将类对象存储到数组中

Question

I'm trying to store objects that have a username and password from the class "Driver" into an array list. When I try to print every value in the array to test whether they're being stored, it only prints the last value declared, numerous times. I have tried nearly every other solution on thee forums related to this issue and it just wont work :(

Code below:

package eDepotSystem;

import java.util.ArrayList;


public class Driver {

    protected static String driverUserName;
    protected static String driverPassWord;

    public Driver (String userName, String passWord) {
        driverUserName = userName;
        driverPassWord = passWord;

    }

    public static void driverArray() {

        ArrayList<Driver> driverList = new ArrayList<Driver>();

        Driver driver = new Driver(driverUserName, driverPassWord);
        driver.setUserName("driver1");
        driver.setPassword("123");
        driverList.add(driver);

        driver = new Driver(driverUserName, driverPassWord);
        driver.setUserName("driver2");
        driver.setPassword("321");
        driverList.add(driver);

        Driver tempDriver = new Driver(driverUserName, driverPassWord);
        for (int i = 0; i < driverList.size(); i++) {
            tempDriver = driverList.get(i);
            System.out.println(tempDriver);
        }   
    }

    public void setPassword(String password) {
        driverPassWord = password;
    }

    public static String getUserName() {
        return driverUserName;
    }

    @Override
    public String toString() {
        return driverUserName + driverPassWord;
    }
}

I don't know whether my loop is wrong or the way I'm declaring the objects is wrong? Any help would be grand and thanks in advance!

Answer 1

your field variables should not be static. It is being shared by all instances of Driver class (ie objects), hence it is printing the last value which you added.

Answer 2

Problem 1:

Your "instance variables" (username and password) are static. Therefore you only have one instance of them. If you print them out you must always get the same value.

Problem 2:

You only add one object. You add it once, change it's values and add it a second time. If you print it out you must get the same values ... even if you remove the static keywords.

You should instead try something like this:

package eDepotSystem;

import java.util.ArrayList;
import java.util.List;

public class Driver {
    private final String driverUserName;
    private final String driverPassWord;

    public Driver (String userName, String passWord) {
        driverUserName = userName;
        driverPassWord = passWord;
    }

    public static void driverArray() {
        List<Driver> driverList = new ArrayList<Driver>();
        driverList.add(new Driver("drv1", "pw1"));
        driverList.add(new Driver("drv2", "pw2"));

        for (Driver tempDriver : driverList) {
            System.out.println(tempDriver);
        }   
    }
}

Answer 3

The static keyword forces the driverUserName and driverPassWord variables to be instantiated only once in memory. While they are not constants, it makes any further additions to your list reference that first and only instance in memory, hence why it keeps showing the same value.

https://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html