'void' type is not allowed here

I'm in the works of creating a program that dishes out basic arithmetics with fractions, i.e., add, subtract. However, I found myself stumped over this error.

Test.java:18: 'void' type not allowed here
System.out.println(r1.add(r2));
^
1 error

My code is as follows:

\\ The constructor \\
public class RationalNumber
{
	int num;
	int den;
	
	private int gcd(int x, int y)
	{
		while(y != 0)
		{
			int temp = x % y;
			x = y;
			y = temp;
		} 		
		return x;
	}
	
	public RationalNumber(int numerator, int denominator)
	{
		if(denominator == 0)
		{
			throw new IllegalArgumentException("You can't divide by zero.");
		}
		else
		{
			num = numerator;
			den = denominator;
		}
	}
	
	public RationalNumber()
	{
		this (0,1); 
	}
	
	public boolean equals(Object o)
	{
		if (o instanceof RationalNumber)
		{
			RationalNumber other = (RationalNumber) o;
			int r1 = this.gcd(num,den);
			int r2 = this.gcd(other.getNumerator(),other.getDenominator());
			return num/r1 == other.num/r2 && den/r1 == other.den/r2;	
		}
		else
		{
			return false;
		}
	}
	
	public int getDenominator()
	{
		return den;
	}
	
	public int getNumerator()
	{
		return num;
	}
	
	public void add(RationalNumber r1)
	{
		int commonDen = den * r1.getDenominator();
		int num1 = num * r1.getDenominator();
		int num2 = r1.getNumerator() * den;
		int sum = num1 + num2;
	}
	
	public void sub(RationalNumber r1)
	{
      int commonDen = den * r1.getDenominator();
		int num1 = num * r1.getDenominator();
		int num2 = r1.getNumerator() * den;
		int sum = num1 - num2;
	}
	
	public String toString()
	{
		if (den == 1)
		{
			return num + "";
		}
		else	
		{
			return num + "/" + den;
		}
	}
}

\\ The client code \\
public class Test
{
   public static void main (String[] args)
   {
		RationalNumber r1 = new RationalNumber(1,3);
		RationalNumber r2 = new RationalNumber(1,6);
		
		System.out.println(r1.add(r2));
	}
}

What is it I must do to rid myself of this problem? Also is my add and subtract methods correct?

[Eric Cute]

System.out.println()

will try to print anything inside the parentheses.

But as you can see your

r1.add(r2)

is a method that returns void.

You cannot print a 'void'.

if you want to show the result of r1.add(r2) the you need to change the return parameter of your add method. try

public int add(RationalNumber r2) {

// then add the bottom of your method you add a return statement

return sum;
}

this will return your sum and then you can print it using system.out.

(",)

Thank you very much. That solved the compile error.