COMP213 - Advanced Object-Oriented Programming

Question 1

Define ‘immutable’

Answer 1

An object whose state cannot be modified after the object is created

Question 2

What is an object?

Answer 2

An allocated region of storage.

In procedural programming it can contain data or instructions but not both, in object-oriented programming it may contain data and instructions for that data.

Question 3

What is an array?

Answer 3

A container object that holds a fixed number of values of a single type.

Question 4

What is an element?

Answer 4

An item in an array

Question 5

What is a class?

Answer 5

A template/blueprint that will be used in constructing objects. It may contain fields that describe its state or methods that determine its behaviour

Question 6

What is public/private data?

Answer 6

Public data can be accessed from any class or method (e.g. card.name to access the public 'name' variable in the card class)
Private data can only be accessed within its class

Question 7

What are access modifiers?

Answer 7

A command word that specifies whether a variable is public, private or secret

Question 8

What is API?

Answer 8

Application Program Interface (e.g. Javadoc)

Question 9

What tags are available in Javadoc?

Answer 9

Any HTML tags as well as:
@author your name
@param the parameters of the method (on separate lines)
@return what the method holds
@throws what exceptions can be thrown
@version number or date of code
@since when this part has first existed

Question 10

What are local variables?

Answer 10

Variables declared within a method

Question 11

What are instance variables?

Answer 11

Variables declared within a class, not within a method

Question 12

What does the keyword ‘this’ do?

Answer 12

References the current object (e.g. this.name is the instance variable ‘name’, as opposed to the local variable ‘name’)

Question 13

What is the initial value of an integer?

Answer 13

0

Question 14

What is the initial value of a float?

Answer 14

0.0

Question 15

What is the initial value of a boolean?

Answer 15

false

Question 16

What is the initial value of a reference variable?

Answer 16

null

Question 17

What does a class constructor do?

Answer 17

Creates objects from the class blueprint

Question 18

How are identical methods distinguished?

Answer 18

By the types of their parameters

Question 19

What is a nested class?

Answer 19

A class declared inside another class (not a subclass or extension of a class)

Question 20

What are the two types of nested class?

Answer 20

Static nested classes with the modifier ‘static’

Non-static nested classes called ‘Inner’

Question 21

What are the motivations for implementing nested classes?

Answer 21

logical grouping: if a class is only useful to one other class, it is logical to nest it within
encapsulation: if a nested class is private within its top-level enclosing it is invisible to the outside world
more readable code: nesting small classes within top-level classes places the code closer to where it is used

Question 22

What is a queue?

Answer 22

A first-in-first-out (FIFO) data structure

Question 23

What is a java interface?

Answer 23

An abstract type that is used to specify a behaviour that classes must implement (e.g. a queue specifies that methods are needed to (a) take element from the head and (b) add an element to the back)

Question 24

What is inheritance?

Answer 24

The process where one class gets non-private properties (methods and fields, but not constructors) of another.

Question 25

What is a child class?

Answer 25

The class that inherits the properties of another (e.g. an extended class, a subclass)

Question 26

What is a parent class?

Answer 26

A class whose properties are inherited (e.g. base class, superclass)

Question 27

The keyword ‘extends’…

Answer 27

Makes a child class an extension of a parent class

Question 28

The keyword ‘super’…

Answer 28

Refers to the parent class to avoid confusion for classes with the same name or to invoke constructors of the parent class

Question 29

What is polymorphism?

Answer 29

The ability to process objects of various types and classes through a single, uniform interface

Question 30

How is static polymorphism achieved?

Answer 30

Through method overloading: several methods having the same name but different types/order/number of parameters (note: it’s not enough just to change the return type)

Question 31

How is dynamic polymorphism achieved?

Answer 31

Method overriding: a child class overrides a method with the same signature in the parent (e.g. both classes have a void method that prints something, the child class’s method will run instead of the parent class’s)

Question 32

The keyword ‘final’…

Answer 32

Restricts the user in the context of variables, parameters, methods, and classes (but not constructors)
A class declared 'final' can't be extended
A method declared 'final' in a parent class is still inherited to child classes, but cannot be overridden
A variable/parameter declared 'final' has a fixed value and becomes a constant

Question 33

Can a constructor be declared private?

Answer 33

Yes, but only if other constructors (i.e. with different parameters) are not private

Question 34

What are generic methods/classes?

Answer 34

Methods or classes that have type (template)
parameters, which can be replaced by actual
types, so one code works for many types

Question 35

What is an abstract method?

Answer 35

A method (with the keyword abstract) that has only a declaration, but no code.
A class containing an abstract method is also abstract and should be extended before objects can be created

Question 36

What is the point of abstract classes?

Answer 36

Help save time and effort by implementing essential methods that should be shared by several types of data

Question 37

What is an interface in java?

Answer 37

An abstract class with three types of methods: abstract, static and default

Question 38

How is an interface declared?

Answer 38

public interface Player {
….
}
Can either be public or have no modifier,
All methods in an interface are implicitly public
All fields in an interface are implicitly public static final (constants)

Question 39

The keyword ‘implements’….

Answer 39

Allows a class to implement all abstract methods of an interface
(e.g. public class Human implements Player{})
All methods in a class implementing an interface should be declared public

Question 40

What are default methods?

Answer 40

Methods declared default in an interface are to be inherited by a class implementing the interface or by another interface extending the interface. Interface, interface interface. Interface.

Question 41

What is a local class?

Answer 41

A class defined within a method (e.g. in an if statement or for loop), they are non-static and can’t contain static declarations (exception for constants (public static final))

Question 42

What is an anonymous class?

Answer 42

A one-use-only, nameless class that is declared and initiated at the same time (typically used for initialising local variables)

Question 43

What is encapsulation?

Answer 43

All values of a data object are hidden in a class so that unauthorised parties cannot access them

Question 44

What does concurrency mean?

Answer 44

That software can handle many transactions “simultaneously” even on a single processing unit

Question 45

What is a thread?

Answer 45

A lightweight process that acts as a path to be followed during program execution

Question 46

What is an exception (in programming)?

Answer 46

An event which occurs during the execution of the program that disrupts the normal flow of the program’s instructions.

Question 47

What is an exception (in Java)?

Answer 47

An object that wraps an error event that occurred within a method. It has information about the error, a state of the code when the error occurred, an potentially other custom information

Question 48

What is the call stack?

Answer 48

A stack of information about the active subroutines of a program

Question 49

What does it mean to ‘throw’ an exception?

Answer 49

When an error occurs in a method, the method creates an exception object and hands it to the runtime system, which then attempts to find a block of code (exception handler) to handle the exception - it searches through the call stack in reverse order, and terminates if there is no suitable handler whose type matches the exception

Question 50

What does it mean to ‘throw’ an exception?

Answer 50

When an error occurs in a method, the method creates an exception object and hands it to the runtime system, which then tries to find a block of code (exception handler) to handle the exception - the search begins in the method where the error occurred and goes backwards through the call stack. The program terminates if no handler is found

Question 51

Advantages for using exceptions

Answer 51

Error handling is separate from regular code
Cleaner algorithms, less clutter
Exceptions propagate errors up the call stack (more reliable, less work)
Exception classes group and differentiate error types

Question 52

The keywords ‘try’ and ‘catch’…

Answer 52

Separate code from error handling, by placing risky code in the try block and exception handling in the catch block (multiple catch blocks can be used for different exceptions)

Question 53

The keyword ‘finally’….

Answer 53

Denotes a block of code that is to be executed regardless of the exceptions in the try…catch preceding it

Question 54

The keyword ‘throws’….

Answer 54

declares and propagates an exception up the call stack,
is followed by class of an exception,
is used with the method signature after the parameter list,
can declare multiple exceptions.

Question 55

What does it mean to ‘rethrow’ an exception?

Answer 55

To throw an exception already caught in the catch block to pass it to a higher level function
E.g.
try{…}
catch ArithmeticException e{throw e}

Question 56

Of the keywords ‘throw’ and ‘throws’, which is used to explicitly throw an exception, and which to declare and propagate an exception up the call stack?

Answer 56

Throw explicitly throws an exception

Throws declares and propagates an exception up the call stack

Question 57

What is a ‘checked’ exception?

Answer 57

An exception that is checked at compile time, and the method in which it was thrown must either handle the exception or specify a handler with the ‘throw’ keyword

Question 58

The keywords ‘try’ and ‘catch’

Answer 58

By putting risky code in the ‘try’ block, we can handle any exceptions with the ‘catch’ block.

Question 59

The keyword ‘finally’…

Answer 59

Denotes code that will be executed regardless of exceptions thrown in the ‘try’ block (it’s a block that goes after the catch)

Question 60

The keyword ‘throws’…

Answer 60

is used to declare and propagate an exception up the call stack
is followed by class of an exception
is used with the method signature after the name with the parameter list
can be used to declare multiple exceptions
can be used for checked or unchecked exceptions