public static void main(String[]args) {

int n = 0;

Scanner sc = new Scanner(System.in);

boolean invalidCheck = false;

System.out.println("Enter an integer sum to find pairs for: ");

while (!invalidCheck) {

String line = sc.nextLine();

try {

n = Integer.parseInt(line);

invalidCheck = true;

} catch (NumberFormatException e) {

System.out.println("Input must be an integer, please re-enter.\n");

invalidCheck = false;

}

}

System.out.println("Enter an integer list separated by spaces, to check sum pairs for: ");

invalidCheck = false;

String input;

String inputString;

input = sc.nextLine();

inputString = input.trim();

String[] inputs = inputString.split(" ");

int length = inputs.length;

int[] intInputs = new int[length];

for(int i = 0; i < length; i++){

try{

intInputs[i] = Integer.parseInt(inputs[i]);

invalidCheck = true;

} catch (NumberFormatException e) {

invalidCheck = false;

}

}

while (length < 2 || !invalidCheck) {

System.out.println("Input must be at least 2 integer values,\nplease re-enter list of integers separated by spaces,\nalso enter X or x to exit");

input = sc.nextLine();

inputString = input.trim();

if(inputString.equalsIgnoreCase("X")){

break;

} else {

inputs = inputString.split(" ");

length = inputs.length;

intInputs = new int[length];

for(int i = 0; i < length; i++){

try{

intInputs[i] = Integer.parseInt(inputs[i]);

invalidCheck = true;

} catch (NumberFormatException e) {

System.out.println("Input must be all integers,\nplease re-enter list of integers separated by spaces,\nalso enter X or x to exit");

invalidCheck = false;

}

}

}

}

sc.close();

HashMap<Integer, Integer> pairsMap = findPairs(n, intInputs);

int size = pairsMap.size();

System.out.println("Number of pairs found: " + size);

if(size > 0){

for (Map.Entry<Integer, Integer> entry : pairsMap.entrySet()) {

Integer key = entry.getKey();

Integer value = entry.getValue();

System.out.print("(" + key + ", " + value + ") ");

}

}

System.out.println("");

}

private static HashMap findPairs(int sum, int[] array){

HashMap<Integer, Integer> pairsMap = new HashMap<Integer, Integer>();

HashMap<Integer, Integer> singlesMap = new HashMap<Integer, Integer>();

for(int element:array){

if(!singlesMap.containsKey(element)){

singlesMap.put(element, 0);

}

singlesMap.put(element, singlesMap.get(element) + 1);

}

for(int i = 0; i < array.length; i++) {

int diff = sum - array[i];

//System.out.println("diff: " + diff + ", int: " + array[i]);

if (pairsMap.containsKey(array[i])) {

System.out.println("Pairs Contains: " + array[i] + " with value " + pairsMap.get(array[i]));

System.out.println("Singles Contains: " + diff + " with value " + singlesMap.get(diff));

if (pairsMap.get(array[i]) == diff && singlesMap.get(array[i]) == 1) {

System.out.println("Removing: " + diff + ", " + pairsMap.get(array[i]));

pairsMap.remove(array[i]);

}

}

if (singlesMap.containsKey(diff)) {

pairsMap.put(diff, array[i]);

}

}

return pairsMap;

}