Using an object as a variable for a constructor

Question

I have a question about using an object as the variable in a constructor. It might be simple but I just really can't think of what to do and my java book isn't really helping. Say i wanted to do this

Fraction f3 = new Fraction(1, 2);
Fraction f5 = new Fraction(f3);

my constructor for the first object is:

public Fraction(int n, int d)

{
    if (d == 0)
    {
        numerator = 0;
        denominator = 1;
        System.err.println("Error: Invalid Denominator (" + d + ")");
    }
    else if (d < 0)
    {
        int nn = Math.abs(n) * (-1);
        numerator = nn;
        denominator = Math.abs(d);
    }
    else
    {
    numerator = n;
    denominator = d;
    }

}

my constructor for the second object is this:

public Fraction(Fraction f) 

{

}

I can't think of how to define the constructor to get it to set a new object as the object given. If anyone could give me a hand or maybe some advice to put me on the path to figuring it out I would greatly appreciate it.

Is it intentional that if for n=-1, d=-1, numerator is set to -1 and denominator to 1? Note that Math.abs does nothing but test for "argument<0?" and returns -argument if yes, argument if not. Since you already established d<0 where you call Math.abs(d), why not write -d? — arne.b
– arne.b, Commented Nov 27, 2011 at 0:02

ziesemer · Accepted Answer · 2011-11-26 23:55:00Z

6

public Fraction(Fraction f){
  this(f.numerator, f.denominator);
}

answered Nov 26, 2011 at 23:55

ziesemer

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Comments

corsiKa · Accepted Answer · 2011-11-26 23:56:20Z

4

I would use "constructor chaining" to ensure the changes made to one are reflected in the other.

// copy constructor
public Fraction(Fraction f) {
    this(f.numerator, f.denominator); // call the regular constructor with f's params
}

answered Nov 26, 2011 at 23:56

corsiKa

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Comments

kgautron · Accepted Answer · 2011-11-26 23:53:29Z

2

public Fraction(Fraction f){
  this.n = f.n;
  this.d = f.d;
}

answered Nov 26, 2011 at 23:53

kgautron

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5 Comments

Traveling Tech Guy Over a year ago

Of course, this assumes 'numerator' and 'denominator' are defined as public members of Fraction

corsiKa Over a year ago

I wouldn't recommend this - if the logic changes, you're going to find a bug soon enough.

corsiKa Over a year ago

@TravelingTechGuy No, because you're inside the file. You have full access to members of the same class, even if they aren't members of this object.

kgautron Over a year ago

@Traveling Tech Guy. No because the constructor is inside Fraction so it sees members of all visibilities.

lostprophet1 Over a year ago

Thank you. It worked perfectly. I had instance variables set up for numerator and denominator already.

hmakholm left over Monica · Accepted Answer · 2011-11-26 23:56:10Z

1

Do you want something such that

Fraction a = .... ;
Fraction b = new Fraction(a);
System.out.println(a==b);

prints true? That is not possible in Java; new always produces a fresh object that is separate from all previously existing objects.

(This is in contrast to, say, ECMAScript, where a constructor is allowed to return an existing object and discard the fresh object the runtime created and passed as the constructor's this).

answered Nov 26, 2011 at 23:56

hmakholm left over Monica

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