#Object Identifier as a variable

1 messages · Page 1 of 1 (latest)

cold sigil
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Hello, i want to create a method to acces a property of an object but without saying which property i want from the beginning. The property is the transfer parameter a. (a is not a property of my object)

I have tried everything i know but i just started java so thanks for helping :/

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candid novaBOT
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<@&987246399047479336> please have a look, thanks.

candid novaBOT
#

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uneven pumice
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Have you ever heard of reflection?

cold sigil
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nah but i will look it up, if it leads to the solution :)

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i hope its not too hard xD

uneven pumice
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It's a pretty advanced Java feature.

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But that's what reflection was made for.

cold sigil
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well thanks a lot, now i know at least, with what tools to work

uneven pumice
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Or perhaps, if you share some more of your code I can suggest an alternative approach. Reflection should only be used where it's needed, not where it can be used.

cold sigil
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okay i gotta explain a little bit

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so this is my object with these properties

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which i end up saving here

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uneven pumice
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Okay, so why do you need to access a property with an arbitrary name instead of creating getters for each field?

cold sigil
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now i display this in a table and i want to sort these for each property

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i wanted to create a method that sorts all the strings, as the name, last name, etc and a method to sort all integer

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uneven pumice
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You should definitely not use reflection here.

radiant dragon
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so you want to sort the items by arbitrary fields or ?

cold sigil
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and as the sort criterium should be my String a == the selected property

radiant dragon
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how do you get this string a ?

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do you decide at some point to sort in a way

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or is it totally arbitrary

cold sigil
radiant dragon
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ah so it is arbitrary

cold sigil
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and then press a button or so

radiant dragon
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wait no

radiant dragon
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so you only want to sort by name ?

cold sigil
radiant dragon
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ah no

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it's the fields

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right

cold sigil
radiant dragon
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either you do a case for each

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it's a single line by field

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or you use reflection

cold sigil
radiant dragon
radiant dragon
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sort is a thing that already exist

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customers.sort(Comparator.comparing(Customer::getName))

Assuming customers is a List<Customer> and Customer has a method getName, this will sort the list by name

cold sigil
radiant dragon
cold sigil
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so with String and int?

radiant dragon
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As long as the type implements Comparable, it's fine

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and for int or other primitive, you would use for example

customers.sort(Comparator.comparingInt(Customer::getAge))
cold sigil
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ok some last questions

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it is right that its :getAge not :getAge() ? i thought methods always with brackets

radiant dragon
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that's called a method reference

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do you know interfaces and inheritance ?

cold sigil
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ok sorry for my bad skills :/

cold sigil
radiant dragon
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right

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so this is a bit advanced

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but basically

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sort method wants a Compaartor

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so you just need to give it a comparator

cold sigil
radiant dragon
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a comparator is an object that has a single method, which takes two objects and returns a negative, 0 or positive int depending if the first is smaller, equal or bigger than the second

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so you could make your own comparator

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but there is also a neat method which make it a bit easier : Comparator.comparing

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this method takes a method reference and will use it as a key to create the comparator

cold sigil
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ooookokok didnt understand every word however the sense

radiant dragon
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for example, if I use the method reference getName, then it will use getName on the first and second object in the comparator to find which one is greater

cold sigil
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gonna try to implement it later, thanks a lot :)

radiant dragon
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class CustomerNameComparator implements Comparator<Customer> {
  @Override
  public int compare(Customer a, Customer b){
    return a.getName().compareTo(b.getName());
  }
}
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this is a comparator for example

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and it will compare the name of two customers

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so you could do

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customers.sort(new CustomerNameComparator());
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this will work

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do you understand this code @cold sigil ?

cold sigil
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yes actually i does

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and with that i could build comparator for every property i want

cold sigil
radiant dragon
cold sigil
radiant dragon
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so you may notice that in this comparator, if you want to change it with for example lastName, or whatever, you only need to change getName method right ?

cold sigil
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yesyesyes i understand omg

radiant dragon
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class CustomerLastNameComparator implements Comparator<Customer> {
  @Override
  public int compare(Customer a, Customer b){
    return a.getLastName().compareTo(b.getLastName());
  }
}
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so what Comparator.comparing does

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is that it takes a reference to the method you want to use

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and build this whole class for you

cold sigil
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thanks for your help, i gotta leave, but answere later <3

radiant dragon
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(I am oversimplificating here but it's basically that)

cold sigil
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still thank you, i'm gonna try it out

cold sigil
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@radiant dragon thank you so much, it works!!!!!! :)