swap() in Java

Introduction to swap() in Java

It refers to a method provided by java.util.Collections to swap the elements of a list present at 2 distinct positions in the List given as arguments while calling a method along with the collection reference, and gives the list with the elements interchanged, in case the two positions specified are same then the list remains unchanged and in case the index specified in the argument is greater than the length of the list IndexOutOfBoundsException is thrown.

Syntax:

public static void swap(List list1, int pos1, int pos2)

• list1: Represents the reference variable of java.util.List type having number of elements.
• pos1: This variable represents the index of first element.
• pos2: This variable represents the index of second element.

How swap() works in Java?

Swap method is functionality provided to us to swap the elements of a list at 2 different indexes provided in the arguments to while calling the functions.

Lets see the working of swap method using a pseudocode:

1. First the indexes given in the arguments is checked if they are not null otherwise compile time error is thrown .
2. Size of list is checked and compared with the indexes given if they lie in the bounds of list size or not, in case no then IndexOutOfBounds run time exception is thrown and exit.
3. List is traversed and list1[i], list1[j] element is taken and swapped.

Lets consider below list of elements and different scenarios in that:

Scenario 1: Swap elements at index 3 and 5.

Scenario 2: Swap elements at index 2 and 7.

Here since we are unable to find element with index =7 thus IndexOutOfBound exception is thrown.

Examples of swap() in Java

Given below are the examples mentioned:

Example #1

Lets see the example to check if a string is palindrome or not using swap() method.

Code:

Import java.util.*;
public class Office {
public static void main(String[] args)
throws Exception
{
try {
List<String>list1 = new ArrayList<String>();
list1.add("R");
list1.add("E");
list1.add("P");
list1.add("A");
list1.add("P");
list1.add("E");
list1.add("R");
System.out.println("String Before swap: " + list1);
int n = list1.size();
for(int i=0;i<list1.size()/2;i++){
Collections.swap(list1, i, n-1-i);
}
System.out.println("\nString After swap: " + list1);
}
catch (IndexOutOfBoundsException e) {
System.out.println("\nExceptionthrown : " + e);
}
}
}

Output:

Explanation:

• In the above program we used swap method to get the reverse of the list to see if the current and reversed list is same or not, thus called as a palindrome string.
• Here for loop is run upto middle of the list and swapping ith element with n-i-1th element in the list. As we can see the string before and after swap operation is same thus it is a palindrome.

Example #2

For IndexOutOfBoundsException.

Lets see one example to access an index in the list which is greater than the total number of elements in the list.

Code:

import java.util.*;
public class Office {
public static void main(String[] args) throws Exception
{
try {
List<String>list1 = new ArrayList<String>();
list1.add("Lets");
list1.add("Start");
list1.add("our");
list1.add("Work");
list1.add("Now");
System.out.println("Before swap: " + list1);
System.out.println("\nLets reverse the list using swap elements ");
for(int i=0;i<list1.size();i++){
Collections.swap(list1, i, list1.size());
}
System.out.println("After swap: " + list1);
}
catch (IndexOutOfBoundsException e) {
System.out.println("Exception thrown : " + e);
}
}
}

Output:

Explanation:

• Here while swapping the elements at positions I and list1.size() in the loop, since we know the index for a list starts from 0 and ends uptolist.size() -1 thus accessing index out of these bounds eventually results in exceptions of IndexOutOfBound Exception as shown in Output screen.
• To make above code working a small tweak is required by changing swap command to Collections.swap(list1,I,list.size() -1).
• Now the index will not go out of the bounds of the list size, thus no exception will be thrown.

Code:

import java.util.*;
public class Office {
public static void main(String[] args) throws Exception
{
try {
List<String>list1 = new ArrayList<String>();
list1.add("Lets");
list1.add("Start");
list1.add("our");
list1.add("Work");
list1.add("Now");
System.out.println("List Before swap: " + list1);
System.out.println("\nLets reverse the list using swap elements ");
for(int i=0;i<list1.size()/2;i++){
Collections.swap(list1, i, list1.size()-1-i);
}
System.out.println("After swap: " + list1);
}
catch (IndexOutOfBoundsException e) {
System.out.println("Exception thrown : " + e);
}
}
}

Output:

Conclusion

Swap method is a functionality given by java.util.Collections class to interchange the values present at different indexes in the list which are specified in the arguments while calling the method along with the reference to the list. In case both the indexes are same the list will remain unchanged. This method throws IndexOutOfBoundsException in case indexes specified are greater than the size of the list.

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