From UML to Java, Simple association

Question

This is the Image that I'm going to transfer from UML to Java, I don't know how to lock them together and i don't know how to make one bankAccount locked to only one person.

How do i connect the 2 classes??

Here is my code so far

My main method

    public class Upp5 {

    public static void main(String[] args) {

        Person2 david = new Person2();
        BankAccount acc1 = new BankAccount();
        BankAccount acc2 = new BankAccount();

        david.setName("David");
        david.setPnr("551012-8978");

        acc1.setBnr("37");
        acc2.setBnr("38");

        System.out.println("Namn: " + david.getName() + " \nPersonnummer:" + david.getPnr());
        System.out.println(acc1.getBnr() + "\n" + acc2.getBnr());
    }

}

BankAccount.java:

public class BankAccount {
    private String bnr;
    private double balance;

    public void credit() {

    }

    public void withdraw(){

    }
    public String getBnr(){
        return bnr;
    }

    public void setBnr(String newAccount){
        bnr = newAccount;
    }

    public void createAccount(String newNbr){
        bnr = newNbr;
    }

}

and Person2.java

public class Person2 {
    private String pnr;
    private String name;

    //Koppla konto till pnr
    public void addAccount(BankAccount a){

    }

    //Skapa Pnr och Namn
    public void setPnr(String newPnr) {
        pnr = newPnr;
    }
    public void setName(String newName){
        name = newName;
    }
    // Hämta Pnr och Namn
    public String getPnr(){
        return pnr;
    }
    public String getName(){
        return name;
    }


}

Answer 1

You didn't put any arrows in your diagram, so we don't know if the bankaccount knows the person it is linked to. The most logical thing to do is; let the Person have a List which you always initiate in the Person's constructor.

If you want the bankaccount to know the person that owns him (which seems logical to me), let the Bankaccount have the property "Person owner" and let the constructor be require a Person to exist. I don't understand why you have made a Person2 instead of a Person class.

Tips for you: Never (!!!) use parameters like; 'a' or properties like 'nBr' because other people wanna see in an instance what they are instead of guessing. :-) And try to make some security rules (that's why I put booleans in the classes).

Solution:

Person:

public class Person {
    List<BankAccount> bankAccounts;
    private String name;
    private String pNbr;

    public Person(String name, String pNbr) {
        this.name = name;
        this.pNbr = pNbr;
    }

    public void addAccount(BankAccount newAccount){
        bankAccounts.Add(newAccount);
    }
}

Bankaccount:

public class BankAccount {
    private String nBr; //maybe make this final?
    private double balance;
    private Person owner;

    public BankAccount(String nbr, Person owner) {
        this(nbr, 0, owner); // If you also want to support new empty accounts
    }

    public BankAccount(String nbr, double balance, Person owner) {
        this.name = name;
        this.pNbr = pNbr;
        this.owner = owner;
    }

    public boolean Credit(double amount)
    { // TODO: write code 
        boolean result = false;
        return result;
    }

    public boolean Withdraw(double amount)
    { // TODO: write code 
        boolean result = false;
        return result;
    }
}

Answer 2

The easiest way to "link" classes in java is to add one of said classes as an attribute. For example:

// Make it private to maintain encapsulation
private BankAccount myBankAccount;

But this only works if you have a 1..1 (One to one) relationship.

Your UML diagram indicates a 1..n (One to many) relationship between Person and BankAccount classes. In other words,

A Person may have multiple BankAccounts.

This means you'll a have to use a structure to "keep" multiple BankAccounts in a single Person. Java already provides you with some handy classes:

• ArrayList: Easy, quick, insertion-ordered list. Allows as many itens as you need.

• Hashmap: Hash implementation of the Map interface. Provides a way to find itens using a "key" (Ex: An account's number). Very efficient.

A suggest reading some of those classes documentation. And you can always look for some neat examples on the internet ;)

Answer 3

You need to define a List<BankAccount> to your Person2 entity:

public class Person2 {
    private String pnr;
    private String name;

    // list of bank accounts (from 0 to n) the Person can have.
    private List<BankAccount> accounts;

    //Koppla konto till pnr
    public void addAccount(BankAccount a){
         if (accounts = null) accounts = new ArrayList<BankAccount>();
         accounts.add(a);
    }

    //Skapa Pnr och Namn
    public void setPnr(String newPnr) {
        pnr = newPnr;
    }
    public void setName(String newName){
        name = newName;
    }
    // Hämta Pnr och Namn
    public String getPnr(){
        return pnr;
    }
    public String getName(){
        return name;
    }

    // include getters setters
}

EDIT1: as suggested by @NathanCastlehow if you want double relationship, BankAccount.java must have a Person2 attibute

public class BankAccount {
    private String bnr;
    private double balance;

    // one bank account can only be owned by a single Person
    private Person2 person;

    public void credit() {

    }

    public void withdraw(){

    }
    public String getBnr(){
        return bnr;
    }

    public void setBnr(String newAccount){
        bnr = newAccount;
    }

    // generate getters setters
    public Person2 getPerson(){
           ......
    }
}

Answer 4

So generally when this is done in models such as a relational model you would have an association class. So a class that has like an ID from a bank account and an ID of the person. You can lock variables using the final keyword in front of them which forces them to only be initialized once.