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Trying to create OO based bank app

 
Tushar Goel
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That means if user enters 06-01-1986 and if return back is 6-1-1986 then it is fine.... Oh great ....

Now if you think everything is fine then let us move towards Customer class....


Customer Class:

state: customerName, dateOfBirth, customerId, customerIdGenerator, CUSTOMER_ID_START_FROM
behaviour: getCustomerName(), setCustomerName(), getDateOfBirtth(), getCustomerId(), equals() and hashcode()






 
Tushar Goel
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I also thought that in Customer class , customerId should be saved some where so that if program is terminated for some reason then it should assign customer id greater than the already
assigned. It should not start again from the initial starting level.

Due to this i am as of now storing maximum customerid used into the file when program terminates. To do the same i again defined another utility which is reading and writing into the file.


 
Tushar Goel
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Modified Customer class:

Customer Class:

state: customerName, dateOfBirth, customerId, customerIdGenerator, CUSTOMER_ID_START_FROM
behaviour: getCustomerName(), setCustomerName(), getDateOfBirtth(), getCustomerId(), equals() and hashcode(), initializeCustomerId()




 
Tushar Goel
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Campbell, just wondering if you got time to look at my last 2 posts?
 
Tim Cooke
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Tushar, please EaseUp (<- click link and read). You are getting some incredible help here from Campbell but please do remember that we are all volunteers and have other demands on our time, like actual paying jobs, family, hobbies, whatever. Remember PatienceIsAVirtue so sit tight, I'm sure you'll get a response in good time.
 
Tushar Goel
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Sorry Tim.. I completely understand your views and having completely respect to Campbell.. He already supported me very much and cannot explain thanks to him in words..

 
Campbell Ritchie
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You have a dreadfully incorrect version of equals().
 
Tushar Goel
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Thanks Campbell, i dont know what i thought at that time to create such method.. I just did some unit testing to see what error will i get if i use incorrect one and things is clear to me now..

Corrected one is as below:

 
Paweł Baczyński
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Tushar Goel wrote:Corrected one is as below:

This is almost correct.
There is one problem. You attempt to cast obj to Customer before you check that you can do this.
 
Tushar Goel
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Thanks Pawel, I have corrected that one too now..

 
Campbell Ritchie
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That makes the hash code method incorrect. What will happen if you have this?
Customer:id=1234567890 name=Campbell Ritchie dob=12/3/45
Customer:id=1234567890 name=Tushar Goel dob=23/4/56
 
Tushar Goel
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But i considering unique id.. No 2 customer can have same id...

I used following implementation to achieve the same:-



Is it wrong?? Also once application is down the maximum id used is being written in the file so that whenever it is up and id grater than already should be assigned:



Bank Class: To save already used customer id to the file




DataReadWrite Utility: Writing data into the file



 
Campbell Ritchie
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Tushar Goel wrote:But i considering unique id.. No 2 customer can have same id...
. . .
We are trying to make you think about it.

If the ID uniquely identifies the Customer, why not use only the ID in the hash code method?
 
Tushar Goel
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Understood Sir.. That's means it will help to provide unique value

 
Paul Clapham
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So it's possible to have two customers with the same ID but different names?
 
Tushar Goel
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It is not possible to have same ID for two customers but by adding extra checks i think it is making doubly sure to have unique object only.
 
Campbell Ritchie
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I would suggest you confirm that you cannot get duplicate IDs, then you only need to use ID in equals and hash code.

You should always use the same fields in hash code and equals.
 
Tushar Goel
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confirm that you cannot get duplicate IDs

I tried to do some testing and it always give me unique case.. I am unable to think about the cases in which customerId is duplicated(except the case when application is shutdown and restarted again or the case of
multithreaded.). Also while testing i realized that my current hashCode implementation is wrong as when i passed null to the fields it throws NullPointerException.. I need to correct that one too...




You should always use the same fields in hash code and equals.

Thanks for it. Earlier was not sure about it...
 
Paul Clapham
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Tushar Goel wrote:It is not possible to have same ID for two customers


Yes, right, that's what I thought.

... but by adding extra checks i think it is making doubly sure to have unique object only.


In my opinion the equals() method is not the place to be making those "extra checks". If they aren't done correctly in the place where they should be done, then making your equals() method incompatible with your hashCode() method is just going to lead to more confusion.
 
Tushar Goel
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If they aren't done correctly in the place where they should be done, then making your equals() method incompatible.


I tried to make them unique. I tested several cases on it to check and every time i am getting unique value. What other cases i need to test? Because if my implementation is
right then as Campbell suggested i need not to use anything else except customerID in equals() and hasCode().

I have added few more test cases to the existing one:


 
Campbell Ritchie
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Tushar Goel wrote: . . . I am unable to think about the cases in which customerId is duplicated(except the case when application is shutdown and restarted again or the case of
multithreaded.). . . .
In which case you need to handle those conditions. I can think of three ways to handle the shutdown problem but one involves a database and we decided earlier not to use databases in this instance. Consider a shutdown hook on the runtime which does one of these things
  • 1: Persist the state in the database.
  • 2: Persist state in an XML or similar file.
  • 3: Persist the state by serialising the bank to a .ser file.
  • Remember shutdown hooks only hae a short time to operate in. If you were using Swing, you could add a Listener to listen for the app closing.
    If you are using AtomicInteger, doesn't that obviate any problems caused by threading? Have you checked?

    It is a valid decision at this stage to decide to delay persistence until later in the proceedings, as long as you do it eventually.
     
    Campbell Ritchie
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    Tushar Goel wrote: . . . my current hashCode implementation is wrong as when i passed null to the fields it throws NullPointerException.. I need to correct that one too...
    . . .
    And the way to correct it is to make sure a null never gets that far.
     
    Campbell Ritchie
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    Tushar Goel wrote: . . . Because if my implementation is right then as Campbell suggested i need not to use anything else except customerID in equals() and hasCode().
    . . .
    You need to go through the constructor and tease it apart with pencil and paper and confirm whether there are any circumstances in which you can get a duplicate ID. I presume that is what you did earlier when you said shut-down or threading. In which case check what you did to confirm whether your conclusion is correct.
    What if you have two Bank instances in different threads? Can you get duplicate IDs in those instances?
     
    Tushar Goel
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    Consider a shutdown hook on the runtime which does one of these things


    Thanks Campbell.. Let me read about shutdown hooks and get back to you. Also i think in shutdown hook i will save state into the file.

    If you are using AtomicInteger, doesn't that obviate any problems caused by threading? Have you checked?


    Yeah i did.. AtomicInteger is thread safe and it helps to maintain a unique customer id.

    you did earlier when you said shut-down or threading. In which case check what you did to confirm whether your conclusion is correct.


    As of now when the application is shutting down i am saving the customerId into the file and whenever services is up then i reading entry from the file so
    this way i am making sure that customerId should not again start from initial value and new customer will get oldValue +1.

    What if you have two Bank instances in different threads? Can you get duplicate IDs in those instances?

    i am using Atomic Integer which is thread safe so i think this will handle the case as well and no duplicate. Though i have not tested in multi threaded environment.


     
    Tushar Goel
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    Consider a shutdown hook on the runtime which does one of these things

    I have added shutdown Hook up in my BankStarter Class. As of now i am just storing customerId into the file. Once i will implements other i will
    add them to. I defined a inner class in my Bank class for the same.



     
    Campbell Ritchie
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    Looks good
    Remember you are adding the shutdown hook to the Runtime, not a particular class. Also remember the Runnable must not do anything taking a long time because the lifetime of the JVM after the shutdown hook is activated is very short.

    Now let's see you write that in Java8 notation without defining a Runnable class!
     
    Tushar Goel
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    Now let's see you write that in Java8 notation without defining a Runnable class!

    Do you mean using Lambda expression? I do not have any idea but let me read about it.
     
    Campbell Ritchie
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    Search my posts for the last week and look out for a spactecular misspelling.
     
    Campbell Ritchie
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    Try here for the wrong spelling.
     
    Tushar Goel
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    Try here for the wrong spelling.


    SingUtilities or SwingUtilities... Syntax are quite different for JRE-8, i need to read whole story about expressions... Pardon me for later replying as i was very busy from last 2 -3 days due
    to family issue.. I will finish my work by today evening and start again concentrate on it....
     
    Tushar Goel
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    Now let's see you write that in Java8 notation without defining a Runnable class!

    Finally, i am able to get it. It looks better ad short. Is it what you referring to?




    Also i have removed local inner class from the Bank and instead defined a static method to store data.
     
    Campbell Ritchie
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    Agree it seems better without the inner class. But why is the store data method static?
     
    Tushar Goel
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    Because i think it is independent to the instances though i agree there will be only one instance of the Bank class. So i thought both are equal but i think you are pointing
    to possible risk to access the method directly from the Class?
     
    Campbell Ritchie
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    … and where will the data come from, if not from the instance?
     
    Tushar Goel
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    got your point. Thanks for clearing it.

     
    Campbell Ritchie
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    You're welcome
     
    Tushar Goel
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    So my customer class is now looks like this. Is this correct now?

     
    Campbell Ritchie
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    Looks all right.

    But…
    Is LocalDate immutable?
    Why are you using a method rather than the field in equals? Why are you using else after a return? I prefer only to use return once per method so I would reduce that method to
    return other == this || other != null && other.getClass().equals(this.getClass()) && ((Customer)other).id == id;
    You do have to keep the terms in that order and use || && not | & to avoid a null exception.
    Do you really want to reset names with set methods? What validation of names are you doing in those set methods?

    Search these fora to find out about the controversy about whether you should or shouldn't use getClass in an equals method.
     
    Tushar Goel
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    Is LocalDate immutable?

    Yes it is..

    Do you really want to reset names with set methods?

    No. Removed now.

    whether you should or shouldn't use getClass in an equals method.

    Yes we should use getClass instead of instanceOf because instanceOf will not return false in case if argument is subclass of the original class but getClass will
    return false. I refer this link http://www.javaranch.com/journal/2002/10/equalhash.html

    What validation of names are you doing in those set methods?

    No validation in set method but i think validation in terms of only alphabets and single whitespace allowed in name should be done in Bank class which accepting string from the utility class.



    Why are you using a method rather than the field in equals? Why are you using else after a return?

    for better readability. Thanks for the suggestion, i almost forgot this way to get fields from the object. I have update the code as suggested as well.


     
    Campbell Ritchie
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    Tushar Goel wrote:
    Is LocalDate immutable?

    Yes it is.. . . .
    You may have checked that before, but I couldn't remember, so I asked it again.

    Coming on nicely

    It is still worth reading the controversy about the equals method. It means that once you have an overridden equals method you should not add any fields in subclasses.
     
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