how to check if an object already exists

[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

[oll3i]

public boolean equals(Object o) {
    if (!(o instanceof Klient)) return false;
    Klient k = (Klient) o;
    return  nazwa ==k.nazwa && pieniadze == k.pieniadze;
  }

i did it like this is it okey

[JosAH]

public boolean equals(Object o) {
    if (!(o instanceof Klient)) return false;
    Klient k = (Klient) o;
    return  nazwa ==k.nazwa && pieniadze == k.pieniadze;
  }

i did it like this is it okey

If 'nazwa' and 'pieniadze' can be compared for equality using the '==' operator,
I think all is fine. If one of them is an object (i.e. not a primitive) you have to
compare those members using the equals() method again (think about Strings).
Don't forget to implement the hashCode() method too.

kind regards,

Jos

[oll3i]

but i have a problem here cos when one button is clicked i create a client and add it to the list then when other button is clicked that client adds to the cart but the client created with the first button is not seen in the other portion of the code (the code for the second button)

[JosAH]

but i have a problem here cos when one button is clicked i create a client and add it to the list then when other button is clicked that client adds to the cart but the client created with the first button is not seen in the other portion of the code (the code for the second button)

That's an entirely different problem. Forget about GUIs for now and just take
care that your equals() and hashCode() methods are correctly implemented.
Build a little main() method that sticks Klients in a List and tries to find them
again afterwards. If all that works, build your GUI.

kind regards,

Jos

[oll3i]

i wdnt have to add to the list if the equals method worked :)

[JosAH]

i wdnt have to add to the list if the equals method worked :)

True, but my guess is that your equals() method doesn't work yet and the less
circumstancial code around (such as buttons etc.) the less difficult it is to
debug your code. Note that a list can store duplicate objects (the equals()
method returns true) so you should test before adding:

if (!list.contains(klient)) list.add(klient);

... or you could use a Set for this but still then: your equals() method should
work correctly.

kind regards,

Jos

[oll3i]

the equals method works i compared two clients

[JosAH]

the equals method works i compared two clients

Good; did you override the hashCode() method too?

kind regards,

Jos

[oll3i]

public int hashCode()
{
int result = 17;
result = 37 * result + pieniadze.hashCode();
result = 37 * result + nazwa.hashCode();

return result;
}

pieniadze is a double and i have a problem
and i dont know if it a good definition

[JosAH]

public int hashCode()
{
int result = 17;
result = 37 * result + pieniadze.hashCode();
result = 37 * result + nazwa.hashCode();

return result;
}

pieniadze is a double and i have a problem
and i dont know if it a good definition

primitives don't have methods so 'double.hashCod e()' doesn't make sense; but
why not simply cast the double value to an int and use that as the double's
hash value:

int result = 17;
result = 37 * result + (int)pieniadze;
result = 37 * result + nazwa.hashCode();

Is 'nazwa' an object? If so you have to use its 'equals()' method in your own
'equals()' method (see a couple of replies back).

kind regards,

Jos