使用另一个类为对象分配ID会返回null而不是数字

So I have a Store that I created and I wanted to give employees, prodcuts and customers IDs. I created a class called ObjectIDs that I call in my other classes. Problem is, it gives out null when I actually go and create another class object.

I have a parent class called Person that has 2 classes which inherit from him: Employee and Customer.

So this is Person:

class Person implements Output {
    private String firstName;
    private String lastName;

    private static final String VALID_PERSON_NAME = "^[a-zA-Z]{2,50}$";

    Person(String firstName, String lastName) throws Exception {
        this.setNames(firstName, lastName);
    }

    private void setNames(String firstName, String lastName) throws Exception {
        if ( !firstName.matches(VALID_PERSON_NAME) || !lastName.matches(VALID_PERSON_NAME)) {

            throw new IllegalArgumentException ("Please only use English");
        }
        this.firstName = firstName;
        this.lastName = lastName;
    }

    String getNames() {
        return this.firstName+" "+lastName;
    }

    public String createOutput() {
        return this.firstName+","+this.lastName;
    }
}

Now that we got that out of the way, we'll take Customer as an example for the null ID I'm getting:

class Customer extends Person{
    private String customerID;

    Customer(String firstName, String lastName, String customerID) throws Exception {
        super(firstName, lastName);
        this.customerID = ObjectIDs.generateID(customerID);
    }

    public String getCustID() {
        return customerID;
}
}

And here is the ObjectIDs class:

import java.util.Random;

public class ObjectIDs {
    String id;

    public static String generateID(String id) throws Exception {
        Random rand = new Random();
        int randInt = rand.nextInt(1000) + 100;
        String randStr = Integer.toString(randInt);
        return randStr;
    }
}

我有一个菜单，可以在其中调用相关功能来创建新客户，例如：

Customer newCustomer(String id, String firstName, String lastName) throws Exception {
        for ( Customer c : customers) {
            if ( id.equals(c.getCustID())) {
                throw new IllegalArgumentException("Customer ID already exist.");
            }
        }
        Customer c = new Customer(id, firstName, lastName);
        this.customers.add(c);
        System.out.println("Customer "+c.createOutput()+" was added to the list");
        return c;
    }

Now, like I said, when I create a new customer for example, I get null instead of a number and I'm not sure what I'm missing. I didn't want to overwhem here with huge walls of code so I believe the issue lies within the pieces of code I shared here.

谢谢 ：）