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How to swap or exchange objects in Java?

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  • Difficulty Level : Medium
  • Last Updated : 19 Nov, 2021

In order to understand how to swap objects in Java, let us consider an illustration below as follows:

Illustration: 

Let’s say we have a class called “Car” with some attributes. And we create two objects of Car, say car1 and car2, how to exchange the data of car1 and car2?

Methods: 

  1. Using concepts of OOPS
  2. Using Wrapper classes of java

Method 1: Using concepts of OOPS 

Here we will be simply swapping members for which let us directly take a sample ‘Car’ illustration with which we will play. So if the class ‘Car’ has only one integer attribute say “no” (car number), we can swap cars by simply swapping the members of two cars.

Example 1-A

Java




// Java program to demonstrate that we can swap two
// objects be swapping members
 
// Class 1
// Number class Car
class Car {
    // Attributes associated with car
    int no;
    Car(int no) { this.no = no; }
}
 
// Class 2
// Uses Car objects
class GFG {
    // Method 1
    // To swap
    public static void swap(Car c1, Car c2)
    {
        int temp = c1.no;
        c1.no = c2.no;
        c2.no = temp;
    }
 
    // Method 2
    // Main driver method
    public static void main(String[] args)
    {
        // Creating car class objects(creating cars)
        Car c1 = new Car(1);
        Car c2 = new Car(2);
 
        // Calling method 1
        swap(c1, c2);
 
        // Print and display commands
        System.out.println("c1.no = " + c1.no);
        System.out.println("c2.no = " + c2.no);
    }
}

Output

c1.no = 2
c2.no = 1

Note: Geek, what if we don’t know members of Car? 

The above solution worked as we knew that there is one member “no” in Car. What if we don’t know members of Car or the member list is too big. This is a very common situation as a class that uses some other class may not access members of other class. Does below solution work? 
 

Example 1-B

Java




// Java program to demonstrate that we can swap two
// objects be swapping members
// Where it does not work
 
// Class 1
// A car with number and name
class Car {
      
    // Attributes of Car class
    int model, no;
 
    // Constructor
    Car(int model, int no)
    {
      // This keyword is used to refer
      // current instance itself
        this.model = model;
        this.no = no;
    }
 
    // Method of this class
    // To print Car
    void print()
    {
       // Printing number and model of car
        System.out.println("no = " + no +
                           ", model = " + model);
    }
}
 
// Class 2
// 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

Output explanation:  As we can see from the above output, the objects are not swapped. We have discussed in a previous post that parameters are passed by value in Java. So when we pass c1 and c2 to swap(), the function swap() creates a copy of these references.

Method 2: Wrapper Class 

If we create a wrapper class that contains references of Car, we can swap cars by swapping references of the wrapper class.

Example 

Java




// Java program to Demonstrate that Wrapper Classes
// Can be Used to Swap two Objects
 
// Class 1
// A car with model and no.
class Car {
    // Attributes associated with car
    int model, no;
 
    // Constructor of class 1
    Car(int model, int no)
    {
        // This refers to current instance itself
        this.model = model;
        this.no = no;
    }
 
    // Method
    // To print object details
    void print()
    {
        System.out.println("no = " + no
                           + ", model = " + model);
    }
}
 
// Class 2
// Wrapper over class that is used for swapping
class CarWrapper {
    Car c;
 
    // Constructor
    CarWrapper(Car c) { this.c = c; }
}
 
// Class 3
// Uses Car class and swaps objects of Car
// using CarWrapper
class GFG {
    // 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;
    }
 
    // Main 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

So a wrapper class solution works even if the user class doesn’t have access to members of the class whose objects are to be swapped.
This article is contributed by Anurag Rai. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 


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