Objects and Classes, Input and Tokens: ISC Class 12 Computer Science

What Objects is about

Weightage: CISCE publishes no chapter or unit weightage for ISC Computer Science. Objects is part of Section B, 5 of the 2027 syllabus and can be asked anywhere in the 70-mark Paper 1 (Part I compulsory: ten 1-mark items and a 10-mark short-answer question; Part II: two of three 10-mark questions in each of Sections A and B, and two of three 5-mark questions in Section C).Source: CISCE ISC Computer Science (868) syllabus, examination year 2027; latest Paper 1 specimen (2025) and the 2025 board paper

Key concepts

Objects and classes

Object: a software entity with state (data, held in data members or attributes) and behaviour (methods that act on the data). Example: a bank account object has holder name and balance (state) and can deposit, withdraw and show (behaviour).

Class: a blueprint or template that defines the data members and methods common to all objects of a kind. An object is an instance of a class, created with new:
Account a = new Account("Riya", 5000);
Here a is a reference variable that refers to the object stored in memory.

Members:

• Instance variables: declared in the class without static; each object has its own copy (holder, balance).
• Class (static) variables: declared static; one copy shared by all objects; accessed as ClassName.variable (Account.count).
• Instance methods act on a particular object (a.deposit(1500)); static methods belong to the class.

Why objects: they model real-world things, keep data and the code that uses it together (encapsulation), hide details behind methods, and let classes be reused.

Worked example

[1] A class Student declares static int total; and int marks;. If 40 Student objects have been created, how many copies of total and of marks exist in memory? (Understand)

Try it first, then show the solutionSolution

1. total is a class (static) variable: one copy shared by all objects; marks is an instance variable: one copy per object, so 40 copies - 1 mark.

Answer: One copy of total (shared by every object and accessed as Student.total) and 40 copies of marks (one inside each object).

Constructors and the this keyword

A constructor is a special method that initialises a new object. It:
1. has the same name as the class;
2. has no return type (not even void);
3. is called automatically when the object is created with new.

Types:

• Default (non-parameterised) constructor: takes no arguments; if you write no constructor, Java supplies one that sets fields to default values (0, 0.0, false, null).
• Parameterised constructor: takes arguments to set initial values.
• Constructor overloading: several constructors with different parameter lists in the same class.

this: a reference to the current object (the object whose method or constructor is running). Uses:
1. To distinguish a data member from a parameter with the same name: this.balance = balance;
2. To call another constructor of the same class: this("Unknown", 0.0); (must be the first statement).
3. To pass the current object as an argument or return it.
this can be used in any instance method, not only constructors; it cannot be used in a static method.

// An object = data (attributes) + behaviour (methods).
// Shows instance variables, a class (static) variable, overloaded constructors and this.
public class Account
{
    String holder;            // instance variable: one copy per object
    double balance;           // instance variable
    static int count = 0;     // class variable: one copy shared by all objects

    Account()                 // default constructor
    {
        this("Unknown", 0.0); // calls the parameterised constructor
    }

    Account(String holder, double balance)   // parameterised constructor
    {
        this.holder = holder;     // this.holder is the data member; holder is the parameter
        this.balance = balance;
        count++;
    }

    void deposit(double amt)
    {
        balance = balance + amt;
    }

    boolean withdraw(double amt)
    {
        if (amt > balance)
            return false;
        balance = balance - amt;
        return true;
    }

    void show()
    {
        System.out.println(holder + " : " + balance);
    }

    public static void main(String[] args)
    {
        Account a = new Account("Riya", 5000);
        Account b = new Account();
        a.deposit(1500);
        boolean ok = b.withdraw(200);
        a.show();
        b.show();
        System.out.println("Withdrawal from b allowed? " + ok);
        System.out.println("Accounts created: " + Account.count);
    }
}

Output:

Riya : 6500.0
Unknown : 0.0
Withdrawal from b allowed? false
Accounts created: 2

Both objects increase the shared counter, so count is 2; b was created through the default constructor, which calls the parameterised one with this(...).

Worked example

[2] Give two differences between a constructor and a method. (Understand)

Try it first, then show the solutionSolution

1. A constructor has the class's name and no return type; a method has its own name and a return type (or void) - 1 mark.
2. A constructor runs automatically once when the object is created; a method runs only when called, any number of times - 1 mark.

Answer: A constructor has the same name as the class and no return type, and it is invoked automatically when an object is created with new. A method has its own name and a return type (or void), and runs only when it is called, as often as needed.

Analysing real-world problems with objects

To design a class from a description:
1. Identify the nouns that describe the thing and its properties: these become data members (Book: title, author, price, copies).
2. Identify the actions: these become methods (issue, returnBook, display).
3. Decide constructors: what must be known when an object is created.
4. Decide what is common to all objects (static) and what differs (instance).
5. Keep data private where possible and provide methods to use it.

Examples:

Real-world thing	Data members	Methods
Student	roll, name, marks[ ]	input( ), average( ), grade( )
Library book	title, author, copies	issue( ), returnBook( ), display( )
Bank account	holder, balance	deposit( ), withdraw( ), show( )
Train ticket	from, to, distance, class	fare( ), print( )

ISC Section B questions do this for you: they give the class name, data members and methods. Your job is to implement them exactly.

Worked example

[2] Identify suitable data members and methods for a class Ticket that calculates a bus fare from distance and passenger age. (Create)

Try it first, then show the solutionSolution

1. Data members: name, age, distance, fare - 1 mark.
2. Methods: Ticket( ) constructor, accept( ), calculate( ) (applying any concession by age), display( ) - 1 mark.

Answer: Data members: String name; int age; double distance, fare. Methods: a constructor, accept( ) to read details, calculate( ) to compute the fare with any age concession, and display( ) to print the ticket.

Input and output: Scanner, print classes and exceptions

Scanner (java.util.Scanner) reads tokens from an input source such as the keyboard (System.in).
Common methods: nextInt( ), nextLong( ), nextDouble( ), nextBoolean( ), next( ) (one word), nextLine( ) (rest of the line), hasNextInt( ) and hasNext( ) (check before reading).
Note: after nextInt( ), the newline remains in the input; a following nextLine( ) returns an empty string unless an extra nextLine( ) is called.

Output: System.out is a PrintStream with print( ), println( ) and printf( ). A PrintWriter can wrap System.out (or a file) and offers the same methods; with autoflush set to true, println( ) flushes the output.

Input/output exceptions:

• InputMismatchException: the next token does not match the type requested (typing 'seventeen' for nextInt( )).
• NoSuchElementException: no more input.
• IOException: a checked exception for failures in input/output (for example with BufferedReader or files); it must be caught or declared with throws.

Handling: put risky code in try, handle in catch; finally runs in either case.

import java.io.PrintWriter;
import java.util.InputMismatchException;
import java.util.Scanner;

// Reads an integer safely: a non-numeric token causes InputMismatchException, which is caught.
// Output goes through a PrintWriter wrapped around System.out.
public class SafeInput
{
    public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out, true);   // true: flush after println
        int age = -1;
        while (age < 0)
        {
            out.println("Enter your age:");
            try
            {
                age = sc.nextInt();
            }
            catch (InputMismatchException e)
            {
                String bad = sc.next();               // remove the bad token from the stream
                out.println("'" + bad + "' is not a whole number. Try again.");
            }
        }
        out.println("Age accepted: " + age);
    }
}

Input: seventeen, then 17

Enter your age:
'seventeen' is not a whole number. Try again.
Enter your age:
Age accepted: 17

Worked example

[2] What happens if a user types 'twelve' when a program calls sc.nextInt( ), and how can the program recover? (Analyse)

Try it first, then show the solutionSolution

1. nextInt( ) throws InputMismatchException because the token is not an integer - 1 mark.
2. Catch it in a try-catch block, discard the bad token with sc.next( ), and ask again (or check with hasNextInt( ) first) - 1 mark.

Answer: nextInt( ) throws an InputMismatchException. The program can catch it, remove the bad token with sc.next( ) and prompt again, or test sc.hasNextInt( ) before reading.

Tokens, whitespace and StringTokenizer

A token is a meaningful unit of input separated from others by delimiters. By default the delimiters are whitespace: spaces, tabs and newlines. Scanner's next( ) returns the next whitespace-separated token.

StringTokenizer (java.util.StringTokenizer) splits a string into tokens:

• new StringTokenizer(str) uses whitespace as delimiters.
• new StringTokenizer(str, " ,.") uses the given characters as delimiters.
• countTokens( ): number of tokens remaining.
• hasMoreTokens( ): true if tokens remain.
• nextToken( ): returns the next token.

Delimiters themselves are not returned (unless a third argument true is given), and consecutive delimiters do not produce empty tokens.

import java.util.Scanner;
import java.util.StringTokenizer;

// Reads a sentence and uses StringTokenizer to count words and find the longest word.
public class WordStats
{
    String sent;

    WordStats()
    {
        sent = "";
    }

    void accept()
    {
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter a sentence:");
        sent = sc.nextLine();
    }

    void analyse()
    {
        StringTokenizer st = new StringTokenizer(sent, " ,.?!");   // space and punctuation are delimiters
        int words = st.countTokens();
        String longest = "";
        while (st.hasMoreTokens())
        {
            String w = st.nextToken();
            if (w.length() > longest.length())
                longest = w;
        }
        System.out.println("Number of words: " + words);
        System.out.println("Longest word: " + longest);
    }

    public static void main(String[] args)
    {
        WordStats ob = new WordStats();
        ob.accept();
        ob.analyse();
    }
}

Input: Java runs anywhere, thanks to bytecode and the JVM.

Enter a sentence:
Number of words: 9
Longest word: anywhere

The comma and full stop are delimiters, so 'anywhere' and 'JVM' come out clean. 'anywhere' and 'bytecode' both have 8 letters; the strict > comparison keeps the first one found.

Worked example

[2] How many tokens does new StringTokenizer("a,b,,c d", ", ") produce, and what are they? (Apply)

Try it first, then show the solutionSolution

1. Delimiters are comma and space; consecutive delimiters (,,) do not create empty tokens - 1 mark.
2. Tokens: a, b, c, d → countTokens( ) = 4 - 1 mark.

Answer: Four tokens: a, b, c and d.

Objects 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

• Classes, Objects and Constructors in Java
• Scanner Input, Tokens and the StringTokenizer Class

Objects: syntax and key facts

Creating an object

ClassName ref = new ClassName(arguments);

new allocates memory and calls the constructor.

Constructor rules

Same name as class; no return type; runs automatically

Can be overloaded.

this

this.member = parameter; this(args) as first statement

Refers to the current object.

Scanner methods

nextInt( ), nextDouble( ), next( ), nextLine( ), hasNextInt( )

InputMismatchException on wrong type.

StringTokenizer

new StringTokenizer(s, delims); countTokens( ); hasMoreTokens( ); nextToken( )

Default delimiters are whitespace.

How the board asks Objects

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

Tricks to remember Objects

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

• Object = State + Behaviour

  S + B: what it has, what it does.

  Use it when: Definition of object.

• Constructor 'SNA'

  Same name, No return type, Automatic call.

  Use it when: Constructor rules.

• Tokenizer trio

  count, has, next: countTokens, hasMoreTokens, nextToken.

  Use it when: StringTokenizer methods.

Objects: 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 D style)

Read the situation, then answer every part. In the board paper this is a 6-mark question split into 1 + 2 + 2 + 1 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 Objects

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

• ✗Mistake: Vague difference between instance and class variables

  CISCE examiners' comments, 2024: candidates 'Gave vague explanation of difference between static and non-static variables'; the expected answer was a separate copy per object versus a single shared copy, referred to by object name or class name.

  ✓Do this: State: instance = one copy per object (object.name); static = one shared copy (ClassName.name).

  Usually costs: 1 mark.

• ✗Mistake: Choosing wrong combinations in 'which statements are valid' items

  CISCE examiners' comments, 2025: on statements about this, candidates were 'Confused by the combinations of statements and chose wrong option'.

  ✓Do this: Mark each statement true or false separately first, then pick the option that lists exactly the true ones.

  Usually costs: 1 mark.

• ✗Mistake: Constructor with a return type, or not initialising all members

  A constructor with void is no longer a constructor; Section B marks require every data member to receive a legal initial value.

  ✓Do this: No return type; initialise every member (0, "", null-safe values, or as the question says).

  Usually costs: 1 mark.

• ✗Mistake: Losing input after nextInt( )

  The leftover newline makes the next nextLine( ) return an empty string.

  ✓Do this: Insert an extra sc.nextLine( ) after reading a number when a full line is read next.

  Usually costs: Wrong program behaviour.

Practice questions on Objects

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 · Very short answerBoard level

   What is an object?

   Hint

   State and behaviour.

   Answer

   An instance of a class that has its own state (data members) and behaviour (methods).

2. Q2 · 1 mark · Very short answerBoard level

   Which keyword creates an object?

   Hint

   Allocates memory.

   Answer

   new

3. Q3 · 1 mark · Very short answerBoard level

   Name the package that contains Scanner and StringTokenizer.

   Hint

   Utilities.

   Answer

   java.util

4. Q4 · 1 mark · Very short answerHarder board level

   Can a constructor be declared with return type void? What happens?

   Hint

   It stops being a constructor.

   Answer

   If void is added, it becomes an ordinary method that happens to have the class's name and is not called automatically by new; if the class has no other constructor, Java supplies the default one.

5. Q5 · 2 marks · Short answerHarder board level

   [2] What is constructor overloading? Give an example of two constructors for a class Point.

   Hint

   Different parameter lists.

   Answer

   Having more than one constructor with different parameter lists. Point() { x = 0; y = 0; } and Point(int x, int y) { this.x = x; this.y = y; }

6. Q6 · 2 marks · Short answerHarder board level

   [2] Write code using StringTokenizer to print each word of a sentence s on a new line.

   Hint

   while hasMoreTokens.

   Answer

   StringTokenizer st = new StringTokenizer(s); while (st.hasMoreTokens()) System.out.println(st.nextToken());

7. Q7 · 5 marks · Long answerHOTS / competency

   [5] Write a class Counter with an instance variable clicks and a static variable total; a method click( ) that increases both. Create two objects, call click( ) twice on the first and once on the second, and state the values printed.

   Hint

   Shared versus own.

   Answer

   class Counter { int clicks; static int total; void click() { clicks++; total++; } } After c1.click(); c1.click(); c2.click(); the values are c1.clicks = 2, c2.clicks = 1, Counter.total = 3 (checked by running the code).

8. Q8 · 10 marks · Long answerHOTS / competency

   [10] Design a class Initials with data member name; a constructor; void accept( ); and void make( ) that uses StringTokenizer to print the initials of all words except the last, followed by the last word in full (for 'Mohandas Karamchand Gandhi' print 'M. K. Gandhi'). Write main( ).

   Hint

   Count tokens first.

   Answer

   Constructor sets name = ""; accept( ) reads the full name with nextLine( ); make( ): StringTokenizer st = new StringTokenizer(name); int n = st.countTokens(); for (int i = 1; i < n; i++) result += st.nextToken().charAt(0) + ". "; result += st.nextToken(); print result; main( ) creates the object and calls accept( ) and make( ). For 'Mohandas Karamchand Gandhi' it prints 'M. K. Gandhi' (checked by running the code).

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.Two features of a constructor?Show answer

   Same name as the class; no return type (and it runs automatically).

2. 2.What does this refer to?Show answer

   The current object.

3. 3.Exception for a wrong input type with Scanner?Show answer

   InputMismatchException.

4. 4.Default delimiters of StringTokenizer?Show answer

   Whitespace (space, tab, newline and similar).

5. 5.How is a static variable accessed?Show answer

   Through the class name, e.g. Account.count.

Quick revision checklist

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

• ☐Object = state (data members) + behaviour (methods); instance of a class.
• ☐Instance variable: per object. Static variable: shared, ClassName.var.
• ☐Constructor: class name, no return type, automatic; overloading allowed.
• ☐this: current object; this.x = x; this(...) first statement.
• ☐Scanner: nextInt, nextDouble, next, nextLine, hasNextInt.
• ☐InputMismatchException; IOException is checked.
• ☐StringTokenizer(s, delims): countTokens, hasMoreTokens, nextToken.

Objects: frequently asked questions