Q:

Arraylist swap elements

How do I swap the the first and last elements of an ArrayList?
I know how to swap the elements of an array: setting a temporary value to store the first element, letting the first element equal the last element, then letting the last element equal the stored first element.
int a = values[0];
int n = values.length;
values[0] = values[n-1];
values[n-1] = a;

So for an ArrayList<String> would it be like this?
String a = words.get(0);
int n = words.size();
words.get(0) = words.get(n-1);
words.get(n-1) = a

A:

You can use Collections.swap(List<?> list, int i, int j);

A:

In Java, you cannot set a value in ArrayList by assigning to it, there's a set() method to call:
String a = words.get(0);
words.set(0, words.get(words.size() - 1));
words.set(words.size() - 1, a)

A:

Use like this. Here is the online compilation of the code. Take a look http://ideone.com/MJJwtc 
public static void swap(List list,
                        int i,
                        int j)

Swaps the elements at the specified positions in the specified list. (If the specified positions are equal, invoking this method leaves the list unchanged.)
Parameters:
    list - The list in which to swap elements.
    i - the index of one element to be swapped.
    j - the index of the other element to be swapped. 
Read The official Docs of collection
http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#swap%28java.util.List,%20int,%20int%29
import java.util.*;
import java.lang.*;

class Main {
    public static void main(String[] args) throws java.lang.Exception       
    {    
        //create an ArrayList object
        ArrayList words = new ArrayList();

        //Add elements to Arraylist
        words.add("A");
        words.add("B");
        words.add("C");
        words.add("D");
        words.add("E");

        System.out.println("Before swaping, ArrayList contains : " + words);

        /*
      To swap elements of Java ArrayList use,
      static void swap(List list, int firstElement, int secondElement)
      method of Collections class. Where firstElement is the index of first
      element to be swapped and secondElement is the index of the second element
      to be swapped.

      If the specified positions are equal, list remains unchanged.

      Please note that, this method can throw IndexOutOfBoundsException if
      any of the index values is not in range.        */

        Collections.swap(words, 0, words.size() - 1);

        System.out.println("After swaping, ArrayList contains : " + words);    

    }
}

Oneline compilation example http://ideone.com/MJJwtc 

