How to create all combinations for items in an Array?

Issue

I did see this and this post on it, but it is not in Java or Kotlin, so it doesn't help me. Now, coming to the problem, I want to be able to create all the possible combinations for an array. For example, I have this array:

[1,2,3,4]

And then all the possible combinations give us this:

[1,2,3,4]
[1,2,4,3]
[1,3,2,4]
[1,3,4,2]
[1,4,2,3]
[1,4,3,2]

[2,1,4,3]
[2,1,3,4]
[2,3,1,4]
[2,3,4,1]
[2,4,3,1]
[2,4,1,3]

[3,1,2,4]
[3,1,4,2]
[3,2,4,1]
[3,2,1,4]
[3,4,1,2]
[3,4,2,1]

[4,1,2,3]
[4,1,3,2]
[4,2,3,1]
[4,2,1,3]
[4,3,2,1]
[4,3,1,2]

I don't want to have a space between these two different starts or numbers. I just added it for readability.

It should not only work for Integers but also for objects like strings, our classes, etc...

So, how can I achieve this in Java or Kotlin?

---

I also just came across this* post, but it wanted for two arrays and needed for one. Also, it was closed, so I felt it better to ask another one.

• The post is now deleted

Solution

If the task is not to implement the algorithm which generates the permutations you need, I would recomend to use a library like combinatoricslib3. If you happen to use Guava or Apache Commons in your projects, those have also some methods to generate combinations / permutations from a given collection. With combinatoricslib3 your code could look something like:

import org.paukov.combinatorics3.Generator;

public class Example {

    public static void main(String[] args) {
        Integer[] array = {1,2,3,4};
        Generator.permutation(array)
                .simple()
                .stream()
                .forEach(System.out::println);

        //Example with strings

        Generator.permutation("apple", "orange", "cherry")
                .simple()
                .stream()
                .forEach(System.out::println);
    }
}

output:

[1, 2, 3, 4]
[1, 2, 4, 3]
[1, 4, 2, 3]
[4, 1, 2, 3]
[4, 1, 3, 2]
....

and

[apple, orange, cherry]
[apple, cherry, orange]
[cherry, apple, orange]
[cherry, orange, apple]
[orange, cherry, apple]
[orange, apple, cherry]

Answered By - Eritrean