how to check if an object already exists

[oll3i]

how to check if an object already exists and return reference to that object

[oll3i]

i get nullpointerexce ption

[oll3i]

i changed it to what You suggested but it doesnt change that it doesnt find the klient in the list
the client is added to the list when a button is clicked then that client adds to the cart but before he adds to the cart i need to check if it isnt a client that already exists if it is then return him with klient = klienci.get(ind ex);<< but it doesnt find that client

there is probably a better solution for that but unfortunately i can not think of one right one

[JosAH]

Does your Klient class override the equals() and hashCode() methods?

kind regards,

Jos

[oll3i]

no it doesnt shd it?

[JosAH]

no it doesnt shd it?

Yes it should. Read all about it in the Object.equals() and Object.hashCode ()
methods documentation.

By default equality is considered reference equality and you want value equality,
e.g. if two distinct objects contain identical member values you want to consider
the two distinct objects as equal.

kind regards,

Jos

[oll3i]

if i define an equal method for klient (client in english) how shd i use it to compare if that client already exists cos i created a list of clients and i need to find that client in a list or shd i do in in some other way

[hirak1984]

can I see how you have defined the object variable?

Did you set it to null , initially?

i get nullpointerexce ption

[JosAH]

if i define an equal method for klient (client in english) how shd i use it to compare if that client already exists cos i created a list of clients and i need to find that client in a list or shd i do in in some other way

That's the convenient part of it all: the List does it for you when you call a
List.contains() of List.indexOf() method. Have a look at the contains() method:
(this is from the 1.6 JDK)

    public boolean contains(Object o) {
    Iterator<E> e = iterator();
    if (o==null) {
        while (e.hasNext())
        if (e.next()==null)
            return true;
    } else {
        while (e.hasNext())
        if (o.equals(e.next()))
            return true;
    }
    return false;
    }

See how it uses the equals() method when it needs to find an object.

kind regards,

Jos

[oll3i]

if i define method equals for klient how i can compare if that klient(client in english) already exists i created a list of clients client doesnt inherits from any other class so i can not compare it to a class it inherits from

[JosAH]

if i define method equals for klient how i can compare if that klient(client in english) already exists i created a list of clients client doesnt inherits from any other class so i can not compare it to a class it inherits from

Your Klient class equals() method just needs to compare your 'this' Klient
with another Klient object:

public class Klient {
   .
   .
   public boolean equals(Object obj) {
      if (obj == null || !(obj instanceof Klient)) return false;
      Klient that= (Klient)obj;
      // compare 'this' Klient with 'that' Klient and return true or false;
      return ...;
   }

Don't forget to implement the hashCode() method too. The two methods always
come in pairs, if you implement one you have to implement the other one too.

kind regards,

Jos