How to reduce fractions in Java?

[uselessparadigm]

I'm new to these forums and I'm not sure if this is the right place to post this question, I hope that's alright. I'm writing a code for my intro to Java Class that manipulated fractions through the different opertions (add, subtr, mult, divide) I have everything done but I am stuck when it comes to reducing fractions I have absolutely no idea what I am supposed to do. My professor gave us this code:

 private int gcf (int a , int b ) {
    	return a%b == 0 ? b : gcf ( b , a%b );
    	}

and he says we have to invoke it to simplify the fractions but I have no idea how to incorporate it into my code.

Here is my code (please excuse it's noobiness, I'm still learning):
Class 1:

public class Fraction {
	
	
    private int numerator;
    private int denominator;
    
    
 //constructor for building fractions
    public Fraction(int num, int denom){
        numerator = num;
        denominator = denom;
        }
//simplifying fractions
    private int gcf (int a , int b ) {
    	return a%b == 0 ? b : gcf ( b , a%b );
    	}
   
   
//addition   
    public static Fraction add(Fraction F1, Fraction F2){
    	if(F1.denominator==0 || F2.denominator==0){
    		System.out.println("Undefined, you cannot divide by zero.");
    	}
    	if((F1.numerator*F2.denominator + F1.denominator*F2.numerator)==0 || (F1.numerator+F2.numerator)==0){
    		System.out.println("Sum is: 0");    		
    	}
    	if(F1.denominator!=F2.denominator)
    		return new Fraction((F1.numerator*F2.denominator + F1.denominator*F2.numerator),(F1.denominator*F2.denominator));
    	else if (F1.denominator==F2.denominator)
    		return new Fraction((F1.numerator+F2.numerator),(F1.denominator));
		return null;
    	}
//subtraction
   public static Fraction subtract(Fraction F1, Fraction F2){
	   if(F1.denominator==0 || F2.denominator==0){
   		System.out.println("Undefined, you cannot divide by zero.");
   	}
   	if((F1.numerator*F2.denominator - F1.denominator*F2.numerator)==0 || (F1.numerator-F2.numerator)==0){
   		   		
   	}
   	if(F1.denominator!=F2.denominator)
   		return new Fraction((F1.numerator*F2.denominator - F1.denominator*F2.numerator),(F1.denominator*F2.denominator));
   	else if (F1.denominator==F2.denominator)
   		return new Fraction((F1.numerator-F2.numerator),(F1.denominator));
		return null;
   	}
//multiply   
   public static Fraction multiply(Fraction F1, Fraction F2){
	   if(F1.denominator==0 || F2.denominator==0){
	   		System.out.println("Undefined, you cannot divide by zero.");
	   	}
	   return new Fraction((F1.numerator*F2.numerator),(F1.denominator*F2.denominator));
   }
//divide
   public static Fraction divide(Fraction F1, Fraction F2){
	   return new Fraction((F1.numerator*F2.denominator),(F1.denominator*F2.numerator));
   }
//comparing fraction
   
   public static Fraction compare(Fraction F1, Fraction F2){	   
	   if((F2.denominator*F1.numerator) > (F2.numerator*F1.denominator)){
		  System.out.println("Comparing Fraction 1 & Fraction 2: " + F1.numerator+"/"+F1.denominator + ">"+
	   F2.numerator+ "/"+ F2.denominator);
		 }
		 	else if((F2.denominator*F1.numerator) < (F2.numerator*F1.denominator)){
			  System.out.println("Comparing Fraction 1 & Fraction 2: " + F1.numerator+"/"+F1.denominator + "<"+
					   F2.numerator+ "/"+ F2.denominator);
			  }
		 	else if((F2.denominator*F1.numerator) == (F2.numerator*F1.denominator)){
				  System.out.println("Comparing Fraction 1 & Fraction 2: " + F1.numerator+"/"+F1.denominator + "="+
						   F2.numerator+ "/"+ F2.denominator);
				  }
	return null;
	   
		  }
		   
  //printing the fraction
    public String toString(){
        return  numerator + "/" + denominator ;
    }
	
}

Class 2:

public class TestFraction {


    public static void main ( String[] args ){
    	
//fraction code (num, den)     
       Fraction fr1 = new Fraction((int)(Math.random()*10), (int) (Math.random()*10)+1);
       Fraction fr2 = new Fraction((int)(Math.random()*10), (int) (Math.random()*10)+1); 
       
       
       
      
 //print the fractions in multiple forms      
        System.out.println("Fraction 1: " + fr1 + "\t\tFraction 2: " + fr2);
        
		System.out.println("Sum is: " + Fraction.add(fr1, fr2));
        System.out.println("Difference is: " + Fraction.subtract(fr1,fr2));	
        System.out.println("Product is: " + Fraction.multiply(fr1, fr2));
        System.out.println("Quotient is: " + Fraction.divide(fr1, fr2));
        System.out.print(Fraction.compare(fr1, fr2));
    }
    
}

[pbrockway2]

gcf stands for greatest common factor. Eg the gcf of 24 and 16 is 8. Given a fraction like n/m we reduce it by diving the top and bottom by the gcd of n and m. Eg 16/24 becomes 2/3 after diving by 8.

[uselessparadigm]

I don't understand how to write the gcd divided by numerator and denominator. Every time I try, it comes out to an error or it makes my two random fractions the same fraction. I don't know what I'm doing wrong, a lot of my code related to gcf ends up being guess work and stabs in the dark, hoping it will work out eventually.

[JosAH]

If you're doing things right (i.e. make your Fractions immutable) you only have to use the gcf( ... ) method in the constructor, i.e.

public Fraction(int n, int d) {
   int gcf= gcf(n, d);

   this.n= n/gcf;
   this.d= d/gcf;
}

kind regards,

Jos

[kosmos890]

I am a beginner. This might help

public class Main{
	
	public static void main(String[] args){
		
		Fraction f1=new Fraction(3,8);
		Fraction f2=new Fraction(5,6);

		System.out.println(add(f1,f2));
	}

	public static int gfc(int a,int b){
		return a%b==0?b:gfc(b,a%b);
	}

	public static Fraction add(Fraction f1,Fraction f2){
		
		Fraction result=new Fraction(0,1);

		int num=f1.num*f2.denom+f2.num*f1.denom;
		int denom=f1.denom*f2.denom;

		int x=gfc(num,denom);
						
			result.num=num/x;
			result.denom=denom/x;
		
		return result;	
	}
}	
class Fraction{
	
	public int num;
	public int denom;

	Fraction(int n,int d){
		num=n;
		denom=d;
	}

	public String toString(){
		return num+"/"+denom;
	}
}

[JosAH]

@kosmos: carefully reread my reply #4.

kind regards,

Jos

[kosmos890]

@JosAH
" you only have to use the gcf( ... ) method in the constructor"

 
	public static Fraction add(Fraction f1,Fraction f2){

		int num=f1.num*f2.denom+f2.num*f1.denom;
		int denom=f1.denom*f2.denom;

		return new Fraction(num,denom);
	}

I do not understand this "i.e. make your Fractions immutable" . What do you mean?
Τhank you very much.

[JosAH]

@JosAH
" you only have to use the gcf( ... ) method in the constructor"

 
	public static Fraction add(Fraction f1,Fraction f2){

		int num=f1.num*f2.denom+f2.num*f1.denom;
		int denom=f1.denom*f2.denom;

		return new Fraction(num,denom);
	}

I do not understand this "i.e. make your Fractions immutable" . What do you mean?
Τhank you very much.

Never change the member variables n and d; a fraction is a type of number and numbers don't change their value.

kind regards,

Jos

[uselessparadigm]

If you're doing things right (i.e. make your Fractions immutable) you only have to use the gcf( ... ) method in the constructor, i.e.

public Fraction(int n, int d) {
   int gcf= gcf(n, d);

   this.n= n/gcf;
   this.d= d/gcf;
}

kind regards,

Jos

I don't know what to do with this since my fractions are randomly generated using Math.random(

[JosAH]

I don't know what to do with this since my fractions are randomly generated using Math.random(

Fine; something randomly generates a numerator and a denominator and then a Fraction object is generated; in the constructor of your Fraction you apply the logic I outlined in my previous reply.

kind regards,

Jos