Java Insertion Sorting with ArrayLists of Strings

Question

I have an assignment on sorting which requires me to sort a list of random words by putting letters that start with the same letter in a group/zones and sorting that group alphabetically. My code sorts the words but my problem is that some of the words have changed. For example instead of having an output as

• a ngela
• A PPLE
• A pple
• a pple
• B aboon
• B all
• C at
• c at
• P INK
• P ink
• S teve

I would have an output of:

• apple
• apple
• apple
• apple
• Ball
• Ball
• cat
• cat
• Pink
• PINK
• Steve

As you can see, some of the words have been changed and in some cases, words with a capital letter are turned into lower cases like "cat" and I can't seem to find where my mistake is.

This is my sorting code; my driver class just takes in the list of random words:

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

public class ZoneSort 
{


ArrayList[] arrayOfZones;
ArrayList<String> words; 

public ZoneSort(ArrayList<String> words)
{
    arrayOfZones = new ArrayList [ 26 ];
    
    for(int index = 0; index < 26;index++)
        arrayOfZones [ index ] = new ArrayList();
    
    this.words = words; 
    
    putWordsIntoZones();
}

private  void putWordsIntoZones()
{
    for(String word: words)
    {
        int index = Character.toLowerCase(word.charAt(0)) - 97; 
        ArrayList<String> zoneAtIndex = arrayOfZones[index];
        zoneAtIndex.add(word);
    }
}

public  void sortTheArrayOfZones() 
    {
        for(ArrayList<String> zone : arrayOfZones )
        {
            sortZone(zone);
        }
    }


private  void sortZone(ArrayList<String> zone)
{
        for(int i = 1; i < zone.size(); i++)
        {
            String key = zone.get(i);
            int j = i-1;
            while(j>=0 && key.compareTo(zone.get(j)) > 0)
            {
                String x = zone.get(j+1);
                zone.set(j, x);
                j--;
            }
            
            String x = zone.get(j+1);
            x = key;
        }
}   

public   void printArrayOfZones()
{
    System.out.println("The sorted words are");
    for(ArrayList<String> zone:arrayOfZones)
    {
        for(String word: zone)
        {
            
            System.out.println(word);
        }
    }
}

Answer 1

Reading your code and viewing your results, it seems that your code overwrites the values instead of swapping them. To fix this you need to take a look at the function sort. I have modified your code so that instead of overwriting, you swap the two elements:

private  void sortZone(ArrayList<String> zone){
    for(int i = 1; i < zone.size(); i++){
        String key = zone.get(i);
        int j = i-1;
        while(j>=0 && key.compareTo(zone.get(j)) > 0){
            String x = zone.get(j+1);
            zone.set(j+1,zone.get(j)); // line added
            zone.set(j, x);
            j--;
        }
        String x = zone.get(j+1);
        x = key;
    }
}

I hope this fixed your problem.

Answer 2

If I compare your sortZone implementation with a reference insertion sort implementation such as https://www.baeldung.com/java-insertion-sort I see the following differences - see inline comments

        for(int i = 1; i < zone.size(); i++)
        {
            String key = zone.get(i);
            int j = i-1;

// The sort order is reversed.
// You've used "key > zone[j]" when it should be "key < zone[j]"

            while(j>=0 && key.compareTo(zone.get(j)) < 0)
            {
// This is copying items backwards, towards the beginning of the array. 
//                String x = zone.get(j+1);
//                zone.set(j, x);
// It should be copying items forwards, towards the end, to make room for "key"
// Like this:
                String x = zone.get(j);
                zone.set(j+1, x);
                j--;
            }
            
// You should be setting zone[j+1] = "key" - this does not do it: 
//            String x = zone.get(j+1);
//            x = key;
// This is how you set a value in a list:
            zone.set(j+1, key);
          
        }