Java Combine Lists

import java.util.ArrayList;
import java.util.Scanner;


public class CombineListsOfLetters {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String firstInput = in.nextLine();
        firstInput = firstInput.replaceAll(" ", "");
        String secondInput = in.nextLine();
        secondInput = secondInput.replaceAll(" ", "");

        ArrayList<Character> firstChars = new ArrayList<> ();
        ArrayList<Character> secondChars = new ArrayList<> ();

        for (char c : firstInput.toCharArray()) {
            firstChars.add(c);
        }

        for (char c : secondInput.toCharArray()) {
            secondChars.add(c);
        }

        for (int i = 0; i < firstChars.size(); i++) {
            System.out.print(firstChars.get(i) + " ");
            if (secondChars.contains(firstChars.get(i))) {
                secondChars.remove(firstChars.get(i));
            }
        }
        for (int i = 0; i < secondChars.size(); i++) {
            System.out.print(secondChars.get(i) + " ");
        }
    }

}