Characteristics of a class - class

Question

In OOP, class fields are characterized with several mutually exclusive options:
public vs private
final vs changeable (?)
static vs instance
Are there any other important characteristics of classes in Java or other common OOP languages, such as C++ and C#?

Answer 1

In programming, object oriented programming is a tool that has unleashed the real power of computing. OOP allows a programmer to define their own types and subsets of variables that they are able to use. Types like integers, doubles, strings, are all defined as a certain set of bits in the computers memory, so OOP gives a programmer the power to create their own types of mailable objects that can have different characteristics.
Public vs Private
These words reference the access that one has to variables and methods within a certain class. Public variables and methods are accessible anywhere meaning they can be called outside of the class that it exists in. Private methods and variables on the other hand, can only be called in the class where they exist.
Eg:
public class Vehicle{
private int milesPerGallon;
public int getMPG{
return this.milesPerGallon;
}
}
Vehicle car1 = new Vehicle();
In this example, the variable milesPerGallon is private while the accessor method (getMPG()) that returns this value is public. So, in order to get the miles per gallon for the car1 object, the programming could not call car1.milesPerGallon. They would instead have to call car1.getMPG() because that method is public.
The use of private variables may seem counterintuitive, but private class members are a way to manage complexity. If all members were public, then bugs and problems in code could be linked to any one of these public members. It is difficult to trace back problems to public members becasue they can be called anywhere. If something were to happen to a pri
A private member, by contrast, can only be accessed from inside the same class, so if something goes wrong with that, there is usually only one source file to look at. If you have a million lines of code in your project, but your classes are kept small, this can reduce your bug tracking effort by a factor of 1000.
-tdammers
Final vs Changeable
These two concepts refer to the malleability of variables within a class. If a member is final, this means that it cannot be changed later in the program. But without the final keyword, then the variables are free to change at any time. Say for example, you were writing a cash register class.
public class Register{
private final double valueOfPenny = .01;
private final double valueOfNickel = .05;
ect...
private int numberOfPennies;
private int numberOfNickels;
ect...
}
In the Register class you will obviously need to know the value of a penny, nickel, dime, ect. But these values are going to stay constant through out the program. So we can set the value of each coin to be a final variable so that it does not accidentally get changed on accident. On the other hand, the number of each coin inside of the register may change through out the program, so we cannot make these variables final.
Static vs Instance
A instance variable means that every object has its own copy of each instance variable. An easy way to remember this is to think that every instantiation of a variable has a copy of the instance variable in a class. By default, all variables of a class are instance variables.
Static variables on the other hand, belong only once to the entire class. No matter how many instantiations of the class have been made, the static variable still only belongs to the class as a whole.
Eg:
public class Vehicle{
private double milesPerGallon;
public static int numberOfVehicles;
}
Vehicle car1 = new Vehicle();
Vehicle car2 = new Vehicle();
Both car1 and car2 have a variable called milesPerGallon. This makes sense because different instances of the Vehicle class will have different values of MPG. But, the variable numberOfVehicles is static because it only acts as a counter for how many vehicles there are. Every vehicle does not need to know this information because it is pointless for car1 and car2 to know that there are 2 vehicles. Only the class must know the value of this variable.
Constructors
You asked if there was any additional information about OOP that would be important and I feel as if constructors are at the top of the list. The constructor is a method that gets called whenever a programmer makes a copy of a class. In other words, whenever the programmer makes a call and instantiates a class, the constructor is the first and only method that gets called. In Java, the constructor is required and the name of the constructor is the same name as the class. If you do not provide Java with a constructor, then it will generate a blank constructor for you. But the power of the constructor is very useful and should often be utilized. Let's go back to the Vehicle example. Let's say you want to make a vehicle that has a certain manufacturer and the year it was made. You can save these two pieces of information as private instance variables as we mentioned above. But every time a Vehicle object is created, you want these two variables to be set right away. (i.e. you want to construct the object).
public class Vehicle{
private makeOfVehicle;
private yearOfVehicle;
public Vehicle(String make, int year){ //constructor
this.makeOfVehicle = make;
this.yearOfVehicle = year;
}
}
Vehicle car1 = new Vehicle("Toyota",2005);
Vehicle car2 = new Vehicle("BMW",2002);
By using the constructor, you can set attributes of objects as they are created!
Please let me know if you have any questions concerning these concepts/code or any other OOP concepts