使用对象作为构造函数的变量

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.

Answer 1

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

Answer 2

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
}

Answer 3

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

Answer 4

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 ).