Basic Objects

Question 1

What is OOP?

Answer 1

Object Oriented Programming: a programming model based on the construction and use of objects- a program built under this model can be seen as a set of objects interacting together

Question 2

What makes up a class?

Answer 2

When you create a class, you’re actually defining a new type of data (i.e. an object)
Like any other type, a class has:
- a name (starts with an uppercase by convention
- some kind of data that is stored: a collection of variables
- some kind of actions that can be performed on the data: a collection of methods

Question 3

What are instance variables?

Answer 3

The object’s data: each instance will have its own specific set of values for the instance variables

Question 4

What do variables of an object type contain?

Answer 4

A reference, not the object itself: just like we have seen for arrays and Strings, because they are objects as well

Question 5

What are instance methods?

Answer 5

methods that can only be used on instances. These methods have access to the instance variables of that specific instance (static keyword is omitted)

Question 6

Why do we use the ‘this’ keyword?

Answer 6

In an instance method, if you have a local variable with the same name as an instance variable, the local one will be used. You use the keyword to represent the current instance of the object.

Question 7

What is the toString() method?

Answer 7

public String toString()

This method is automatically called by Java to get a String representation of your object (called when printing or concatenation is needed with your object)

Question 8

What are constructors?

Answer 8

A special method that is used to instantiate an object. We normally use it to initialize the instance variables. They have no return type at all (not even void) and must have the exact same name as the class. You can define multiple constructors so long as they have different signatures

Question 9

What is the purpose of the public keyword?

Answer 9

makes the method/variable accessible outside of the class

Question 10

What is the default constructor?

Answer 10

Provided by Java in case no constructor is defined in a class. Does not do anything except instantiate the object. It disappears as soon as one constructor is defined in the class. This means that you cannot use a constructor with no parameters if you declared only one constructor with parameters

Question 11

What is the distinction between functions and methods?

Answer 11

functions are static, methods work with instance variables because they are not static

Question 12

What are the access modifiers?

Answer 12

public, private, protected-private

Question 13

What do public and private mean?

Answer 13

public: means any code, anywhere, can access or use it
private: means only methods in this same class can access or use it

use private for instance variables and public for most instance methods

Question 14

What is encapsulation?

Answer 14

The idea that you can restrict access to some of the object’s fields. It protects the internals, preventing other classes from misusing the object.

Question 15

What are fancy names for get/set methods?

Answer 15

accessor/mutator

Question 16

What is a mutable object?

Answer 16

An object whose contents can be changed after the object is constructed. That is, it has a mutator method.

Question 17

What happens when you pass a mutable object to a method?

Answer 17

The method can change the contents of the object. the change is permanent: the object stays changed after the method ends. In Java, this only happens with mutable objects, not primitives

Question 18

What are immutable objects?

Answer 18

An object whose contents cannot be changed after the object is constructed. It has no mutator or equivalent methods, and all instance variables are private. Only write setter methods if you need them

Question 19

Are Strings mutable or immutable?

Answer 19

Every string is immutable. Once it’s created you cannot change it’s value. Every time you “modify” the value of a String variables, a new string object is created and the new reference to it is returned

Question 20

What are class variables and methods?

Answer 20

Variables and methods which do not refer to any one specific instance but belong to the class as a whole. To create them, we need to use the static keyword

Question 21

What are some standard methods for comparing the actual data inside Objects?

Answer 21

object1.equals(object2)
object1.compareTo(object2)
- gives a negative value if object1 is “smaller
- gives a positive value if object1 is “larger”
- gives a zero if they are “equal”

you can write these methods for your own objects. the default ones are not useful