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Java Java Objects (Retired) Meet Objects Privacy and Methods

Posted by Jonathan Nott
Jonathan Nott
538 Points

What is happening here? why is their a letter m before the variable? public String mColor

Here is the Class:

public class GoKart { public String mColor = "red";

public String getColor(){ return mColor; } }

2 Answers

jcorum
jcorum
71,830 Points

Many consider it "best practice" to prefix instance variables with the letter m. It's not required, but you will see it a lot in these videos and challenges.

Jonathan Nott
Jonathan Nott
538 Points

Great, thanks.

That opens a new question, how do you best describe an instance variable and how is it different to a standard variable?

jcorum
jcorum
71,830 Points
jcorum
jcorum
71,830 Points

Sorry, but there really are no standard variables. Variables can either be class (static) variables, or instance variables. 

public class Circle {
    private double radius;  //instance variable
    private static int numCircles = 0;  //class variable

    public Circle(double radius) { //constructor
        this.radius = radius;
        numCircles++;  //keep track of how many circles are created
    }

    public int getNumCircles() {  //getter method for numCircles
        return numCircles;
    }

    public static void main(String[] args) {
      Circle c1 = new Circle(23.5); 
      System.out.println(c1.getNumCircles());  // prints 1
    }
}

In the above code, whenever a new circle object is created (as an instance of Circle), it has its own copy of the instance variables (here radius, which is the only one). You can see why: every circle can and may have a different radius, which is specified when you call the constructor.

Class variables are different. There is only one value, which is shared by every object created from the class. Again, you can see why: it wouldn't be very valuable information if every circle had a different value for numCircles. Every Circle object can read the class variable numCircles, because of the getter method, but there is only one value at any given time. If there are 10 Circle objects then numCircles would be 10. If there are 100, numCircles would be 100.

Note that it would be a big mistake to write a setter method for numCircles.