The Swap Method in Java

1. Swap Two Elements in a Java List With the swap Method
2. Swap Two Characters in a Java String
3. Swap Two Objects in Java

The swap() method is used to exchange the position of two elements, characters, or objects in Java. This method can be applied to a list, a string, or an object. In this article, we will discuss the use of the swap() method in:

1. Swapping two elements in a list
2. Swapping two characters in a string
3. Swapping two objects

Swap Two Elements in a Java List With the swap Method

This is a method used to exchange two specific elements in defined positions without impacting other elements in a list. If one of the specified indexes is higher than the list’s size, then the method returns an out of bound exception. The swap() will give an output of the list with the elements in the indexes swapped.

Syntax:

public static void swap(List mylist, int m, int n)

This method takes three parameters as its arguments - the list that the swap() method will be applied on and the two indexes which are to be exchanged.

Code example:

import java.util.ArrayList;
import java.util.Collections;

public class PerformSwap {
  public static void main(String[] args) {
    ArrayList<String> lettersList = new ArrayList<String>();
    lettersList.add("a");
    lettersList.add("b");
    lettersList.add("c");
    lettersList.add("d");
    lettersList.add("e");
    lettersList.add("f");
    lettersList.add("g");
    System.out.println("Original List: " + lettersList);
    Collections.swap(lettersList, 0, 6);
    System.out.println("\nSwapped List: " + lettersList);
  }
}

Output:

Original List: [a, b, c, d, e, f, g]

Swapped List: [g, b, c, d, e, f, a]

In the example above, we have exchanged the letter g on index 6 with the letter a on index 0. The swap method has only exchanged these two letters without interfering with any other list elements.

Swap Two Characters in a Java String

One of the string value’s main properties is that it is immutable, meaning it cannot be changed. To perform the swap operation, we first have to copy the String object to a StringBuilder or a character array. These two data types allow us to perform swap operation on the copied object. Below, we’ll perform the swap operation using char array and StringBuilder to create a new string with swapped characters.

Perform String Swap Using Char Array in Java

The swap method has three parameters - the String we are going to perform swap on and the two indexes of the characters we exchange. To perform a character swap, we first create a temporary character storage object - tempo. This temporary object stores the first character as we replace it with the second character and then passes this character to the second character to complete the exchange process.

There are three steps involved:

Code example:

public class SwapString {
  static char[] swap(String mystring, int i, int j) {
    char ch[] = mystring.toCharArray();
    char tempo = ch[i];
    ch[i] = ch[j];
    ch[j] = tempo;
    return ch;
  }

  public static void main(String args[]) {
    String theS = "Favourite";

    System.out.println(swap(theS, 5, 2));
    System.out.println(swap(theS, 0, theS.length() - 1));
    System.out.println(theS);
  }
}

Output:

Farouvite
eavouritF
Favourite

Perform String Swap Using StringBuilder in Java

Code example:

public class SwapString {
  static String swap(String mystring, int i, int j) {
    StringBuilder mysb = new StringBuilder(mystring);
    mysb.setCharAt(i, mystring.charAt(j));
    mysb.setCharAt(j, mystring.charAt(i));
    return mysb.toString();
  }

  public static void main(String args[]) {
    String theS = "Favorite";

    System.out.println(swap(theS, 5, 2));
    System.out.println(swap(theS, 0, theS.length() - 1));

    // Original String
    System.out.println(theS);
  }
}

Output:

Faiorvte
eavoritF
Favorite

Swap Two Objects in Java

The swap method can also be used to swap the two objects’ attributes. Objects swap can be performed on objects with one attribute and also on objects with more than one attribute.

Swap Object With One Attribute

Suppose we have a class called House with some attributes such as the number of bedrooms and the number of bathrooms. Let’s create two objects of House - house1 and house2. House has only one attribute - value. Our goal is to swap the data in house1 and house2.

Code example:

public class SwapObjects {
  public static void swap(House house1, House house2) {
    House temp = house1;
    house1 = house2;
    house2 = temp;
  }

  public static void main(String[] args) {
    House house1 = new House();
    House house2 = new House();

    house1.value = 5;
    house2.value = 2;

    // swap using objects
    swap(house1, house2);
    System.out.println(house1.value + ", " + house2.value);
  }
}
class House {
  public int value;
}

Output:

5, 2

Swap Object With More Than One Attribute in Java

We use a Wrapper class to swap the attribute of two objects that have multiple attributes. If we perform the swap without the wrapper class, the function swap will only create a copy of the object references.

public class SwapObjects {
  public static void main(String[] args) {
    House house1 = new House(5, 3);
    House house2 = new House(2, 1);

    Wrapper whs1 = new Wrapper(house1);
    Wrapper whs2 = new Wrapper(house2);

    // swap using wrapper of objects
    swap(whs1, whs2);
    whs1.house.print();
    whs2.house.print();
  }

  public static void swap(Wrapper whs1, Wrapper whs2) {
    House temp = whs1.house;
    whs1.house = whs2.house;
    whs2.house = temp;
  }
}
class House {
  int bedroom, bathroom;

  // Constructor
  House(int bedroom, int bathroom) {
    this.bedroom = bedroom;
    this.bathroom = bathroom;
  }

  // Utility method to print object details
  void print() {
    System.out.println("bedrooms = " + bedroom + ", bathrooms = " + bathroom);
  }
}
class Wrapper {
  House house;
  Wrapper(House house) {
    this.house = house;
  }
}

Output:

bedrooms = 2, bathrooms = 1
bedrooms = 5, bathrooms = 3

The wrapper class swaps objects even if the member’s class doesn’t give access to the user class. While applying the swap() method to an object, you can select the approach to use based on the number of attributes in an object.