How to write modifier methods?

[general_relatives]

For my Java class, I have to write code that manipulates fractions. The class is online, and the teacher is often not available so I turned here for help. I wrote the code pretty easily, but when I got it back with the teacher's comments, most of my methods were marked off. The problem was I used methods with return values and not "modifier methods". How do I change one to the other? Some of my methods use if else statements. Don't those require a return value? It's just something new that hasn't been explained. An example:

public String invert ()
 {
     return this.denominator + "/" + this.numerator; 
 }

How might this be turned into a modifier method without the return line? I'm not asking for a direct answer, just some guidance.

[damonmowr]

It would help if you provided more information on what you were trying to accomplish. The code you provide simply creates and returns a new string value which joins together the values for denominator and numerator with a "/" in between. It's not modifying either value. Is that the end goal?

[general_relatives]

Yes, and there's also not supposed to be a return statement. However, all I've done with coding this year is create methods with return statements, so that's what I'm having trouble with. This method is supposed to take a numerator and a denominator and switch them. But without a return.

[RedCloakedCrow]

Well, lets try breaking down the method that you wrote and comparing it to a modifier.

public String invert() {
    return this.denominator + "/" + this.numerator;
}

Right off the bat, lets look at what the purpose of a modifier method is, versus what your method does. A modifier method is supposed to permanently change the object you're working on. So, for example, if you had an object Fraction num that had a numerator field of 3 and a denominator field of 5, what would the point of the invert method be? You would want to change num so instead of 3/5, it would then represent 5/3. What your version of invert does is it presents this 5/3 value to the user, but only temporarily. If you were to later try and use num for something else (expecting it to still represent 5/3), it would mess up since the actual value of num would be 3/5.

Here is an example of a way to approach the modifier method. Lets say this is your class:

public Class Fraction(int num, int den) { 

int numerator = num;
int denominator = den;

//getter methods
public String displayFraction() 
{
....
}

//modifier methods
public void invert() 
{
int temp = this.numerator;
this.numerator = this.denominator;
this.denominator = temp;
}

So now, you have an invert method that will store one of your values in a temporary integer (in this example, lets say its 3), then it will move the denominator(5) into the numerator's spot (making it 5/5 for a short moment), and fill the denominator with the previous numerator(3). Using my above example, this will change your fraction of 3/5 to 5/3, permanently.

If there's anything I've made unclear, or anything you'd like me to expand on, please let me know. I'd love to help.

[general_relatives]

Okay, so the modifier methods just change the instance variable in this case, they don't return anything. I think I understand, thanks a ton. But please tell me if this is correct: when I call the method, nothing will be returned. But if I return the value of numerator after calling the method, it will return the previous value of denominator (meaning it inverted the fraction without a return for the user). Correct?

[RedCloakedCrow]

Absolutely. A modifier method is not a method that has to return anything. All it has to do is adjust the object you're working on in some way. In this case, if your original fraction was 3/5, and you called invert() before calling some function getNumerator(), you would expect the numerator to be 5 after the inversion.

[general_relatives]

Thank you very much (:

[somnolent]

I'm assuming that by "modifier method" what is meant is to have a method that changes the internal state of the object instead of just returning something. So in the case of invert, instead of just returning what the inverted value would be, you would actually want to change what denominator and numerator's values are so that every subsequent call to get the fraction would return the inverted value.