Java Code That Prints Reverse of Given ArrayList

Question

So I was trying to write a method which returns the reverse of given ArrayList but it gives me Index error and I don't know how to solve it. I appreciate your help. This is my code:

import java.util.ArrayList;

public class question2 {

public static ArrayList<Double> reverse(ArrayList<Double> arraylist) {
    ArrayList<Double> result = new ArrayList<Double>();
    for(int i=0; i<arraylist.size(); i++) {
        result.set((arraylist.size()) - 1 - i, arraylist.get(i));
    }
    return result;
}

public static void main(String[] args) {
    
    ArrayList<Double> doubles = new ArrayList<Double>();
    doubles.add(1.5);
    doubles.add(2.3);
    doubles.add(7.7);
    doubles.add(8.6);
    System.out.println(reverse(doubles));
    
}

}

And this is the error:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index 3 out of bounds for length 0

Read the documentation of the set method.

f1sh
– f1sh

2020-10-07 17:57:42 +00:00
Commented Oct 7, 2020 at 17:57 — f1sh
– f1sh, Commented Oct 7, 2020 at 17:57

Birju B · Accepted Answer · 2020-10-07 18:15:16Z

1

public static ArrayList<Double> reverse(ArrayList<Double> arraylist) {
       ArrayList<Double> result = new ArrayList<Double>(arraylist.size());
        for(int i=arraylist.size()-1; i>=0; i—-) {
            result.add(arraylist.get(i));
        }
        return result;
     }

Hope this will work!

edited Oct 7, 2020 at 18:15

answered Oct 7, 2020 at 18:06

Birju B

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9 Comments

Daixan Over a year ago

I changed the first part with ArrayList<Double> result = ArrayList<Double>(arraylist.size()) and the arraylist.set with arraylist.add but it didn't work.

Birju B Over a year ago

Please upvote and mark it as true answer. Thanks in advance

Daixan Over a year ago

It still doesn't work and throws same exception (I just copy pasted)

Birju B Over a year ago

Can you use Collections.reverse(arraylis);

Daixan Over a year ago

I can't, I have to write it myself because it's for school.

|

Amar Singh · Accepted Answer · 2020-10-07 18:26:59Z

0

Can you try my code , it will work for you

 public static List<Double> reverse(ArrayList<Double> arraylist) {
        List<Double> result = Arrays.asList(new Double[arraylist.size()]);
        for(int i=0; i<arraylist.size(); i++) {
            result.set((arraylist.size()) - 1 - i, arraylist.get(i));
        }
        return result;
    }

answered Oct 7, 2020 at 18:26

Amar Singh

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1 Comment

Daixan Over a year ago

Hmm it gives some kind of error. But I guess that's because I didn't import needed libraries. Thanks for your answer :)

Akash Hingane · Accepted Answer · 2020-10-07 18:40:58Z

0

public static String reverse(ArrayList<Double> arraylist) {
ArrayList<Double> result = new ArrayList<Double>();
for(int i=arraylist.size() -1; i>=0; i--) {
    result.add(arraylist.get(i));
}
return result.toString();

}

the return type of reverse() method should be char[] or String as you are printing on the console from the main method

answered Oct 7, 2020 at 18:40

Akash Hingane

1

Comments

WJS · Accepted Answer · 2020-10-07 19:21:45Z

0

Here is something you may want to try. It does not require a separate List and only loops half as many times as copying the entire list to another one.

It works by swapping the values at the beginning and the end as the loop index moves closer toward the middle.

set size = list.size();
iterate the loop from 0 to i < size/2
save the value at i to a variable.
set the position at i to the value at size - i - 1
set the position at size - i - 1 to the saved value.

This will proceed until the loop has reached the middle. Note that for even sized lists, the inner most values are adjacent and swapped. For odd sized lists, the inner most item is ignored since it is correctly positioned.

edited Oct 7, 2020 at 19:21

answered Oct 7, 2020 at 18:25

WJS

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2 Comments

Daixan Over a year ago

Thanks for your answer. But I can't use pre-made methods since this is some kind of homework. I have to write it myself. But thanks anyway :)

Daixan Over a year ago

Yes, this also works and in a different way. Thanks for your answer. I appreciate it.

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