can i both use both use lcd and serial monitor? if yes, pls help me, i trying to use both serial monitor and lcd in my mega arduino, 4x4 keyad, servo motor,coin acceptor. In my lcd output, it display an words something i didn't know what it is. I also research about serial pins, but i don't quite understand it. 
here my code:
// Coin to Bill System
#include <Servo.h>
Servo myservo;
// constants
const int coinpin = 20,ledpin = 13,button1 = 24; //here's my pins
int servoPin = 22,buttonState =0;
// Variables
volatile int pulse = 0,a=0;
Servo Servo1;
boolean bInserted = false;
boolean b=0,c=0;
//...
// Inventory Systems
#include <LiquidCrystal.h> //include LCD library (standard library)
#include <Keypad.h> //include keypad library - first you must install library (library link in the video description)
#define redLED 10 //define the LED pins
#define greenLED 11
char* password ="1234"; //create a password
int pozisyon = 0; //keypad position
const byte rows = 4; //number of the keypad's rows and columns
const byte cols = 4;
char keyMap [rows] [cols] = { //define the cymbols on the buttons of the keypad
{'1', '2', '3', 'A'},
{'4', '5', '6', 'B'},
{'7', '8', '9', 'C'},
{'*', '0', '#', 'D'}
};
byte rowPins [rows] = {2, 3, 4, 5}; //pins of the keypad
byte colPins [cols] = {6, 7, 8, 9};
Keypad myKeypad = Keypad( makeKeymap(keyMap), rowPins, colPins, rows, cols);
LiquidCrystal lcd (A0, A1, A2, A3, A4, A5); // pins of the LCD. (RS, E, D4, D5, D6, D7)
void setup(){
//Coin to Bill System
Serial.begin(9600);
attachInterrupt(digitalPinToInterrupt(coinpin), coinInterrupt, RISING);
pinMode(ledpin, OUTPUT);
Servo1.attach(servoPin);
pinMode(button1,INPUT);
// Inventory Systems
lcd.begin(16, 2);
pinMode(redLED, OUTPUT); //set the LED as an output
pinMode(greenLED, OUTPUT);
setLocked (true); //state of the password
}
void loop(){
//Coin to Bill System
myservoloop();
checkButton1();
checkcoin();
// Inventory Systems
char whichKey = myKeypad.getKey(); //define which key is pressed with getKey
lcd.clear();
lcd.setCursor(0, 0);
lcd.print(" Welcome");
lcd.setCursor(0, 1);
lcd.print(" Enter Password");
if(whichKey == '*' || whichKey == 'A' || //define invalid keys
whichKey == 'B' || whichKey == 'C' || whichKey == 'D'|| whichKey == '#'){
pozisyon=0;
setLocked (true);
lcd.clear();
lcd.setCursor(0, 0);
lcd.print("TRY AGAIN!");
delay(1000);
lcd.clear();
}
if(whichKey == password [pozisyon]){
pozisyon ++;
}
if(pozisyon == 4){
setLocked (false);
lcd.clear();
lcd.setCursor(0, 0);
lcd.print("*** Verified ***");
delay(3000);
lcd.clear();
systemfunction();
}
delay(100);
}
void systemfunction(){
lcd.setCursor(0,0);
lcd.print("Pls Enter your");
lcd.setCursor(0,1);
lcd.print("options:");
delay(1000);
lcd.setCursor(0,0);
lcd.print ("Bill to coins[1]");
lcd.setCursor(0,1);
lcd.print ("Coins to Bill[2]");
delay(1000);
lcd.clear();
char whichKey = myKeypad.waitForKey();
if(whichKey == '1'){// coin to bill system
lcd.setCursor(0,0);
lcd.print("Pls Enter Number");
lcd.setCursor(0,1);
lcd.print("of Coins:");
char coins;
String convert1,convert_integer1;
while(coins !='#'){
coins = myKeypad.waitForKey();
if(coins =='#'){
return systemfunction();//return to system function
}
if(coins =='*'){
return 0;
}
lcd.print(coins);
if(coins =='1'||coins =='2'||coins =='3'||coins =='4'||coins =='5'||coins =='6'||
coins =='7'||coins =='8'||coins =='9'){
convert1=String(convert1 + coins);
convert_integer1 = convert1.toInt();
}
}
}
else if(whichKey == '2'){ //bill to coins system
char bills;
String convert2,convert_integer2;
lcd.setCursor(0,0);
lcd.print("Pls Enter Number");
lcd.setCursor(0,1);
lcd.print("of Bills:");
while(bills !='#'){
bills = myKeypad.waitForKey();
if(bills =='#'){
return systemfunction();//return to system function
}
lcd.print(bills);
if(bills =='1'||bills =='2'||bills =='3'||bills =='4'||bills =='5'||bills =='6'||
bills =='7'||bills =='8'||bills=='9'){
convert2=String(convert2 + bills);
convert_integer2 = convert2.toInt();
}
}
}
else{
lcd.clear();
lcd.setCursor(0, 0);
lcd.print("Invalid Key!");
delay(1000);
lcd.clear();
return systemfunction();
}
}
void setLocked(int locked){
if(locked){
digitalWrite(redLED, HIGH);
digitalWrite(greenLED, LOW);
}
else{
digitalWrite(redLED, LOW);
digitalWrite(greenLED, HIGH);
}
}
//...
// Coin to Bill System
void checkcoin(){
if( bInserted ){ // for show LED ON when Inserted
bInserted = false;
digitalWrite(ledpin, HIGH);
delay(1000);
}else{
// Turn off LED
digitalWrite(ledpin, LOW);
}
}
void checkButton1() {
buttonState = digitalRead(button1);
}
// Interrupt
void coinInterrupt(){
// Each time a pulse is sent from the coin acceptor,
// interrupt main loop to add 1 and flip on the LED
pulse++ ;
a++;
bInserted = true;
// digitalWrite(ledpin, HIGH);
// Serial.println( pulse );
}
void myservoloop(){
if(a==4 && buttonState == HIGH ){
Servo1.write(0);
delay(1000);
Servo1.write(90);
delay(1000);
a=(a-4);
}
if( bInserted ){ // for show LED ON when Inserted
bInserted = false;
digitalWrite(ledpin, HIGH);
delay(1000);
}else{
// Turn off LED
digitalWrite(ledpin, LOW);
}
}
//...
// merging system codes
