How to remove and replace an item in an array list in Java?

Question

I have the following code in java,

import java.util.ArrayList;
import java.util.Objects;

public class Cars {

    private static ArrayList<String> replaceDuplicates(ArrayList<String> aList) {

        for (int i = 0; i < aList.size(); i++) {
            for (int j = i + 1; j < aList.size(); j++) {
                if (Objects.equals(aList.get(i), aList.get(j))) {
                    aList.remove(i);
                    aList.add(i, "");
                    aList.remove(j);
                    aList.add(j, "");
                }
            }
        }

        return aList;
    }

    public static void main(String[] args) {

        ArrayList<String> cars = new ArrayList<>();

        cars.add("Ford");
        cars.add("Ford");
        cars.add("Hyundai");
        cars.add("Toyota");
        cars.add("Toyota");
        cars.add("Toyota");
        cars.add("Ford");
        cars.add("Honda");
        cars.add("GMC");

        System.out.println(cars);

        cars = replaceDuplicates(cars);

        System.out.println(cars);

    }

}

The output of this code is - [, , Hyundai, , , Toyota, Ford, Honda, GMC]

I want to replace the name of cars that appear more than once in the array list with a " ". For some reason, in my code if a car's name has appeared thrice in the array list, then the third occurrence isn't getting replaced by " ".

My desired output should be like this - [, , Hyundai, , , , , Honda, GMC]

What am I doing wrong here?

Thank you in advance!

Whenever you find a pairwise match, you clear both matched names. What, then, are you comparing against subsequent names? — John Bollinger
– John Bollinger, Commented Dec 5, 2016 at 22:53
What is your desired output? I'm guessing something like [, , Hyundai, , , , , Honda, GMC], but not sure from your description. — clstrfsck
– clstrfsck, Commented Dec 5, 2016 at 23:00

user4668606 · Accepted Answer · 2016-12-05 22:56:49Z

First off: you can simplify this code by using List.set instead of inserting and removing elements.

aList.remove(i);
aList.add(i, "");

would simply become

aList.set(i, "");

You're deleting both entries, if they're duplicate. This leads to the behavior of always deleting an even number of entries. For example:

a  b  a  a  c  a  d  a
b  a  c  a  d  a         #first pair removed
b  c  d  a               #second pair removed

If the number of elements is odd, there will always remain one element in the list.

Obviously you need some way to remember what elements to delete. A simple approach to this would be to use a flag to remember whether a duplicate of an element has been encountered:

for (int i = 0; i < aList.size(); i++) {
    //the flag
    boolean duplicate = false;        

    for (int j = i + 1; j < aList.size(); j++) {
        if (Objects.equals(aList.get(i), aList.get(j))) {
            aList.set(j, "");   //remove all duplicates with an index higher than i
            duplicate = true;   //remember that element at index i is a duplicate
        }
    }

    //remove last duplicate element
    if(duplicate)
        aList.set(i, "");
}

Mikhail Batcer · Accepted Answer · 2016-12-05 23:00:00Z

1

If you still want to use your approach, you can create references to your items before modifying list:

for (int i = 0; i < aList.size(); i++) {
    String a = aList.get(i);
    for (int j = i + 1; j < aList.size(); j++) {
        String b = aList.get(j);
        if (Objects.equals(a, b)) {
            aList.remove(i);
            aList.add(i, "");
            aList.remove(j);
            aList.add(j, "");
        }
    }
}

answered Dec 5, 2016 at 23:00

Mikhail Batcer

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