Chapter 1 - Unit 2

Elementary Concept of Objects and Classes

Class 10 - APC Understanding Computer Applications with BlueJ

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Fill in the blanks

Question 1

Creating object is the fundamental concept in object oriented programming language.

Question 2

A class is also considered as an object factory.

Question 3

A real world object deals with characteristics and behaviours.

Question 4

The objects of a class differs on various characteristics.

Question 5

The characteristics of the real world objects are considered to be the data members of the software objects.

Question 6

An object of a class is created by using a keyword new.

Question 7

Class is a blueprint of the objects.

Question 8

new keyword is used for dynamic allocation of an object.

Question 9

The living things are considered as real world objects.

Question 10

The different objects of a class have common behaviours.

Answer the following questions

Question 1

How will you define a software object?

Answer

A software object replaces the characteristics and behaviours of a real world object with data members and member methods, respectively.

Question 2

Define class and object with an example.

Answer

An object is an entity having a specific identity, specific characteristics and specific behavior. A class is a blue print that represents a set of objects that share common characteristics and behaviour. Take the example of a house. An architect will have the blueprints for a house. Those blueprints will be plans that explain exactly what properties the house will have and how they are all laid out. However, it is just the blueprint, we can't live in it. Builders will look at the blueprints and use those blueprints to make a physical house. They can use the same blueprint to make as many houses as they want. Each house will have the same layout and properties. Each house can accommodate its own families. So, in this example, the blueprint is the class, the house is the object and the people living in the house are data stored in the object's properties.

Question 3

What does the following statement mean?
Employee staff = new Employee ( );

Answer

This statement creates a new object of class Employee. The newly created object is assigned to a variable named staff which is of Employee type. The object can be accessed using staff variable.

Question 4

A class is also referred to as 'Object Factory'. Comment.

Answer

A class has the complete description of the data elements the object will contain, the methods the object can do, the way these data elements and methods can be accessed. A class can create objects of itself with different characteristics and common behaviour just like a factory can produce similar items based on a particular design. Hence, class is also referred to as 'Object Factory'.

Question 5

Why is a class known as composite data type?

Answer

A class can contain data members of various primitive and reference data types. Hence, class is known as composite data type.

Question 6

A statement is given as:
'Study_Table' is an object of the class 'Furniture'. Write down Java statement for the same.

Answer

Furniture Study_Table = new Furniture();

Question 7

Class and Objects are inter-related. Explain.

Answer

A Class is used to create various Objects that have different characteristics and common behaviours. Each object follows all the features defined within a class. That is why class is also referred to as a blue print or prototype of an object. This way we can say that they are inter-related.

Question 8

Why is an Object called an 'Instance' of a class? Explain.

Answer

A class can create objects of itself with different characteristics and common behaviour. So, we can say that an Object represents a specific state of the class. For these reasons, an Object is called an Instance of a Class.

Question 9

Write a statement to create an object 'Keyboard' of the class 'Computer'.

Answer

Computer Keyboard = new Computer();

Question 10

Mention three characteristics and two methods for the following Classes:

(a) Class Mobile Phone

Answer

Characteristics	Methods
colour	makeCall()
model	receiveCall()
IMEI Number

(b) Class Bike

Answer

Characteristics	Methods
colour	startEngine()
model	stopEngine()
Registration Number

(c) Class Fruits

Answer

Characteristics	Methods
name	buy()
quantity	sell()
cost

(d) Class Pen

Answer

Characteristics	Methods
name	write()
type	refill()
Company

Question 11

Design a class program to calculate the discount given to a customer on purchasing LED Television. The program also displays the amount paid by the customer after discount. The details are given as:
Class name : Television
Data members: int cost, int discount, int amount
Member functions:
Accept( ) : to input the cost of Television
Calculate( ) : to calculate the discount
Display( ) : to show the discount and the amount paid after discount

Answer

class Television {
    int cost;
    int discount;
    int amount;

    void Accept() {
        /*
         * Input the cost
         * of Television
         */
    }

    void Calculate() {
        /*
         * Calculate the discount
         */
    }

    void Display() {
        /*
         * Show the discount and
         * the amount paid
         * after discount
         */
    }
}