关于this的使用

public class Rational {
public Rational() {
this(0);
}

public Rational (int n) {
this (n, 1);
}

public Rational(int x, int y) {
int g = gcd(Math.abs(x), Math.abs(y));
num = x / g;
den = Math.abs(y) /g;
if ( y< 0) num = -num;
}

public Rational add(Rational r) {
return new Rational(this.num * r.den + r.num*this.den,
this.den * r.den);
}
public Rational subtract(Rational r) {
return new Rational (this.num * r.den - r.num * this.den, this.den * r.den);
}

public Rational multiply(Rational r) {
return new Rational( this.num * r.num, this.den *r.den);
}

public Rational divide(Rational r) {
return new Rational(this.num * r.den, this.den * r.num);
}

public String toString() {
if (den == 1) {
return "" + num;
}
else {
return num + "/" + den;
}
}

private int gcd(int x, int y) {
int r = x % y;
while (r != 0) {
x = y;
y = r;
r = x % y;
}
return y;
}
private int num;
private int den;
}

我想问一下关于
public Rational add(Rational r) {
return new Rational(this.num * r.den + r.num*this.den,
this.den * r.den);
}中this.num代表的是什么意思？如果可以的话，举个栗子来详细说明它们的参数