Classes and Objects 2

Question 1

What are getters and setters?

Answer 1

Methods that generally initialising/updating/accessing instance variables

aka Accessor/Mutator

Question 2

What are the default values when you create a new object?

Answer 2

The initial value of the instance variables are set to default values based on the data type

Question 3

What are the cons of initializing objects with default values?

Answer 3

You would need 100 lines to call create value for each attribute

Question 4

What is the definition of constructor?

Answer 4

A method used to create and initialise an object

Question 5

What are the features of a constructor?

Answer 5

• The right hand side calls a class’ constructor
• Constructors are methods
• Constructors are used to initialize objects
• Constructors have the same name as the class
• Constructors cannot return values
• A class can have one or more constructors, each with a different set of parameters

Question 6

What is the definition of method overloading?

Answer 6

Ability to define methods with the same name but with different signatures (argument types and/or numbers)

• This is a form of polymorphism (same method - different behaviour)

Question 7

What is the definition of polymorphism?

Answer 7

Ability to process objects differently depending on their data type or class

Question 8

How are methods with the same name distinguished?

Answer 8

• number of arguments
• type of arguments
• position of arguments

Question 9

What is the definition of “this”?

Answer 9

A reference to the calling object, the object that owns/is executing the method

Question 10

What is the definition of instance variable?

Answer 10

A variable in an instance of an object

example: CentreX, CentreY

Question 11

What is the definition of static variables?

Answer 11

A variable that is shared among all objects of the class; a single instance is shared among all objects of the class. Such an attribute using the class name

example: numCircles

Question 12

What is the definition of static members?

Answer 12

Methods and attributes that are not specific to any object of the class

Question 13

What is the definition of Static Method?

Answer 13

A method that doesn’t depend on (access/modify) any instance variables of the class. Such a method is invoked (called) using the class name.

Question 14

What are the constraints of static methods?

Answer 14

• Static methods can only call other static methods
• Static methods can only access static data
• Static methods cannot refer to Java keywords such as, this or super because they are related to objects (class instances)
• Do not make all methods and attributes in your classes static

Question 15

How to decide to make an attribute or a method static?

Answer 15

Consider whether it is a class level member or an instance specific member

Question 16

What does the equals method do?

Answer 16

It compares two objects to see if they are equal

Question 17

Why do we use the equals method?

Answer 17

Using “==” will only check if references are equal as opposed to checking if objects are equal

Question 18

What do we use the toString method for?

Answer 18

It is used to print the strings of the object

Question 19

What does the copy constructor do?

Answer 19

It creates a separate copy of the object sent as input