Using getters with this. in java

Question

I am doing a project for CS, and I just realized that I do not understand the mechanics behind the this. java reference and getters. Specifically, if I have the following:

class Circle{
   private int radius;
}

public Circle(int radius){
   this.radius = radius;
}

public int getRadius(){
   return radius;
}

• Why is it that for the constructor, I use this.radius to reference the data field "radius" in the Circle class, but for the constructor, I have this.radius = radius?

• Does it make a difference whether or not I use the this. so long as it is the only data field named radius?

• I just tested it on Sublime, and it outputs the same result.
  Just according to my own logic, would it not make more sense to use this.radius to return the radius in the getRadius() getter instead of just return radius because I am referring to the data field in the object Circle?

I really appreciate all the help I can get!

Answer 1

Actually when you refer to this.radius that means you use a class field variable. Otherwise (in your code) you may re-assign your radius as an argument in a given constructor that is may be unwanted in your case. To distinguish it you must either use different name of a variable or use this.

Answer 2

You don't need this in the constructor if you do not shadow the local name. That is with

public Circle(int r){
    this.radius = r;
}

You can write

public Circle(int r){
    radius = r;
}

The this is only required when it is used to specify which radius you are referring to.

Answer 3

It's because radius is the name of both parameter of constructor and field of the class. To disambiguate those this keyword is used. In case of getter this is not needed, but also won't hurt. Some formatters add this by default, it's equivalent to:

public int getRadius(){
    return this.radius;
}