Java Method for generating final variable NOT updating the value used for it

Question

Hi I'm working with JAVA by blueJ and I'm having problem with the code below:

public class License {

    private String lastName = "Null";
    private final String ID_NUM = generateIDNumber();
    private static int ID_GEN = 33333;

    public License(String lastName) {
        this.lastName = lastName;
    }

    public String getLastName() {
        return lastName; 
    } 

    private String generateIDNumber() {
        String firstLetter = getLastName().substring(0,1);
        String idNumber = String.format("%d", ID_GEN) + "-" + firstLetter;                       firstLetter;
        ID_GEN += 1;
        return idNumber;
    }
}

So when I put my last name as an input, the program needs to generate the id number
which is composed of 5 digit number and first letter of the input last name.

However, the value of lastName used for generateIDNumber()is not getting updated with the newly entered input,
although when I execute getLastName(), it gets the updated input.


In other words, even if I input the lastName as "James", the value of firstLetter in generateIDNumber() is still "33333-N" for "Null", not "33333-J" for "James".

Why is this? Thank you in advance.

Answer 1

You're setting ID_NUM before the constructor runs, so lastName is still "Null". The solution is to set it within the constructor:

private final String ID_NUM;

public License(String lastName) {
    this.lastName = lastName;
    this.ID_NUM = generateIDNumber();
}

As a side note, CONSTANT_CASE should be reserved for static final variables. None of your variables fall into that category.