Java Tokens, Identifiers, Literals and Escape Sequences (ICSE Class 10)

Understand Tokens, Identifiers, Literals and Escape Sequences

Tokens, Identifiers, Literals and Escape Sequences: key facts

Five types of tokens

• •Keywords
• •Identifiers
• •Literals
• •Punctuators
• •Operators

Classify every symbol of a statement into one of these.

Identifier rules

• •Letters, digits, underscore and dollar sign only
• •Must not start with a digit
• •No spaces
• •Not a keyword
• •Case sensitive

A hyphen is not allowed: it is the minus operator.

Escape sequences in the syllabus

• •\n new line
• •\t tab
• •\\ backslash
• •\" double quote
• •\' single quote

Each is one character.

ASCII anchors

• •'A' = 65
• •'Z' = 90
• •'a' = 97
• •'z' = 122
• •'0' = 48
• •'9' = 57

Lower case is 32 more than upper case.

How to answer Tokens, Identifiers, Literals and Escape Sequences questions

1. 1Split the statement at every symbol and word, left to right.
2. 2Mark each piece: word reserved by Java (keyword), programmer name (identifier), fixed value (literal), ; , ( ) { } (punctuator), symbol that computes (operator).
3. 3For a name, check in order: first character not a digit, only letters digits _ $, no spaces, not a keyword.
4. 4For escape sequences, replace each by its single character and then print the line breaks and tabs in order.
5. 5For char arithmetic, convert characters to their ASCII numbers first, calculate as int, then convert back with (char) if a character is wanted.

Tokens, Identifiers, Literals and Escape Sequences 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 Tokens, Identifiers, Literals and Escape Sequences

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

• KILPO

  Keywords, Identifiers, Literals, Punctuators, Operators: KILPO.

  Use it when: Recalling the five token types.

• Not 1st digit, no gaps

  A name never starts with a digit and never has a gap or hyphen.

  Use it when: Valid or invalid identifier items.

• A65 a97 zero48

  A is 65, a is 97 and the digit zero is 48.

  Use it when: Character arithmetic.

Tokens, Identifiers, Literals and Escape Sequences: solved examples

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

double area = 3.5 * r;

System.out.println("Name:\t\"Ravi\"");
System.out.println("C:\\data\nEnd");

Where students lose marks in Tokens, Identifiers, Literals and Escape Sequences

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

• ✗Mistake: Calling a name with a hyphen or a space valid

  Only letters, digits, _ and $ are allowed; a hyphen is the minus operator.

  Wrong working

  Roll-No is a valid identifier.

  Right working

  Roll-No is invalid; Roll_No is valid.

  ✓Do this: Scan for symbols other than _ and $.

  Usually costs: 1 mark

• ✗Mistake: Writing "C:\Java" for a path in a string

  \J is not an escape sequence; the compiler reports an error. A backslash must be doubled.

  Wrong working

  System.out.println("C:\Java");

  Right working

  System.out.println("C:\\Java"); prints C:\Java

  ✓Do this: Double every backslash you want to print.

  Usually costs: 1 mark

• ✗Mistake: Printing 'A' + 1 as B

  Adding an int to a char gives an int; the cast (char) is needed to get a character.

  Wrong working

  System.out.println('A' + 1); gives B

  Right working

  System.out.println('A' + 1); gives 66; System.out.println((char)('A' + 1)); gives B

  ✓Do this: Look for the (char) cast before writing a letter.

  Usually costs: 1 mark

Practice questions on Tokens, Identifiers, Literals and Escape Sequences

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 of the following is a valid identifier in Java?
   (a) 3rd (b) int (c) total$ (d) a-b

   Hint

   Check the first character, keywords and symbols.

   Answer

   (c) total$

2. Q2 · 1 mark · Very short answerBoard level

   Write the escape sequence that moves the cursor to a new line.

   Hint

   Backslash and a letter.

   Answer

   \n

3. Q3 · 2 marks · Short answerHarder board level

   Write the output (show the line breaks):

   System.out.println("He said \"Hi\"");
   System.out.println("C:\\Java");

   Hint

   Each escape is one character.

   Answer

   He said "Hi"
   C:\Java

4. Q4 · 2 marks · Short answerHarder board level

   Write the output of:

   System.out.println((char)('A' + 5));
   System.out.println('A' + 5);

   (ASCII code of A is 65.)

   Hint

   Which line has the (char) cast?

   Answer

   F
   70

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

   Assertion (A): The word class cannot be used as the name of a variable in Java.
   Reason (R): class is a keyword and keywords have a fixed meaning for the compiler.
   (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

   Is class a keyword?

   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.Name the five types of tokens.Show answer

   Keywords, identifiers, literals, punctuators and operators.

2. 2.What is the ASCII code of capital A?Show answer

   65

3. 3.Which escape sequence prints a tab?Show answer

   \t

4. 4.Can an identifier start with a digit?Show answer

   No.

5. 5.What is Unicode?Show answer

   A character code covering the letters of many languages; Java char uses it.

6. 6.How many characters does \n count as?Show answer

   One.

Tokens, Identifiers, Literals and Escape Sequences: frequently asked questions