Programming Tutorials

Converting Default Function Arguments in C++ to Java

By: Charles in C++ Tutorials on 2007-09-15  

One extensively used feature of C++ that Java does not support is default function arguments. For example, the area() function shown in the following C++ program computes the area of a rectangle if called with two arguments, or the area of a square if called with one argument.

// C++ program that uses default arguments.
#include <iostream>
using namespace std;
/* Compute area of a rectangle. For a square,
pass only one argument.
*/

double area(double l, double w = 0)
{
    if (w == 0)
        return l * l;
    else
        return l * w;
}
int main()
{
    cout << "Area of 2.2 by 3.4 rectangle: ";
    cout << area(2.2, 3.4) << endl;
    cout << "Area of 3.0 by 3.0 square: ";
    cout << area(3.0) << endl;
    return 0;
}

As you can see, when area() is called with only one argument, the second defaults to zero. When this happens, the function simply uses the first argument for both the length and the width of the rectangle.

While convenient, default arguments are not, of course, necessary. In essence, default arguments are actually a shorthand form of function overloading in which one form of the function has a different number of parameters than the other. Thus, to convert a C++ function that contains one or more default arguments into Java, simply create overloaded methods that handle each case. In this example, you need a version of area() that takes two arguments and another that takes only one argument. Using this approach, here is the preceding program rewritten for Java:

// Java version of area program.
class Area {
    // Compute area of a rectangle.
    static double area(double l, double w) {
        if (w == 0)
            return l * l;
        else
            return l * w;
    }

    // Overload area() for a square.
    static double area(double l) {
        return l * l;
    }

    public static void main(String args[]) {
        System.out.println("Area of 2.2 by 3.4 rectangle: " +
                area(2.2, 3.4));
        System.out.println("Area of 3.0 by 3.0 square: " +
                area(3.0));
    }
}