Programming Tutorials

C ++ Reference Parameters Versus Java Reference Parameters

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

The C++ pointer parameter, In Java, became a reference parameter. Of course, C++ also supports reference parameters. As mentioned, most pointer parameters found in C++ code are simply holdovers from C. Nearly all new C++ code will use reference parameters when a function needs access to the argument, itself. (In essence, pointer parameters, although still common, are actually anachronisms in most C++ code.) Since both Java and C++ support reference parameters, you might think that the conversion of a C++ function that uses reference parameters to a Java method would involve few changes. Unfortunately, this is not always the case. To understand why, let's convert the following C++ program, which swaps the contents of two Coord objects using reference parameters:

// Swap coordinates - C++ version.
#include <iostream>
using namespace std;
class Coord
{
public:
    int x;
    int y;
};
// Swap contents of two Coord objects.
void swap(Coord &a, Coord &b)
{
    Coord temp;
    // swap contents of objects
    temp = a;
    a = b;
    b = temp;
}
int main()
{
    Coord ob1, ob2;
    ob1.x = 10;
    ob1.y = 20;
    ob2.x = 88;
    ob2.y = 99;
    cout << "Original values:\\n";
    cout << "ob1: " << ob1.x << ", " << ob1.y << "\\n";
    cout << "ob2: " << ob2.x << ", " << ob2.y << "\\n";
    cout << "\\n";
    swap(ob1, ob2);
    cout << "Swapped values:\\n";
    cout << "ob1: " << ob1.x << ", " << ob1.y << "\\n";
    cout << "ob2: " << ob2.x << ", " << ob2.y << "\\n";
    return 0;
}

Following is the output produced by this program. As you can see, the contents of ob1 and ob2 have been exchanged:

Original values:
ob1: 10, 20
ob2: 88, 99
Swapped values:
ob1: 88, 99
ob2: 10, 20

In Java, all objects are accessed via an object reference variable. Thus, when an object is passed to a method, only its reference is passed. This means that all objects are automatically passed by reference to a Java method. Without thinking any deeper about what is actually occurring, someone might initially try the following (incorrect) conversion of the preceding program:

// Swap program incorrectly converted to Java.
class Coord {
    int x;
    int y;
};

class SwapDemo {
    static void swap(Coord a, Coord b) {
        Coord temp = new Coord();
        // this won't swap contents of a and b!
        temp = a;
        a = b;
        b = temp;
    }

    public static void main(String args[]) {
        Coord ob1 = new Coord();
        Coord ob2 = new Coord();
        ob1.x = 10;
        ob1.y = 20;
        ob2.x = 88;
        ob2.y = 99;
        System.out.println("Original values:");
        System.out.println("ob1: " +
                ob1.x + ", " + ob1.y);
        System.out.println("ob2: " +
                ob2.x + ", " + ob2.y + "\\n");
        swap(ob1, ob2);
        System.out.println("Swapped values:");
        System.out.println("ob1: " +
                ob1.x + ", " + ob1.y);
        System.out.println("ob2: " +
                ob2.x + ", " + ob2.y + "\\n");
    }
}

The output produced by this incorrect program is shown here:

Original values:
ob1: 10, 20
ob2: 88, 99
Swapped values:
ob1: 10, 20
ob2: 88, 99