Java：将对象添加到数组列表并检索它会打印出 null

Question

I have two classes (actually more, but that's not relevant). In my main class I have a method called createEmployee() , in which I create an Employee with user input. Here's the Main class:

import java.util.*;

public class Main {

private static String END_LINE;
private Scanner sc;
public String name;
public String ID;
public double salary;
private ReusaxCorp reusaxcorp;

public Main(){
    sc = new Scanner(System.in);
    END_LINE = System.lineSeparator();
}

public void presentoptions(){
    reusaxcorp = new ReusaxCorp();
    while (true){
        System.out.println("=== Welcome === ");
        System.out.println("Choose an option below: ");
        System.out.println(" ");
        System.out.println("1. Register an employee. ");
        System.out.println("2. Remove an employee. ");
        System.out.println("3. Retrieve all employees information. ");
        System.out.println("4. Quit this program. ");
        int option = sc.nextInt();

        switch (option) {
            case 1:
                System.out.println("What type of employee? " + END_LINE
                        + " - Intern. " + END_LINE
                        + " - Employee. " + END_LINE
                        + " - Manager. " + END_LINE
                        + " - Director." + END_LINE);
                String type = sc.nextLine();

                createEmployee();
                break;

            case 2:
                break;
            case 3:
                reusaxcorp.retrieveEmployee();
                break;
            case 4:
                System.out.println("You've quitted the program.");
                System.exit(0);

            default:
                System.out.println("Error. Please try again.");
                break;
        }
    }
}

String empID;
String empName;
double empSalary;

public void createEmployee(){
    String typeofemployee = sc.nextLine();

        System.out.println("What's the ID of the new " + typeofemployee + "?");
        String ID = sc.nextLine();
        System.out.println("Wheat's the name of the new " + typeofemployee + "?");
        name = sc.nextLine();
        System.out.println("What's the salary of the new " + typeofemployee + "?");
        salary = sc.nextDouble();

        Employee employee = new Employee(ID, name, salary);

        switch (typeofemployee) {

            case "Employee":
                reusaxcorp.registerEmployee();
                break;

            default:
                System.out.println("Error");
                break;
        }
}

public static void main(String[] args) {
    Main runcode = new Main();
    runcode.presentoptions();
}

}

In my ReusaxCorp class I have a method registerEmployee , which should add a new Employee to the Array List and retrieveEmployee , which should print all registered Employees. Here they are:

public class ReusaxCorp extends Main {

   ArrayList<Employee> employees = new ArrayList<Employee>();
   final String END_OF_LINE = System.lineSeparator();

   public void registerEmployee(){
       employees.add(new Employee(ID, name, salary));
   }

   public void retrieveEmployee() {
       Main main = new Main();
       for(Employee employee: employees){
           System.out.println("ID: " + ID + END_OF_LINE + "Name: " + employee.name + END_OF_LINE + "salary: " + employee.grossSalary);
   }
}

The problem I have is that it always prints out ID: null Name: null salary: 0.0 and I don't know what that is. I also tried doing - for instance - this:

String empID;
String empName;
double empSalary;

// in my Main class I wrote getters and setters and this in my createEmployee method:

System.out.println("What's the ID of the new " + typeofemployee + "?");
String newID = sc.nextLine();
setEmpID(newID);

In my retrieveEmployee() method I then wrote getEmpID() instead of ID , but it still printed out null. Where's the mistake?

Answer 1

You are in effect, creating a new Employee object, doing nothing with it, and then calling a method in your ReusaxCorp that creates a new Employee object with no values and adds it to your "employees" list. When you call your "retrieveEmployees" method, it prints out your empty employee.

Solve this by:

in your "createEmployee" class, make the following change in your "switch" statement:

reusaxcorp.registerEmployee(employee); // Now you are actually passing your newly created Employee object

and in your "ReusaxCorp" class, change your "registerEmployee" method:

registerEmployee(Employee employee){ // Here, you are receiving an Employee object
   employees.add(employee);          // and here you are adding it to your list.
}

Ultimately, the problem is that you never used any of the objects you created, and just populated the list with "empty" Employee objects.

Hope this helped.