#include <Keypad.h>

const int ROW_NUM    = 4;
const int COLUMN_NUM = 4;

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

byte pin_rows[ROW_NUM] = {9, 8, 7, 6};
byte pin_column[COLUMN_NUM] = {5, 4, 3, 2};

Keypad customKeypad = Keypad(makeKeymap(MatrixKeyPad), pin_rows, pin_column, ROW_NUM, COLUMN_NUM );

void setup() {
  Serial.begin(9600);
}
void loop() {
  char key = keypad.getKey();
  if (key) {
    Serial.println(key);
  }
}