swap objects in Java?

Suppose we have a class named "Car", which has some properties. And we created two Car objects, such as car1 and car2, how to exchange the data of car1 and car2?

A Simple Solution is to swap members. 

// A Java program to demonstrate that we can swap two
// objects be swapping members.

// A car with number class Car
class Car
{
	int no;
	Car(int no) { this.no = no; }
}

// A class that uses Car objects
class Main
{
	// To swap c1 and c2
	public static void swap(Car c1, Car c2)
	{
		int temp = c1.no;
		c1.no = c2.no;
		c2.no = temp;
	}

	// Driver method
	public static void main(String[] args)
	{
		Car c1 = new Car(1);
		Car c2 = new Car(2);
		swap(c1, c2);
		System.out.println("c1.no = " + c1.no);
		System.out.println("c2.no = " + c2.no);
	}
}

Output:

c1.no = 2
c2.no = 1

What if we don’t know members of Car?

// A Java program to demonstrate that simply swapping
// object references doesn't work

// A car with number and name
class Car
{
	int model, no;

	// Constructor
	Car(int model, int no)
	{
		this.model = model;
		this.no = no;
	}

	// Utility method to print Car
	void print()
	{
		System.out.println("no = " + no +
						", model = " + model);
	}
}

// A class that uses Car
class Main
{
	// swap() doesn't swap c1 and c2
	public static void swap(Car c1, Car c2)
	{
		Car temp = c1;
		c1 = c2;
		c2 = temp;
	}

	// Driver method
	public static void main(String[] args)
	{
		Car c1 = new Car(101, 1);
		Car c2 = new Car(202, 2);
		swap(c1, c2);
		c1.print();
		c2.print();
	}
}

Output:

no = 1, model = 101
no = 2, model = 202

As we can see from the output above, the objects are not swapped. In the previous post we discussed that parameters are passed by value in Java. So when we pass c1 and c2 to swap(), the swap() function makes a copy of this reference.

The solution is to use the Wrapper class. If we create a shell class that contains a reference to Car, we can change the car by swapping the reference to the shell class.

// A Java program to demonstrate that we can use wrapper
// classes to swap to objects

// A car with model and no.
class Car
{
	int model, no;

	// Constructor
	Car(int model, int no)
	{
		this.model = model;
		this.no = no;
	}

	// Utility method to print object details
	void print()
	{
		System.out.println("no = " + no +
						", model = " + model);
	}
}

// A Wrapper over class that is used for swapping
class CarWrapper
{
Car c;

// Constructor
CarWrapper(Car c) {this.c = c;}
}

// A Class that use Car and swaps objects of Car
// using CarWrapper
class Main
{
	// This method swaps car objects in wrappers
	// cw1 and cw2
	public static void swap(CarWrapper cw1,
							CarWrapper cw2)
	{
		Car temp = cw1.c;
		cw1.c = cw2.c;
		cw2.c = temp;
	}

	// Driver method
	public static void main(String[] args)
	{
		Car c1 = new Car(101, 1);
		Car c2 = new Car(202, 2);
		CarWrapper cw1 = new CarWrapper(c1);
		CarWrapper cw2 = new CarWrapper(c2);
		swap(cw1, cw2);
		cw1.c.print();
		cw2.c.print();
	}
}

Output

no = 2, model = 202
no = 1, model = 101