Object Oriented Concepts

Question 1

What is an object?

Answer 1

something that can be described by both data attributes (e.g., age, weight) and behavior
(e.g., eat, talk).

Question 2

Explain this class diagram

Answer 2

The
top compartment contains the name of the thing being modeled…in our case a cat. The next
compartment contains a list of the attributes we are including in our model. And the bottom
compartment contains a list of relevant behaviors.

Question 3

what is a class?

Answer 3

programming construct used to create objects. It defines the structure and behavior (data and code) shared by a set of objects.

It creates a new data type.

Question 4

What is encapsulation?

Answer 4

a protective wrapper that controls access to the code and data through a well-defined interface

Question 5

what does it mean if something is a “member” of a class?

Answer 5

the code and data in the class. Member variables and member methods

Question 6

whats the difference between a public and private method?

Answer 6

public can be accessed outside of the class. Private methods can only be accessed by code that is a member f hte class.

Question 7

What is inheritance

Answer 7

process by which one object acquires the properties of another object.

Important because it supports the concept of hierarchical classification (e.g. dog inherits mammal inherits animal)

Question 8

What is polymorphism?

Answer 8

“one interface multiple methods”

one interface used for a general class of actions

Question 9

What relationship between a class and an object?

Answer 9

object is instance of a class.

Class is template for an object

Question 10

what does ‘new’ operator do

Answer 10

dynamically allocates (meaninning allocates at run time) memory for an object

Box mybox = new Box();
Box mybox; // declare reference to object
mybox = new Box(); //allocates a Box object

Question 11

What is a constructor?

Answer 11

Defines what occurs when an object of a class is created.

class name followed by parentheses specifies the constructor for the class

Question 12

Explain this situation:

Box b1 = new Box();
Box b2 = b1;

Answer 12

Both point to same object. Changing b2 will update for b1 too.

Question 13

Whats difference between parameter and argument?

Answer 13

parameter is the variable defined by the method that receives the value.

argument is the value passed to a method when its invoked

Question 14

How write the constructor method?

Answer 14

same name as the class with capital

Box () {
….
}

Question 15

explain ‘this’

Answer 15

refers to the current object

this.width = w

Question 16

what is garbage collection?

Answer 16

releasing of memory of objects no longer needed.

Java does it automatically

Question 17

What is method overloading

Answer 17

two methods with different parameter declarations. When called, Java executes the one with matching parameters

one way Java supports polymorphism.

Question 18

for overloading, will java do automatic type conversions (e.g. int to double) to execute a method?

Answer 18

yes

if no test(int) exists, it will elevate the argument to a double to execute test(double)

Question 19

Explain what it means what objects passed as parameters are call-by-reference?

Answer 19

When primitive types are passed, it creates a new local variable, and updates to that variable in that method do not affect within the invoking method.

In contract, objects pass the reference to that data. So updates in the current method will update the reference, and thus affect the invoking method.

Question 20

What does recursion mean with respect to a method?

Answer 20

Defining something in terms of itself. A method calling itself.

Question 21

what is a package?

Answer 21

a group of classes

Question 22

How is private and public used for access control to instance variables?

Answer 22

public means it can be accessed and edited outside of the class. object.variable = 10 sets the variable to 10.

private means that variable can only be set within that class through a defined method like setNum. object.setNum(10)

public int a;
private int b;

Question 23

what does static do?

Answer 23

methods and variable declarations that can be used before any instances of the class are created. They run first on their own.

Question 24

what does ‘final’ do?

Answer 24

prevents its contents from being modified.
Useful for parameters and local variables to prevent it from changing within the methods.
NOT useful for object parameters which are reference variables. Can still edit underlying.

final int FILE_NUM = 1;

common to use uppercase for constants

Question 25

What’s an inner class?

Answer 25

A class created within another block scope. Could be created within another class, method, or even loop.

It can access all the attributes from the outer class, but the outer does not have access to the inner.

Question 26

what is a command line argument?

Answer 26

info passed to program when you run it, stored in array args[]

Question 27

what are variable-length arguments and how to use them and why

Answer 27

if want to create a method that takes in zero or more arguments because you’ll just take in what you can get.

used with “…”

static void vaTest(int … v)

v is declared as an array with variable amount of data.

can call with parameters and variable length parameters but variable needs to be last

int test(int a, int b, double c, int … vals)

Question 28

What is containment?

Answer 28

using other objects as class members

Question 29

Class compilation

Answer 29

classes usually maintained as separate files (allows for parallel development)

classes compiled separately

Question 30

this

not sure really

Question 31

Can return an object from local method?

Answer 31

Yes Java will copy the reference before method variable destruction.