how to check if an object already exists

[oll3i]

Good Morning :)

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

public static void main(String[] args) {
    
    Klient k1 = new Klient("Jan",200);
    Klient k2 = new Klient ("Jan",200);
    
    System.out.print(k1.equals(k2));
}
}

prints out
Jan
Jan
200.0
200.0
true

is that what you meant

[JosAH]

Good Morning :)

is that what you meant

Yep, what happens now if you use the List.contains(. ..) method after you've
inserted one of those Klients? The List should call your equals() method for
the actual comparisons (which in turn show you what actually is compared).

kind regards,

Jos

[oll3i]

now it finds a client in a list

...but i get nullpointerexce ption :( and eclipse doesnt say the line

[JosAH]

now it finds a client in a list

...but i get nullpointerexce ption :( and eclipse doesnt say the line

Do you have a 'null' element in your List? Print the entire list (a List can do
it itself):

System.out.println(yourList);

... and see for yourself.

kind regards,

Jos

[oll3i]

the client is in a list the nullointerexcep tion is somewhere else but after i click a button

[oll3i]

please clear me that out so i learn for the future how a list can call the equals method when i defined it for a client ?

[JosAH]

please clear me that out so i learn for the future how a list can call the equals method when i defined it for a client ?

This is what method overriding is all about. The Object class (the top of the
class hierarchy) defines an equals() method. The List calls an equals() method.
You have overridden Object's equals() method so *your* equals() method will
be called by the List because your Klient extends the Object class (implicitly)
and your class defines its own equals() method.

Your NullPointerExce ption is an entirely different problem though ...

kind regards,

Jos

[oll3i]

so how wd that look in use?

[JosAH]

so how wd that look in use?

Sorry, I don't understand your question.

kind regards,

Jos

[oll3i]

cd you give me now the example how i shd use that in my case with my list and equals method
please

[JosAH]

cd you give me now the example how i shd use that in my case with my list and equals method please

You don't have to do anything: method overriding is an intrinsic part of the Java
language. You have implemented your own equals() method and therefore
(automatically) overridden the equals() method in the Object class. You've
seen it happen yourself: the List calls your equals() method instead of the
one defined in the Object class.

kind regards,

Jos

[oll3i]

when the list calls the equal method?

[prometheuzz]

when the list calls the equal method?

Whenever you invoke the contains(...) and indexOf(...) methods from the List, for example.
Please take the time to read this tutorial:

http://java.sun.com/docs/books/tutorial/java/IandI/objectclass.html

it explains in great detail how to override the equals(...) (and hashCode()!) method(s).

[oll3i]

cd you look at my nullpointerexce ption post with nullpointerexce ption in the title
please

[JosAH]

cd you look at my nullpointerexce ption post with nullpointerexce ption in the title
please

I just did; please follow up my advice I gave you in your other thread and then
come back here when your don't understand what the Exception stacktrace has
to tell you.

kind regards,

Jos