how to check if an object already exists

[oll3i]

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

[RedSon]

Can you please describe your question in more detail?

[oll3i]

i have to write a flower shop
i need to check if a client already exists so that i dont add a new one

[hirak1984]

[font=Verdana][size=2]your question is not yet clear.[/size][/font]
[font=Verdana][size=2]In general we compare the instance of the object with null,[/size][/font]
[font=Verdana][size=2]to see if it exists or not...[/size][/font]

i have to write a flower shop
i need to check if a client already exists so that i dont add a new one

[prometheuzz]

i have to write a flower shop
i need to check if a client already exists so that i dont add a new one

Exists in what? An array, a java.util.List some other java.util.Collection?

[oll3i]

in a list of clients

[JosAH]

in a list of clients

Well, read all about it in the API docs for the List.contains(...) method.

kind regards,

Jos

ps. Bookmark The API docs or download them for easy reference.
Keep those docs at your fingertips whenever you want to write code.

[oll3i]

[code]
if(klienci.contains(klient)){
int index=klienci.indexOf(klient);

klient=klienci.get(index);
}

[code]

but it doesnt find the client in the list

[hirak1984]

can we know the errors you are getting?

[code]
if(klienci.contains(klient)){
int index=klienci.indexOf(klient);

klient=klienci.get(index);
}

[code]

but it doesnt find the client in the list

[prometheuzz]

if(klienci.contains(klient)){
int index=klienci.indexOf(klient);

klient=klienci.get(index);
}

but it doesnt find the client in the list

When doing this, you're looping through the list twice: once in the contains(...) method, and another looptwhen using the indexOf(...) method.
I suggest doing this:

int index = klienci.indexOf(klient);
 
if(index != -1) {
  klient = klienci.get(index);
} 
else {
  // 'klient' is not in the list 'klienci'
}

[oll3i]

i get nullpointerexception

[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(index);<< 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 nullpointerexception

[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.hashCode()' 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

[oll3i]

nazwa is a string and yes i used equals(nazwa.equals(k.nazwa)) in the equals method

[oll3i]

but now what to do with the buttons problem

[JosAH]

but now what to do with the buttons problem

I don't know; for starters: are both ActionListeners registered with those Buttons
using the same List? Add a lot of System.out.println(...) debug statements and
see what happens.

kind regards,

Jos

ps. does your equals() and hashCode() methods work correctly now?

[oll3i]

thank u a lot lot lot yes they do :)

[oll3i]

i did use lots of System.out.print() but the prob is that what's created by one button is not seen by the other button code and where to look 4 solution of that?

[JosAH]

i did use lots of System.out.print() but the prob is that what's created by one button is not seen by the other button code and where to look 4 solution of that?

Are both ActionListeners using the same List in which you added your Klient
objects? Print out both lists everytime you access them and see what's in
them. (they both should be two references to one List).

kind regards,

Jos

ps. a List can print itself quite nicely because it overrides the toString() method.

[oll3i]

i walked through the list and i know that the client is in that list but when i do
int index = klienci.indexOf(klient); it returns false

[JosAH]

i walked through the list and i know that the client is in that list but when i do
int index = klienci.indexOf(klient); it returns false

Can't be, the indexOf() method returns an int; -1 possibly if no object was found.
I think you have to recheck your equals() method and your hashCode() method.
Print out what your equals() method has to say about every comparison.

kind regards,

Jos

[oll3i]

that was with contains sorry so it returns -1

but
Klient k1 = new Klient("Jan",200);
Klient k2 = new Klient ("Jan",200);

System.out.print(k1.equals(k2));


returns true


int index = klienci.indexOf(klient);

if(index != -1) {
klient = klienci.get(index);
}
else {
// 'klient' is not in the list 'klienci'
}

[JosAH]

that was with contains sorry so it returns -1

but
Klient k1 = new Klient("Jan",200);
Klient k2 = new Klient ("Jan",200);

System.out.print(k1.equals(k2));


returns true


int index = klienci.indexOf(klient);

if(index != -1) {
klient = klienci.get(index);
}
else {
// 'klient' is not in the list 'klienci'
}

You did do a 'klienci.add(k1)' or 'klienci.add(k2)' did you? If so, can you show
your equals method just to be sure? Add a System.out.println() to your equals()
method to see if it is called by your List.

kind regards,

Jos

[oll3i]

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

i will show u how it is with the buttons

m_add_client.addActionListener( new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
 
try{
 
 //Klient k = new Klient((String)m_name.getText(),Double.valueOf(m_money.getText().trim()).doubleValue()); 
 
 
klienci.add(new Klient((String)m_name.getText(),Double.valueOf(m_money.getText().trim()).doubleValue()));                       
 
}catch(NumberFormatException exception){
System.err.println("Nie podana suma pieniedzy");
}  
catch(NullPointerException exception){
System.err.println("?");
}
}
});

then the other button

 m_add.addActionListener( new ActionListener() {
                    public void actionPerformed(ActionEvent e) {
 
                        try{
                        Cennik c= Cennik.getInstance();
                        Klient klient = new Klient((String)m_name.getText(),Double.valueOf(m_money.getText().trim()).doubleValue()); 
 
 
 
 
 
 
 
//if(k.equals(klient)) {klient=k;}  // k is not seen here but that wd solve my problem  i wd only compare the two client objects 
 
//but i have to find a client in a list 
                       int index = klienci.indexOf(klient);        
 
                        if(index != -1) {
                          klient = klienci.get(index);
 
                        } 
                        else {
                          // 'klient' is not in the list 'klienci'
                        }
 
                      and the rest of the code
 

[oll3i]

BTW Happy Easter

[JosAH]

BTW Happy Easter

Happy Easter to you too and to everybody else reading this.

About your equals() method: it seems ok (as well as the rest of your code)
except for one thing: you're comparing two double values; two double values
are almost never equal to each other. Can you add some System.out.println()
methods in your equals() method showing exactly *what* is compared to *what*.
Also print out what the equals() method wants to return. To be sure remove those
doubles from your hashCode() method, i.e. return the hashCode() if those Strings
only.

I think you're almost there. ;-)

kind regards,

Jos

[oll3i]

but first i have the equals method but where in the code is place for that method if i already have a list and i can get the object with klient = klienci.get(index);

also two doubles that may be the reason why int index = klienci.indexOf(klient); returns -1

[JosAH]

but first i have the equals method but where in the code is place for that method if i already have a list and i can get the object with klient = klienci.get(index);

also two doubles that may be the reason why int index = klienci.indexOf(klient); returns -1

Good morning; you could (and should) put these System.out.println() statements
in your own equals() method, i.e. print the values of the two objects to be
compared as well as the boolean that is to be returned to the caller. The List
is the caller of course and you can't change anything in the List code itself.

kind regards,

Jos

[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 nullpointerexception :( and eclipse doesnt say the line

[JosAH]

now it finds a client in a list

...but i get nullpointerexception :( 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 nullointerexception is somewhere else but after i click a button