Create different types of combination with a given set of elements.

CombinationGenerator.java

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.Stack;

public class CombinationGenerator {
	private void func3(int[] arr, Stack<Integer> stack, ArrayList<String> list, int j) {
		if (j < arr.length) {
			for (int value : arr) {
				stack.push(value);
				StringBuilder stringBuilder = new StringBuilder();
				stack.forEach(stringBuilder::append);
				list.add(stringBuilder.toString());
				func3(arr, stack, list, j + 1);
				stack.pop();
			}
		}
	}
	
	/**
	 * Generate combination of all the elements
	 *
	 * @param arr elements
	 */
	private ArrayList<String> generatorType3(int[] arr) {
		ArrayList<String> list = new ArrayList<>();
		func3(arr, new Stack<Integer>(), list, 0);
		return list;
	}
	
	private void func2(int[] arr, LinkedHashSet<Integer> set, ArrayList<String> list, int j) {
		if (j < arr.length) {
			for (int value : arr) {
				if (!set.contains(value)) {
					set.add(value);
					StringBuilder stringBuilder = new StringBuilder();
					set.forEach(stringBuilder::append);
					list.add(stringBuilder.toString());
					func2(arr, set, list, j + 1);
					set.remove(value);
				}
			}
		}
	}
	
	/**
	 * Generate combination of all the elements
	 * but without repetition
	 *
	 * @param arr elements
	 */
	private ArrayList<String> generatorType2(int[] arr) {
		ArrayList<String> list = new ArrayList<>();
		func2(arr, new LinkedHashSet<>(), list, 0);
		return list;
	}
	
	private void func1(int[] arr, LinkedHashSet<Integer> set, ArrayList<String> list, int j) {
		if (j < arr.length) {
			for (int value : arr) {
				if (!set.contains(value)) {
					set.add(value);
					if (j == arr.length - 1) {
						StringBuilder stringBuilder = new StringBuilder();
						set.forEach(stringBuilder::append);
						list.add(stringBuilder.toString());
					}
					func1(arr, set, list, j + 1);
					set.remove(value);
				}
			}
		}
	}
	
	/**
	 * Generate combination of all the elements
	 * of size equal to the size of elements
	 *
	 * @param arr elements
	 */
	private ArrayList<String> generatorType1(int[] arr) {
		ArrayList<String> list = new ArrayList<>();
		func1(arr, new LinkedHashSet<>(), list, 0);
		return list;
	}
	
	public static void main(String[] args) {
		int[] arr = {1, 2, 3};
		Main main = new Main();
		ArrayList<String> list1 = main.generatorType1(arr);
		ArrayList<String> list2 = main.generatorType2(arr);
		ArrayList<String> list3 =  main.generatorType3(arr);
		
		System.out.println("First type of combination..");
		list1.forEach(System.out::println);
		System.out.println("Size of the list: " + list1.size());
		
		System.out.println("Second type of combination..");
		list2.forEach(System.out::println);
		System.out.println("Size of the list: " + list2.size());
		
		System.out.println("Third type of combination");
		list3.forEach(System.out::println);
		System.out.println("Size of the list: " + list3.size());
	}
}