Class as the Basis of All Computation ICSE Class 10 Java

What Class as the Basis of All Computation 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

Member variables and member methods

Member variables (data members, fields, attributes) are declared inside the class but outside all methods. They define the state of each object and are available to every method of the class. Member methods (functions) define behaviour: what an object can do with its data.

class Account
{
double balance; // member variable (state)
void deposit(double amt) // member method (behaviour)
{
balance = balance + amt;
}
}

Local variables are declared inside a method; they exist only while the method runs and are not shared with other methods. A common board error is declaring the member variables again inside accept(), which creates new local variables and leaves the real members unchanged.

Worked example

(2 marks) What is the difference between a member variable and a local variable?

Try it first, then show the solutionSolution

1. Member variable: declared in the class outside methods; available to all methods; part of the object's state - 1 mark.
2. Local variable: declared inside a method; exists only during that method - 1 mark.

Answer: A member variable is declared inside the class but outside every method, belongs to the object and can be used by all its methods; a local variable is declared inside a method and exists only while that method runs.

Instance variables and class (static) variables

Instance variables (non-static members): each object gets its own copy. Two Student objects have their own names.
Class variables (static members): declared with static; only one copy is shared by all objects of the class. Used for counts and common values.

class Visitor
{
static int total; // shared
int id; // one per object
void enter()
{
total++;
id = total;
}
public static void main(String args[])
{
Visitor a = new Visitor();
Visitor b = new Visitor();
a.enter();
b.enter();
System.out.println(a.id + " " + b.id + " " + Visitor.total);
}
}
Output: 1 2 2

Rules: a static method can directly use only static members; to use instance members it needs an object (that is why main creates an object). An instance (non-static) method can use both static and instance members.

Worked example

(2 marks) Why can't a static method directly use an instance variable?

Try it first, then show the solutionSolution

1. A static method belongs to the class and can run without any object - 1 mark.
2. Instance variables exist only inside objects, so there is no object whose copy it could use - 1 mark.

Answer: A static method belongs to the class and may be called without creating any object, while an instance variable exists separately in each object; without an object there is no copy to refer to, so it must create or be given an object first.

Primitive and composite data types

Primitive (built-in) types: byte, short, int, long, float, double, char, boolean. They hold a single value of fixed size and are predefined by Java.
Composite (non-primitive, reference) types: classes, arrays, interfaces and String. They are built from primitive and other types, may be defined by the user, and their variables hold references to objects.

Declarations: int count = 0; (primitive) and Student s = new Student(); (composite: the variable s refers to an object created with new).

Differences: (1) predefined vs user-defined; (2) single value vs group of values and methods; (3) fixed size vs size depends on members; (4) stored directly vs accessed through a reference; (5) default value 0/false vs null.

Worked example

(2 marks) Give two differences between primitive and composite data types.

Try it first, then show the solutionSolution

1. Primitive types are predefined by Java; composite types are built from them, often by the user - 1 mark.
2. Primitive variables hold one value directly; composite variables refer to objects that may hold many values and methods - 1 mark.

Answer: (1) Primitive types such as int are predefined by Java, whereas composite types such as a class are built from primitive types, often by the user. (2) A primitive variable holds a single value directly, whereas a composite variable refers to an object that can hold several values and methods.

Class as abstraction and object factory

A class is an abstraction for a set of similar objects: it keeps only the features relevant to the program. A class Patient in hospital software may keep name, age and illness, ignoring hair colour. From this one description, any number of objects can be produced (object factory), each an instance with its own state.

Real-life analogy: 'Mobile phone' is the class; your phone and your friend's phone are objects. They share features (screen, battery, call()) but have different states (different contacts, charge levels).

Worked example

(2 marks) Explain 'objects are instances of a class' with an example.

Try it first, then show the solutionSolution

1. A class is a general description; an instance is one particular object made from it - 1 mark.
2. Example: class Car; myCar and taxi are instances with their own colours and speeds - 1 mark.

Answer: An instance is a particular object created from the general description in a class. For class Car, Car myCar = new Car(); and Car taxi = new Car(); create two instances, each with its own colour and speed.

Writing a class-based Section B program

Board pattern: 'Define a class with the following specifications: class name, member variables, member methods.' Steps:
(1) Use exactly the class, variable and method names given.
(2) Declare member variables in the class, outside methods (not inside accept()).
(3) accept(): create a Scanner and read each member variable with the right method.
(4) calculate(): use an if-else-if ladder for slabs; store the result in the member variable.
(5) display(): print every member variable asked for, with labels.
(6) main(): create an object and call accept(), calculate() and display() in order.
(7) Write a variable description or comments.

Worked example

(2 marks) A student writes void accept() { Scanner sc = new Scanner(System.in); double weight = sc.nextDouble(); } for a member variable weight. What goes wrong?

Try it first, then show the solutionSolution

1. double weight inside accept() declares a new local variable that hides the member variable - 1 mark.
2. The member weight stays 0.0, so calculate() uses 0.0; write weight = sc.nextDouble(); instead - 1 mark.

Answer: Redeclaring weight inside accept() creates a local variable; the member variable weight remains 0.0, so later calculations are wrong. Write weight = sc.nextDouble(); without the type.

Class as the Basis of All Computation 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

• Instance and Static Variables, Primitive and Composite Types

Class as the Basis of All Computation: syntax and key facts

Class template

class Name { member variables; void accept() { } void calculate() { } void display() { } main() { Name ob = new Name(); ob.accept(); ob.calculate(); ob.display(); } }

Use the given names exactly.

Static rule

static method → static members directly; instance members only through an object

Instance methods can use both.

Slab template

if (x <= a) c = x * r1; else if (x <= b) c = a * r1 + (x - a) * r2; else c = a * r1 + (b - a) * r2 + (x - b) * r3;

How the board asks Class as the Basis of All Computation

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

Tricks to remember Class as the Basis of All Computation

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

• A-C-D-M

  Accept, Calculate, Display, Main: the four parts of every class program.

  Use it when: Section B structure.

• Static = one for all

  One static copy for all objects; one instance copy for each object.

  Use it when: Static vs instance.

Class as the Basis of All Computation: 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 Class as the Basis of All Computation

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

• ✗Mistake: Not using the names given in the question

  Both the 2024 and 2025 examiners reported changed class, variable or method names.

  ✓Do this: Copy every name exactly, including case.

  Usually costs: 1 to 2 marks of 15.

• ✗Mistake: Declaring member variables inside accept()

  Reported by the 2025 examiners in the CloudStorage program.

  ✓Do this: Declare members once in the class; inside methods assign without a type.

  Usually costs: 2 to 3 marks.

• ✗Mistake: Wrong slab logic or accepting the bill as input

  In 2025 candidates did not use the tier method, or accepted the bill instead of calculating it.

  ✓Do this: Compute the bill in calculate() using the slabs given.

  Usually costs: 4 to 6 marks.

• ✗Mistake: Not creating an object or calling methods

  Reported in both 2024 and 2025.

  ✓Do this: main(): ClassName ob = new ClassName(); then call each method.

  Usually costs: 2 to 3 marks.

• ✗Mistake: Confusion over static access

  The 2024 examiners found candidates unclear about static members and accessibility.

  ✓Do this: Static methods use instance members only through an object.

  Usually costs: 1 mark.

Practice questions on Class as the Basis of All Computation

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

   Variables declared inside a class but outside all methods are called:
   (a) local variables
   (b) member variables
   (c) parameters
   (d) constants

   Hint

   Members.

   Answer

   (b) member variables

2. Q2 · 1 mark · MCQBoard level

   A variable shared by all objects of a class is declared with the keyword:
   (a) final
   (b) static
   (c) new
   (d) void

   Hint

   One copy.

   Answer

   (b) static

3. Q3 · 1 mark · MCQHarder board level

   What is the default value of a String member variable that has not been assigned?
   (a) ""
   (b) 0
   (c) null
   (d) error

   Hint

   Reference type.

   Answer

   (c) null

4. Q4 · 2 marks · Very short answerBoard level

   Distinguish between an instance variable and a class (static) variable.

   Hint

   Copies.

   Answer

   Each object has its own copy of an instance variable, whereas a class variable, declared static, has a single copy shared by all objects of the class and can be used with the class name.

5. Q5 · 2 marks · Short answerHOTS / competency

   Give the output:
   class P
   {
   static int s = 0;
   int t = 0;
   void inc() { s++; t++; }
   public static void main(String args[])
   {
   P a = new P(), b = new P();
   a.inc(); a.inc(); b.inc();
   System.out.println(a.t + " " + b.t + " " + s);
   }
   }

   Hint

   s shared, t separate.

   Answer

   2 1 3

6. Q6 · 2 marks · Very short answerHarder board level

   Why is a class called an abstraction for a set of objects?

   Hint

   Relevant features.

   Answer

   Because it describes only the common, relevant features (variables and methods) of a group of similar objects, ignoring details that do not matter to the program; each object is a particular case of that description.

7. Q7 · 15 marks · Long answerBoard level

   Define a class Parking with member variables vno (int), hours (int) and bill (double). accept() reads them; calculate() charges ₹30 for the first hour and ₹20 for each additional hour; display() prints vno, hours and bill. Write main().

   Hint

   First hour is a flat charge.

   Answer

   import java.util.Scanner;
   class Parking
   {
   int vno, hours;
   double bill;
   void accept()
   {
   Scanner sc = new Scanner(System.in);
   System.out.print("Vehicle number and hours: ");
   vno = sc.nextInt();
   hours = sc.nextInt();
   }
   void calculate()
   {
   if (hours <= 1)
   bill = 30;
   else
   bill = 30 + (hours - 1) * 20;
   }
   void display()
   {
   System.out.println(vno + " " + hours + " " + bill);
   }
   public static void main(String args[])
   {
   Parking p = new Parking();
   p.accept();
   p.calculate();
   p.display();
   }
   }
   Trace 4521, 5 hours: 30 + 4 × 20 = 110.0; prints 4521 5 110.0.

8. Q8 · 2 marks · Short answerHOTS / competency

   In the Storage program, what is the bill for 31 GB? Show the working.

   Hint

   Third slab then tax.

   Answer

   225 + 195 + 1 × 11 = 431; not more than 500, so no tax; bill = 431.0.

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

   State two differences between a class and a primitive data type.

   Hint

   Methods; definer.

   Answer

   (1) A class can contain several variables of different types together with methods; a primitive holds one value and has no methods. (2) Primitive types are predefined by Java; classes can be defined by the user.

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.Where are member variables declared?Show answer

   Inside the class, outside all methods.

2. 2.How many copies of a static variable exist?Show answer

   One, shared by all objects.

3. 3.Is String primitive or composite?Show answer

   Composite (a class).

4. 4.What must main() do in a class program?Show answer

   Create an object and call the methods.

5. 5.Default value of an int member?Show answer

   0

Quick revision checklist

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

• ☐Class = member variables (state) + member methods (behaviour).
• ☐Instance: one per object; static: one shared.
• ☐Static methods need an object for instance members.
• ☐Primitive: single predefined value; composite: classes, arrays, String.
• ☐Section B: given names, members outside methods, accept-calculate-display, main with object.

Class as the Basis of All Computation: frequently asked questions