第23行和24行哪个 r.a * this.b + r.b*this.a是咋回事，为啥有r.a还有this.??

Question

import java.util.Scanner;  public class Main {       public static void main(String[] args) {          Scanner in = new Scanner(System.in);          Fraction a = new Fraction(in.nextInt(), in.nextInt());          Fraction b = new Fraction(in.nextInt(),in.nextInt());             a.print();          b.print();          a.plus(b).print();              }  }  class Fraction{            private int a;      private int b;            public Fraction(int a,int b){          this.a = a;          this.b = b;       }            public Fraction plus(Fraction r){          int n = r.a * this.b + r.b*this.a;          int fm = r.b * b;          return  new Fraction(n,fm);       }  public void print(){          int t = getReduce(a,b);          int fz = a/t;          int fm = b/t;          if(fm==fz)              System.out.println(fm/fz);          else              System.out.println(fz+"/"+fm);      }     }