How to swap items in a list?

Question

I've been trying to figure out how to swap items in an arraylist; so first I created an arraylist using this code:

import java.util.ArrayList;
import java.util.List;

public class ListADT {
public static void main (String[] args){
    List <String> myList = new ArrayList<String>(); {
    myList.add("A");
    myList.add("B");
    myList.add("C");
    myList.add("D");
    myList.add("E");
System.out.println(myList);}}}

This returns [A,B,C,D,E] on the console.
Then I opened up another class and used this code I found online and tried to make modifications to get it to work, but it has not been working.

import java.util.List;


public class swap {
    //precondition: x and y are >= 0 and < mylist.size()
    //precondition: mylist is not null
    void swap(int x, int y, List <String> myList) {
        String s = myList.get(x);
        myList.add(x, myList.get(y));
        myList.add(y, s);
System.out.println(myList);
} }

If I try to replace x and y with position numbers of the elements in the list, then that just causes a host of new problems. As you can probably tell, I'm a beginner to this and any help would be appreciated.

Answer 1

You need to use set instead of add.

if you use set it will replace the element in that position with the new element.

// arrayList.set(index i,String replaceElement);

so your swap function should look like this:

public class swap {
    void swap(int x, int y, List <String> myList) {
        String s = myList.get(x);
        myList.set(x, myList.get(y));
        myList.set(y, s);
System.out.println(myList);
} }

Answer 2

Take a look at the ArrayList API, specifically the set() method. http://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#set(int,%20E)

public E set(int index, E element)

"Replaces the element at the specified position in this list with the specified element."

Give that method a shot instead of add(), probably as follows:

• You can get and store the String from position x (like you're doing),
• Then set the value at position y into position x,
• Finally, set the value at position y from the String you stored earlier.

Basically, whenever you're stuck, it can't hurt to browse the API to see if any of the other methods might make the task a bit easier. None of us know the API completely by heart, and going back to that doc is a common thing for everyone. Well, almost everyone.

Answer 3

What your code does is:

1. get the string at x
2. insert string at y, at position x
3. insert previous string at x, at position y

So if you have a list with content ["A", "B", "C", "D", "E"], and you called swap(1, 3, list), following the pseudocode, you'd get:

1. s becomes "B"
2. list becomes ["A", **"D"**, "B", "C", "D", "E"]
3. list becomes ["A", "D", "B", **"B"**, "C", "D", "E"]

What you want to do is swap the values at x and y by SETTING them, not adding them:

void swap(int x, int y, List <String> myList) {
    String s = myList.get(x);

    myList.set(x, myList.get(y));
    myList.set(y, s);

    System.out.println(myList);
}

This will actually replace the values at index x and y with each other

Answer 4

The Collections.swap(List, int, int) JDK 1.4+ method does just this. Here's an example:

Collections.swap(myList, 0, 1)