Elementary Concept of Objects and Classes ICSE Class 9 Computer Applications

What Elementary Concept of Objects and Classes is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Computer Applications. Class IX is examined by your school: one 100-mark theory paper of 2 hours, plus 100 marks of internal assessment (at least 15 programming assignments and a written project on ethical computing). This is topic 2 of the 2028 syllabus.Source: CISCE ICSE 2028 Computer Applications syllabus, Class IX

Key concepts

Objects: modelling real entities

An object is an identifiable entity that has:

• State (characteristics or attributes): the data that describe it at a moment. In Java these are stored in variables (data members, also called fields or instance variables).
• Behaviour: the actions it can perform or that can be performed on it. In Java these are methods (member functions).
• Identity: each object is distinct, even if two objects have the same state.

Examples:

• Real-life object: a car. State: colour, model, speed, fuel. Behaviour: start, accelerate, brake.
• A student. State: name, roll number, marks. Behaviour: attend class, write exam, show result.
• A mobile phone. State: brand, battery level, contacts. Behaviour: call, message, charge.

Software objects mirror real ones: the data members hold the state and the methods define the behaviour. The object's methods are the only proper way to change its state, which is encapsulation.

Worked example

(2 marks) Identify two attributes and two behaviours of an object 'Fan'.

Try it first, then show the solutionSolution

1. Attributes (state): speed, colour, brand, isOn (any two) - 1 mark.
2. Behaviours (methods): switchOn(), switchOff(), changeSpeed() (any two) - 1 mark.

Answer: Attributes: speed and colour. Behaviours: switching on (switchOn()) and changing speed (changeSpeed()).

A class as a specification and blueprint

A class is a template or blueprint that specifies the data members (attributes) and member methods (behaviour) common to all objects of one kind. It is a description, not a thing: no memory is used for an object's data until an object is created.

An object is an instance of a class. Many objects can be made from one class; each has the same set of attributes, but its own values.

Real-life analogy: the architect's plan of a house is the class; the houses built from it are objects. Each house has rooms in the same places, but the owners and colours can differ.

Java form:
class Lamp
{
boolean isOn; // state
String room;
void switchOn() { isOn = true; } // behaviour
void switchOff() { isOn = false; }
void show() { System.out.println(room + " lamp on? " + isOn); }
}

The class name begins with a capital letter by convention; data members and methods go inside the class braces.

Worked example

(2 marks) Distinguish between a class and an object.

Try it first, then show the solutionSolution

1. Class: a blueprint/template describing attributes and methods common to a set of objects; a logical entity - 1 mark.
2. Object: an instance of a class with its own values, occupying memory; a physical (actual) entity - 1 mark.

Answer: A class is a blueprint that defines the attributes and methods common to a group of objects; it is a logical description. An object is a specific instance of a class, with its own values of those attributes, created in memory.

A class as an object factory: creating objects with new

A class can be seen as a factory that produces similar objects on demand. Each object is created using the new operator:
Lamp a = new Lamp();

• Lamp a declares a reference variable a of type Lamp.
• new Lamp() creates an object in memory (allocates space for its data members) and returns its reference.
• = stores the reference in a.

Members are reached with the dot (.) operator: a.room = "Kitchen"; a.switchOn();

Each object has its own copy of the data members. As run in this study guide:
Lamp a = new Lamp();
Lamp b = new Lamp();
a.room = "Kitchen";
b.room = "Bedroom";
a.switchOn();
a.show();
b.show();
b.switchOn();
a.switchOff();
a.show();
b.show();
Output:
Kitchen lamp on? true
Bedroom lamp on? false
Kitchen lamp on? false
Bedroom lamp on? true

A boolean data member that has not been given a value starts as false; an int starts as 0; a String reference starts as null.

Worked example

(2 marks) Explain the statement: Student s = new Student();

Try it first, then show the solutionSolution

1. Student s declares a reference variable s of class type Student - 1 mark.
2. new Student() creates a Student object in memory; its reference is stored in s - 1 mark.

Answer: Student s declares a variable s that can refer to a Student object. new Student() creates a new Student object in memory, and the assignment stores a reference to that object in s, so its members can be used as s.member.

Class as a user-defined data type

Java has built-in (primitive) data types such as int, double and char. A class lets the programmer create a new data type with its own data and functionality.

Once a class Lamp is defined, Lamp can be used like any type to declare variables: Lamp a, b; just as int x, y;

Differences from a primitive type:

• A primitive variable holds a single value (int x = 5;).
• A class-type variable holds a reference to an object that may contain many values of different types and methods to work on them.

Why it is called user-defined: it is designed by the programmer (the user of the language), not built into Java. String and Scanner are classes too, written by Java's designers and supplied in its libraries.

Worked example

(2 marks) Why is a class called a user-defined data type?

Try it first, then show the solutionSolution

1. It is created by the programmer, not built into the language - 1 mark.
2. Once defined, its name is used to declare variables (objects), just like int or double, and it combines data with methods - 1 mark.

Answer: A class is designed by the programmer rather than built into Java, and once defined its name can be used to declare variables (objects) just like a built-in type; it bundles several data members with methods, so it is a user-defined data type.

Message passing: computation as method calls

In OOP, objects interact by sending messages to one another. Sending a message means calling a method of an object: the receiving object performs the action using its own data.

Form: objectName.methodName(arguments)
Examples:

• p.addCoins(50); sends the message 'add 50 coins' to the object p.
• a.switchOn(); asks lamp a to switch itself on.
• System.out.println("Hi"); sends the message println with the argument "Hi" to the object System.out (the screen output stream).

A message has three parts: the receiving object, the method name and any information (arguments) needed.

Real life: a customer (object) tells a waiter (object) 'bring one dosa' (message with information); the waiter tells the cook; the cook prepares it. Each object does its own job.

Why this matters: the caller does not need to know how the receiving object does the job (abstraction), and the data stays inside the object (encapsulation).

Worked example

(2 marks) What is meant by message passing? Identify the object, method and argument in p.addCoins(50);

Try it first, then show the solutionSolution

1. Message passing: objects communicate by calling each other's methods - 1 mark.
2. Object: p; method: addCoins; argument: 50 - 1 mark.

Answer: Message passing is the way objects communicate, by calling (invoking) one another's methods. In p.addCoins(50); the receiving object is p, the method is addCoins and the argument is 50.

Objects encapsulate state and behaviour

An object bundles its state (data members) and behaviour (methods) together. This is encapsulation in action.

Why it helps:

• The data of one object is separate from another's: changing a.isOn does not change b.isOn.
• Methods keep the data valid, e.g. a spend() method can refuse to spend more coins than there are.
• The rest of the program uses the object through its methods and does not need to know the details.

Two references to one object: if Counter c3 = c1; then c3 and c1 refer to the same object; a change through one is seen through the other. As run in this study guide:
Counter c1 = new Counter();
Counter c2 = new Counter();
c1.tick(); c1.tick(); c2.tick();
Counter c3 = c1;
c3.tick();
System.out.println(c1.count + " " + c2.count + " " + c3.count);
Output: 3 1 3
(tick() adds 1 to count.)

Worked example

(2 marks) In the program above, why does c1.count print 3 although c1.tick() was called only twice?

Try it first, then show the solutionSolution

1. Counter c3 = c1; makes c3 refer to the same object as c1; no new object is created - 1 mark.
2. c3.tick() therefore increases the same count to 3, which both c1 and c3 show - 1 mark.

Answer: c3 = c1 copies the reference, not the object, so c1 and c3 point to the same Counter. The call c3.tick() raises that object's count from 2 to 3, so c1.count and c3.count are both 3.

Elementary Concept of Objects and 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

• Class as a Blueprint, an Object Factory and a User-Defined Data Type
• Object State, Behaviour, References and Message Passing with Traces

Elementary Concept of Objects and Classes: syntax and key facts

Object

Object = state (data members) + behaviour (methods) + identity

Instance of a class.

Creating an object

ClassName obj = new ClassName();

new allocates memory; obj holds the reference.

Message passing

obj.method(arguments); obj.dataMember

Dot operator reaches members.

Three views of a class

Blueprint (specification); object factory; user-defined data type

All three in the syllabus.

How the school exam asks Elementary Concept of Objects and Classes

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

Tricks to remember Elementary Concept of Objects and Classes

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

• Blueprint, Factory, Type

  A class is a BFT: Blueprint, Factory, Type.

  Use it when: Three views of a class.

• SBI

  An object has State, Behaviour and Identity.

  Use it when: Features of an object.

• Dot means 'ask'

  a.switchOn() reads 'ask a to switch on'.

  Use it when: Message passing.

Elementary Concept of Objects and Classes: exam-level solved questions

Ordered from exam 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 a school exam set on the board pattern this is a 7-mark question split into 2 + 2 + 3 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 Elementary Concept of Objects and Classes

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

• ✗Mistake: Calling a method without an object

  Forgetting that methods belong to objects.

  Wrong working

  switchOn(); inside main() of another class

  Right working

  a.switchOn();

  ✓Do this: Create the object with new and use objectName.method().

  Usually costs: 1 to 2 marks.

• ✗Mistake: Forgetting new

  Declaring is mistaken for creating.

  Wrong working

  Lamp a; a.switchOn();

  Right working

  Lamp a = new Lamp(); a.switchOn();

  ✓Do this: A declaration only makes a reference; new makes the object.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Mixing up attributes and behaviours

  Verbs and nouns confused.

  ✓Do this: Attributes are things an object has (nouns); behaviours are things it does (verbs).

  Usually costs: 1 mark.

• ✗Mistake: Assuming s2 = s1 copies the object

  Thinking of primitive variables.

  ✓Do this: It copies the reference; both names refer to one object.

  Usually costs: 1 to 2 marks in output questions.

Practice questions on Elementary Concept of Objects and 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 · MCQExam level

   An object is an ______ of a class.
   (a) attribute
   (b) instance
   (c) method
   (d) keyword

   Hint

   Example made from the plan.

   Answer

   (b) instance

2. Q2 · 1 mark · MCQExam level

   The operator used to call a method of an object is:
   (a) new
   (b) =
   (c) . (dot)
   (d) ;

   Hint

   obj.method()

   Answer

   (c) . (dot)

3. Q3 · 1 mark · Very short answerHarder exam level

   Which of the following is a class and which are objects: Animal, Tiger in Ranthambore, Your pet dog Bruno?

   Hint

   General vs particular.

   Answer

   Animal is a class; a particular tiger in Ranthambore and the pet dog Bruno are objects (instances).

4. Q4 · 2 marks · Very short answerExam level

   Why is a class called an object factory?

   Hint

   Many similar objects.

   Answer

   Because, like a factory producing many similar items from one design, a class can produce any number of objects with the same attributes and methods, each with its own values, using new.

5. Q5 · 2 marks · Very short answerHarder exam level

   Write a statement to create an object m of class MobilePhone and call its method charge() with the argument 30.

   Hint

   new, then dot.

   Answer

   MobilePhone m = new MobilePhone();
   m.charge(30);

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

   What value does an int data member have if an object is created and nothing is assigned to it? What about a boolean data member?

   Hint

   Default values.

   Answer

   An int data member is 0 and a boolean data member is false by default.

7. Q7 · 2 marks · Short answerHOTS / competency

   Two objects s1 and s2 of class Student are created. s1.marks = 80; s2 = s1; s2.marks = 95; What is s1.marks now? Explain.

   Hint

   Reference copy.

   Answer

   95. After s2 = s1, both names refer to the same object, so setting s2.marks to 95 changes the marks seen through s1 too.

8. Q8 · 15 marks · Long answerHarder exam level

   Define a class Fan with data members speed (int) and isOn (boolean) and methods switchOn(), setSpeed(int s) and show(). In main(), create a fan, switch it on, set its speed to 3 and show it.

   Hint

   Same pattern as Lamp.

   Answer

   class Fan
   {
   int speed;
   boolean isOn;
   void switchOn()
   {
   isOn = true;
   }
   void setSpeed(int s)
   {
   speed = s;
   }
   void show()
   {
   System.out.println("On: " + isOn + ", speed: " + speed);
   }
   public static void main(String args[])
   {
   Fan f = new Fan();
   f.switchOn();
   f.setSpeed(3);
   f.show();
   }
   }
   Output (as run): On: true, speed: 3

9. Q9 · 3 marks · Short answerHOTS / competency

   A game has a class Player. 'Every player has a name and a score, and can jump and collect coins.' Explain how this sentence helps you design the class, and why the score of one player does not affect another's.

   Hint

   Nouns and verbs.

   Answer

   The nouns describing a player (name, score) become data members, and the verbs (jump, collect coins) become methods such as jump() and collectCoins(). Each Player object created with new gets its own copy of name and score, so collecting coins changes only that object's score.

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.What are the two parts of an object's description?Show answer

   State (data members) and behaviour (methods).

2. 2.Operator that allocates memory for an object?Show answer

   new

3. 3.A class used to declare variables is a ______ data type.Show answer

   User-defined.

4. 4.Calling a method of an object is called?Show answer

   Message passing.

5. 5.Default value of a boolean data member?Show answer

   false

Quick revision checklist

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

• ☐Object: state (data members) + behaviour (methods) + identity.
• ☐Class: blueprint, object factory, user-defined data type.
• ☐ClassName obj = new ClassName(); members via obj.member.
• ☐Message passing: obj.method(arguments).
• ☐Each object has its own copy of data; copying a reference shares one object.

Elementary Concept of Objects and Classes: frequently asked questions