Library Classes ICSE Class 10: Wrapper Classes and Character Methods

What Library Classes is about

Weightage: CISCE publishes no chapter weightage. Section A (40 marks) is compulsory: 1-mark MCQs and 2-mark short answers including output and error questions; in Section B you answer any four of six 15-mark programming questions written in Java.Source: CISCE ICSE Computer Applications (661) specimen paper structure for 2027

Key concepts

Wrapper classes and composite types

A wrapper class wraps a primitive value inside an object, so that it can be treated as an object and has useful methods. Every primitive type has one, all in java.lang:
byte - Byte, short - Short, int - Integer, long - Long, float - Float, double - Double, char - Character, boolean - Boolean.
(Note the full names Integer and Character.)

A class is a composite (non-primitive) type: it groups data and methods, can be created by the user, and its variables refer to objects. Primitive types are built in, hold one value and have no methods. Wrapper classes bridge the two: Integer is a class whose objects hold one int.

Worked example

(2 marks) Name the wrapper classes of char and int, and the package in which they are found.

Try it first, then show the solutionSolution

1. char → Character; int → Integer - 1 mark.
2. Package java.lang, imported by default - 1 mark.

Answer: Character (for char) and Integer (for int); both are in java.lang, the default package.

Parse methods: string to number

Parse methods convert a String containing a number into the primitive value:
int Integer.parseInt(String s): Integer.parseInt("45") → 45
long Long.parseLong(String s): Long.parseLong("9876543210") → 9876543210
float Float.parseFloat(String s): Float.parseFloat("2.5") → 2.5
double Double.parseDouble(String s): Double.parseDouble("25") → 25.0

Why needed: input or data read as text cannot be used in arithmetic directly: "12" + "8" gives "128", but Integer.parseInt("12") + Integer.parseInt("8") gives 20.

If the string is not a valid number ("12a"), a runtime error occurs (NumberFormatException). Method names are case sensitive: parseDouble, not Parsedouble, and the argument must be a String: Double.parseDouble(25) is wrong.

Worked example

(1 mark) Which converts "25" to 25.0?
(a) Double.Parsedouble("25")
(b) Double.parse("25")
(c) Double.parseDouble("25")
(d) Double.parseDouble(25)

Try it first, then show the solutionSolution

1. Correct name and case, with a String argument.

Answer: (c) Double.parseDouble("25")

Character test methods

These static methods of Character return boolean:
Character.isDigit(ch): true if ch is '0' to '9'. isDigit('7') → true.
Character.isLetter(ch): true for a letter. isLetter('g') → true; isLetter('5') → false.
Character.isLetterOrDigit(ch): true for a letter or a digit. isLetterOrDigit('#') → false.
Character.isLowerCase(ch): true for a lower-case letter. isLowerCase('a') → true.
Character.isUpperCase(ch): true for an upper-case letter. isUpperCase('a') → false.
Character.isWhitespace(ch): true for a space, tab or new line. isWhitespace(' ') → true.

They are clearer than writing ranges like ch >= 'A' && ch <= 'Z', and the board may require them ('Use Character and String methods only').

Worked example

(1 mark) The method to check whether a character is a letter or a digit is:
(a) isDigit(char)
(b) isLetterOrDigit()
(c) isLetterOrDigit(char)
(d) isLETTERorDIGIT(char)

Try it first, then show the solutionSolution

1. Correct name and case, with a char argument.

Answer: (c) isLetterOrDigit(char)

Character conversion methods

char Character.toUpperCase(ch): returns the upper-case form; non-letters are returned unchanged. toUpperCase('b') → 'B'; toUpperCase('B') → 'B'; toUpperCase('3') → '3'.
char Character.toLowerCase(ch): returns the lower-case form. toLowerCase('Q') → 'q'.

These return a char; they do not change the original variable unless you assign: ch = Character.toUpperCase(ch);

Toggle case:
if (Character.isUpperCase(ch)) ch = Character.toLowerCase(ch);
else if (Character.isLowerCase(ch)) ch = Character.toUpperCase(ch);

Worked example

(2 marks) Give the output:
char c = 'm';
System.out.println(Character.toUpperCase(c));
System.out.println(c);
System.out.println(Character.isUpperCase(Character.toUpperCase(c)));

Try it first, then show the solutionSolution

1. toUpperCase returns 'M' but c is not changed - 1 mark.
2. isUpperCase('M') is true - 1 mark.

Answer: M
m
true

Autoboxing and unboxing

Autoboxing: the automatic conversion of a primitive value into an object of its wrapper class by the compiler.
Integer obj = 25; // int 25 boxed into an Integer object
Character c = 'x'; // char boxed into Character

Unboxing: the automatic conversion of a wrapper object back into its primitive value.
int n = obj; // Integer unboxed to int
Double price = 4.5; // boxed
double d = price; // unboxed to 4.5

Boxing = primitive → object; unboxing = object → primitive. Before autoboxing, programmers had to write these conversions by hand.

Worked example

(2 marks) Differentiate between boxing and unboxing with an example of each.

Try it first, then show the solutionSolution

1. Boxing: primitive to wrapper object, e.g. Integer a = 10; - 1 mark.
2. Unboxing: wrapper object to primitive, e.g. int b = a; - 1 mark.

Answer: Boxing converts a primitive value into an object of its wrapper class (Integer a = 10;), while unboxing converts the wrapper object back into a primitive value (int b = a;).

Library Classes in real life

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

Topics in this chapter

• Wrapper Class Methods, Character Tests and Autoboxing

Library Classes: syntax and key facts

Wrapper classes

byte Byte, short Short, int Integer, long Long, float Float, double Double, char Character, boolean Boolean

All in java.lang.

Parse methods

Integer.parseInt, Long.parseLong, Float.parseFloat, Double.parseDouble (String → number)

Invalid text → NumberFormatException.

Character methods

isDigit, isLetter, isLetterOrDigit, isLowerCase, isUpperCase, isWhitespace (boolean); toLowerCase, toUpperCase (char)

Static: Character.method(ch).

Boxing

1. 1.Autoboxing: primitive
2. 2.object; Unboxing: object
3. 3.primitive

How the board asks Library Classes

The question patterns to expect, and the method that works for each.

Tricks to remember Library Classes

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

• Two odd names

  Only int and char change their names: Integer and Character; the rest just capitalise.

  Use it when: Wrapper class names.

• is...() asks, to...() changes

  is-methods answer yes/no (boolean); to-methods return a converted char.

  Use it when: Return types of Character methods.

Library Classes: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 10-mark question split into 2 + 3 + 5 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Library Classes

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

• ✗Mistake: Using ranges instead of Character methods

  The 2025 examiners reported ch >= 'A' && ch <= 'Z' in the Super string program that required Character methods.

  ✓Do this: Use Character.isUpperCase / isLowerCase when the question says so.

  Usually costs: 2 to 4 marks of 15.

• ✗Mistake: Interchanging boxing and unboxing

  Reported by the 2024 examiners.

  ✓Do this: Boxing puts a primitive in a box (object); unboxing takes it out.

  Usually costs: 2 marks.

• ✗Mistake: Wrong package

  In 2025 many chose java.util for Character; in 2024 some wrote 'wrapper classes' for java.lang.

  ✓Do this: Wrappers: java.lang; Scanner: java.util.

  Usually costs: 1 mark.

• ✗Mistake: Wrong method syntax

  The 2025 MCQ offered Parsedouble, parse and a non-String argument.

  ✓Do this: Double.parseDouble("25"): exact case, String argument.

  Usually costs: 1 mark.

• ✗Mistake: Counters initialised inside the loop; = instead of ==

  Both errors were reported in the 2025 Super string program.

  ✓Do this: Initialise counters once before the loop; compare with ==.

  Usually costs: 2 to 3 marks.

Practice questions on Library Classes

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 wrapper class of char is:
   (a) Char
   (b) Character
   (c) Chr
   (d) String

   Hint

   Full name.

   Answer

   (b) Character

2. Q2 · 1 mark · MCQBoard level

   Character.isUpperCase('a') returns:
   (a) A
   (b) true
   (c) false
   (d) a

   Hint

   boolean.

   Answer

   (c) false

3. Q3 · 1 mark · MCQHarder board level

   Integer p = 7; has already been declared. Which statement causes unboxing?
   (a) Integer a = 5;
   (b) int b = p;
   (c) Double c = 2.0;
   (d) Character d = 'k';

   Hint

   Object to primitive.

   Answer

   (b) int b = p;

4. Q4 · 2 marks · Very short answerBoard level

   State the return type and purpose of Character.isWhitespace(char).

   Hint

   Spaces.

   Answer

   It returns boolean: true if the character is a white-space character such as a space, tab or new line, otherwise false.

5. Q5 · 2 marks · Short answerHOTS / competency

   Give the output: System.out.println(Integer.parseInt("7") * 2 + "7" + 2);

   Hint

   Left to right.

   Answer

   1472 (7 × 2 = 14; then "14" + "7" + "2")

6. Q6 · 2 marks · Very short answerBoard level

   Fill in the blanks: Wrapper classes are part of the ______ package; the Scanner class is part of the ______ package and must be imported.

   Hint

   lang, util.

   Answer

   java.lang; java.util

7. Q7 · 15 marks · Long answerHarder board level

   Write a program to accept a String of digits (e.g. "40213") and print the sum of its digits using Character.isDigit and type conversion.

   Hint

   ch - '0'.

   Answer

   import java.util.Scanner;
   class DigitString
   {
   public static void main(String args[])
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("Enter digits: ");
   String s = sc.next();
   int sum = 0;
   for (int i = 0; i < s.length(); i++)
   {
   char ch = s.charAt(i);
   if (Character.isDigit(ch))
   sum = sum + (ch - '0');
   }
   System.out.println("Sum = " + sum);
   }
   }
   Trace "40213": 4 + 0 + 2 + 1 + 3 = 10 → Sum = 10.

8. Q8 · 2 marks · Short answerHOTS / competency

   Why is it better to use Character.isUpperCase(ch) than ch >= 'A' && ch <= 'Z'?

   Hint

   Clarity; question demand.

   Answer

   It is clearer and less error-prone, it also works for upper-case letters beyond A to Z in Unicode, and when a question says 'use Character methods' the examiners expect the library method (the 2025 examiners listed range comparisons as an error).

9. Q9 · 2 marks · Very short answerHarder board level

   What happens when Integer.parseInt("12a") runs?

   Hint

   Runtime.

   Answer

   A runtime error (NumberFormatException) occurs, because "12a" is not a valid integer string.

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.Wrapper class of int?Show answer

   Integer

2. 2.Character.toUpperCase('5')?Show answer

   '5' (unchanged)

3. 3.Return type of isDigit?Show answer

   boolean

4. 4.Primitive to object automatically?Show answer

   Autoboxing

5. 5.Package of wrapper classes?Show answer

   java.lang

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Wrappers in java.lang: Integer, Character, Double...
• ☐parseInt, parseLong, parseFloat, parseDouble: String → number.
• ☐isDigit, isLetter, isLetterOrDigit, isLowerCase, isUpperCase, isWhitespace → boolean.
• ☐toLowerCase, toUpperCase → char; assign to keep the change.
• ☐Autoboxing: primitive → object; unboxing: object → primitive.

Library Classes: frequently asked questions