Permutation using recursive on Java

recursivePermutations.java

package com.jiahaoliuliu.tools;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class Permutations<T> {

    private List<List<T>> generatePermutations(List<T> seedLists) {
        return generatePermutations(new ArrayList<>(), seedLists);
    }

    private List<List<T>> generatePermutations(
            List<T> existingPermutations, List<T> remainSeeds) {

        List<List<T>> result = new ArrayList<>();
        // A simple optimization
        if (!existingPermutations.isEmpty()) {
            // Safe copy
            result.add(new ArrayList<>(existingPermutations));
        }

        // Stop condition
        if (remainSeeds.isEmpty()) {
            return result;
        }

        for (int i = 0; i < remainSeeds.size(); i++) {
            List<T> existingPermutationsCopy = new ArrayList<>(existingPermutations);
            existingPermutationsCopy.add(remainSeeds.get(i));
            result.addAll(
                    generatePermutations(
                            existingPermutationsCopy,
                            remainSeeds.subList(i+1, remainSeeds.size())));
        }
        return result;
    }

    public static void main(String... args) {
        test1();
        test2();
        test3();
    }

    private static void test1() {
        Permutations<Character> permutations = new Permutations();
        String word = "a";
        List<Character> characterList = word.chars()
                .mapToObj(i -> (char)i).collect(Collectors.toList());
        // [a]
        System.out.println(permutations.generatePermutations(characterList));
    }

    private static void test2() {
        Permutations<Character> permutations = new Permutations();
        String word = "ab";
        List<Character> characterList = word.chars()
                .mapToObj(i -> (char)i).collect(Collectors.toList());
        // [a, ab, b]
        System.out.println(permutations.generatePermutations(characterList));
    }

    private static void test3() {
        Permutations<Character> permutations = new Permutations();
        String word = "abc";
        List<Character> characterList = word.chars()
                .mapToObj(i -> (char)i).collect(Collectors.toList());
        //[a, ab, abc, ac, b, bc, c]
        System.out.println(permutations.generatePermutations(characterList));
    }
}