Lecture 6 and 7 - Constructors , Objects , Key features of OOP

Question 1

What is a java constructor?

Answer 1

constructor is invoked at the time of object creation. It constructs the values i.e. initialises data for the object.

Constructors could also be overloaded.

Question 2

Must for a constructor:

Answer 2

name must be same as its class name

must have no explicit return type

Question 3

What happens if a constructor is omitted in the class?

Answer 3

In this case, the compiler automatically create a default no-arg constructor.

Question 4

How do you create an object?

Answer 4

Examples

Rectangle rectSmall = new Rectangle();
or:
Rectangle rectLarge;
rectLarge = new Rectangle(10.0,15.0);

Question 5

Steps for creating an object:

Answer 5

Declaration
Instantiation
Initialization

Accessing instance members (i.e, attributes and methods):

Question 6

Explain instantiation:

Answer 6

The new keyword is used to create the object, i.e., a block of memory space is allocated for this object.

Question 7

Explain initialization:

Answer 7

The new keyword is followed by a call to a constructor which initializes the new object.

Question 8

Explain how to access instance members:

Answer 8

First, create an instance/object of the class;

Then, use the dot operator (.) to reference the member
attributes or member methods

ex.
Rectangle rect1 = new Rectangle(10,15,2,4);

double w = rect1.width;
rect1.move(2.5,0);

double area = new Rectangle(10,15).getArea();

Question 9

What can instance methods do ?

Answer 9

can access instance attributes and instance methods directly.

can access static attributes and static methods directly.

Question 10

What can static methods do and not do ?

Answer 10

can access static attributes and static methods directly.

CANNOT access instance attributes or instance methods directly, they must use an object reference.

CANNOT use the this keyword as there is no instance for this to refer to

Question 11

The arguments of a method could be:

Answer 11

Primitive Data Type

Reference Data Type (Class, interface, array, enum)

Question 12

Explain passing by value :

Answer 12

The method gets a copy of the variable you pass. If you change the copy inside the method, the original value remains the same.

Question 13

What is auto-boxing and unboxing

Answer 13

Autoboxing Converting a primitive value (an int, for
example) into an object of the corresponding wrapper class(Integer).

Unboxing Converting an object of a wrapper type (Integer) to its corresponding primitive (int) value.

Question 14

How does java apply auto-boxing and unboxing?

Answer 14

Assignment

Method invocation