#include <Keypad.h>

const uint8_t ROWS = 4;
const uint8_t COLS = 4;

char keys[ROWS][COLS] = {
  {'1', '2', '3', 'A'},
  {'4', '5', '6', 'B'},
  {'7', '8', '9', 'C'},
  {'*', '0', '#', 'D'}
};

uint8_t rowPins[ROWS] = {15, 2, 0, 4};
uint8_t colPins[COLS] = {19, 21, 22, 23};

Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

void setup() {
  Serial.begin(9600);
}

void loop() {
  char key = keypad.getKey();
  if (key != NO_KEY) {
    Serial.println(key);
  }
}