

   Give the output of: char x = '8'; int n = x - '0'; System.out.println(n * 2); System.out.println(x + 1); System.out.println((char) (x + 1)); (3 marks)

   Hint

   '8' has code 56.

   Answer

   16
   57
   9

3. Q3 · 4 marks · Short answerHarder board level

   Write a segment to count the digits and the capital letters in a string s = "Room 21 at ISC" and give both counts. (4 marks)

   Hint

   Character.isDigit and isUpperCase.

   Answer

   Loop over s with Character.isDigit(ch) and ch >= 'A' && ch <= 'Z'. Digits: 2 and 1 = 2. Capital letters: R, I, S, C = 4. Output: 2 4

4. Q4 · 4 marks · Short answerHarder board level

   A program converts a string to its "next letter" form: each small letter is replaced by the next letter and z becomes a. Write the code and the output for "hello zoo". (4 marks)

   Hint

   Keep the space.

   Answer

   For each ch, if ch >= 'a' && ch <= 'z' append (char) ((ch - 'a' + 1) % 26 + 'a'), else append ch. "hello zoo" becomes "ifmmp app".

5. Q5 · 3 marks · Short answerHOTS / competency

   A student sees the output 98 for System.out.println('a' + 1) and expects b. Explain, then state the changes needed to print b and the output of System.out.println((char) ('a' + 1) + 1). (3 marks)

   Hint

   char + int is int.

   Answer

   'a' + 1 is an int expression (97 + 1 = 98). Use (char) ('a' + 1) to print b. In the second case (char) ('a' + 1) is 'b' and adding 1 again gives the int 99, so the output is 99.

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.char size in Java?Show answer

   16 bits.

2. 2.char + int gives?Show answer

   int.

3. 3.Digit char to value?Show answer

   ch - '0'.

4. 4.Unicode escape form?Show answer

   \u followed by four hex digits.

5. 5.Default char value?Show answer

   \u0000.

Characters in Java: char, Unicode, Character Arithmetic and Table Lookup: frequently asked questions