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

public class FindPair {
	int totalNum;		//Total number
	Integer[] list;	//The list of integers
	int size;

	//Test functions
	public static void main(String args[]){
		FindPair demo = new FindPair(4);
		demo.readUserInput();
		demo.displayResult();
	}

	//Constructor
	public FindPair(int listSize){
		totalNum = 0;
		size = listSize;
		list = new Integer[size];
	}

	/**
	 * Validate the total number that entered by user
	 * @param num the number that inputed by user
	 * @return true for valid number, false for invalid number
	 */
	private boolean NumVal(String num)
	{
		//Check whether the input is integer
		try {
			Integer.parseInt(num);
		}
		catch(NumberFormatException | NullPointerException e) {
			System.out.println("Please enter a valid integer");
			return false;
		}
		//Check whether the number is greater than 0
		if(Integer.parseInt(num) < 1) {
			System.out.println("Please enter a positive number: ");
			return false;
		}

		else
			return true;
	}

	/**
	 * Read user input for the list and total number
	 */
	public void readUserInput()
	{
		Scanner sc = new Scanner(System.in);
		//Catch the exception when user input the number
		System.out.println("Please enter the first numer as the sum: ");
		String num = sc.nextLine();
		//valid the number, if the result is false than let the user enter the number again
		while(!NumVal(num)) {
			num = sc.nextLine();
		}
		totalNum = Integer.parseInt(num);
		//Read user input for list
		for(int i = 0; i < size; i++)
		{
			System.out.println("Please enter the " + (i+1) + " number in the list: ");
			num = sc.nextLine();
			//Validation of number
			while(!NumVal(num)) {
				num = sc.nextLine();
			}
				list[i] = Integer.parseInt(num);
		}
		sc.close();
	}

	/**
	 * Find pairs from the list
	 * @return the array list of pairs
	 */
	private ArrayList<Integer> findPair()
	{
		ArrayList<Integer> result = new ArrayList<Integer> ();
		//Find pairs by two loops
		for(int i = 0; i < size; i++)
		{
			for(int j = i + 1; j < size; j++)
			{
				if (list[i] + list[j] == totalNum && list[i] != list[j] && !result.contains(list[i]))
				{
					result.add(list[i]);
					result.add(list[j]);
					break;
				}
			}
		}
		return result;
	}

	/**
	 * Display the pairs that have the sum of total number
	 */
	public void displayResult()
	{
		//Invoke the method of findPair
		ArrayList<Integer> result = findPair();
		System.out.println("Result(s): ");
		for(int i = 0; i < result.size() - 1; i+=2)
		{
			System.out.print("[" + result.get(i) + ", " + result.get(i+1) + "] ");
		}
	}
}