Trying to access an object's methods

[Jacob Steingart]

I have an array of objects, and I'm trying to access a getter with code like this:

showName = users[numOfUsers].getName();

By the way, showName is a static variable, not a method or undeclared variable. Right, so when I run this code, I get a NullPointerException in runtime. Does anyone know what I'm doing wrong?

[Wirianto Djunaidi]

very likely the users[numOfUsers] is returning null, so it failed when null is used to call getName()

[Michael Dunn]

> I get a NullPointerException in runtime.

generally indicates this type of structure

Person[] users = new Person[10];
showName = users[numOfUsers].getName();//NPE here

missing is this bit (on its own, or in a loop)
users[x] = new Person();

the first bit declares the space [10]
the missing bit creates each object

[Mark Vedder]

Most likely the snippet:

users[numOfUsers]

is returning a null value. So when you try to call

users[numOfUsers].getName()

you calling getName on a null object, and thus getting a NullPointerException.

Can you show the code where you populate the array?

Remember that object arrays default to null objects. Only arrays of primitives (ints, bytes, booleans, floats, etc) are initialized with default values.

So if I have the code:

User[] users = new User[10];

I get an array of 10 null objects.

[Mark Vedder]

LOL - looks like you had three of us all typing up an answer at the same time. But since we all basically said the same thing... we must be on the right track

[Sagar Rohankar]

may be your array is just declared but not initialized ! , You can always print array element or check for its nullity before invoking any method !

[Sagar Rohankar]

me too Mark, just 2 minutes late in replying !

[Jacob Steingart]

I'm pretty sure I declared it... here is the code I'm using. I have a lot of commented out code, so I hope it isn't too messy .

import java.io.*;
import java.util.*;
 
public class AccountStuff {
//create important statics
static BufferedReader kb = new BufferedReader(new InputStreamReader(System.in));
static User[] users = new User[50];
static int numOfUsers = 0;
static int loadedNum = 0;
static String tempName;
static String tempPass;
static String showName;
static String showPass;
 

public static void main(String[] args) throws IOException {
//start the login
load();
}

public static void load() throws IOException {
System.out.println("Loading vars...");

try {
File logFile = new File("users.txt");
FileReader fileReader = new FileReader(logFile);
BufferedReader reader = new BufferedReader(fileReader);

numOfUsers = Integer.parseInt(reader.readLine());

String line = null;

while((line = reader.readLine()) != null) {
addUser(line,tempName,tempPass);
}

} catch(Exception ex) {
System.out.println("File load error.");
ex.printStackTrace();
}

System.out.println("Vars loaded.");
//System.out.println("numOfUsers: " + numOfUsers);
logIn();
}

public static void addUser(String lineToParse,String name,String pass) {
String[] result = lineToParse.split("/");
users[loadedNum] = new User(name,pass);

loadedNum++;
}

public static void logIn() throws IOException {
System.out.println("Welcome to the PPPLS.");
System.out.println("If you are a registered user, please type your verification code into the box below.");
System.out.println("If you are not registered, please enter 0000000 as the verification code.");

int input = Integer.parseInt(kb.readLine());


if(input == 0000000) {
createAccountDll();
}
}

/*else {
proceedToLogin();
}*/

//get info and pass to method that generates an account with the User class
public static void createAccountDll() throws IOException {
System.out.println("Enter your desired username:");
String tempName = kb.readLine();

createAccount(tempName);

}

public static void createAccount(String tempUName) throws IOException{
//create array for each password digit
int[] digits = new int[7];

digits[0] = (int)(Math.random() * 9 + 1);
digits[1] = (int)(Math.random() * 9 + 1);
digits[2] = (int)(Math.random() * 9 + 1);
digits[3] = (int)(Math.random() * 9 + 1);
digits[4] = (int)(Math.random() * 9 + 1);
digits[5] = (int)(Math.random() * 9 + 1);
digits[6] = (int)(Math.random() * 9 + 1);

//convert to Strings for ordering purposes
String digits0 = Integer.toString(digits[0]);
String digits1 = Integer.toString(digits[1]);
String digits2 = Integer.toString(digits[2]);
String digits3 = Integer.toString(digits[3]);
String digits4 = Integer.toString(digits[4]);
String digits5 = Integer.toString(digits[5]);
String digits6 = Integer.toString(digits[6]);

String tempPass = digits0 + digits1 + digits2 + digits3 + digits4 + digits5 + digits6;

users[numOfUsers] = new User(tempUName,tempPass);

 


numOfUsers++;
saveUser();

}

public static void saveUser() throws IOException{

RandomAccessFile raf = new RandomAccessFile("users.txt","rw");
raf.setLength(0);

try {
FileWriter writer = new FileWriter("users.txt");
//write numOfUsers
writer.write(numOfUsers + "\n");
//write current users
for (int i=0;i<numOfUsers;i++) {
writer.write(users[i].userName + "/");
writer.write(users[i].passWord + "\n");
}
writer.close();
} catch(IOException ex) {
System.out.println("An error in writing your account data to a file ocurred. Please try again later.");
ex.printStackTrace();
}
//logIn();
showName = users[numOfUsers].getName();
showPass = users[numOfUsers].getPass();

//System.out.println("Your username is " + showName);
//System.out.println("Your password is " + showPass);
System.out.println("Press enter to proceed to main menu...");
kb.readLine();

//tellInfo();
}
/*
public static void tellInfo() throws IOException {
System.out.println("Your username is " + users[numOfUsers].getName());
System.out.println("Your password is " + users[numOfUsers].getPass());
System.out.println("Press enter to proceed to main menu...");
kb.readLine();
logIn();
}*/
}
 
public class User {
String userName;
String passWord;

public User(String uName, String pWord) {
userName = uName;
passWord = pWord;
}

public String toString() {
return "Username: " + userName;
}

public String getName() {
return userName;
}

public String getPass() {
return passWord;
}
}

It's probably something really tiny and stupid that I'm missing, but I have a tendency too miss these things. Thanks in advance!

[Michael Dunn]

your problem might be in this bit:

users[numOfUsers] = new User(tempUName,tempPass);
numOfUsers++;//<------------------
saveUser();

in this, you create the User(), then increment numOfUsers.

try it like this, and see if it still errors
showName = users[numOfUsers-1].getName();

(also check showName gets the correct name)

[Amit Ghorpade]

Although Michael has isolated the problem,

showName = users[numOfUsers-1].getName();

this code can throw the ArrayIndexOutOfBoundsException if numOfUsers is zero.
The same thing can happen if numOfUsers is greater than 50.

So I think a better option would be to slightly modify the code so that numOfUsers properly points to the required location.
And also make sure that numOfUsers is always less than the maximum limit, 50 in this case.


Hope this helps

[Ronald Schild]

public static void addUser(String lineToParse,String name,String pass){
  String[] result = lineToParse.split("/");
  users[loadedNum] = new User(name,pass);
  loadedNum++;
}

Unrelated to the question, but, aren't you forgetting to do something with result here?