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I have some doubts about a constructor.

There is a class "Foo" that can be created in three different "status" we could call them "YOUNG", "ADULT" and "OLD".

I want to underline that they cannot be seen as different classes because the object will evolve and if it has been created as "YOUNG" it will become "ADULT" and then "OLD" and so on..

My question is: how can I define one or more constructors to diversify these three typologies??

I see some possibilities, but no one is an "elegant" solution..

1) Create a constructor with an int as input

public Foo(int i) { switch (i) { case 0: . . . case 1: . . . case 2: . . . } }

but i do not like it because it is not so clear to understand if another person see this code.

2) Create a blank constructor and then create three different methods like 

public Foo() { } public void setYoungFoo() { . . . } public void setAdultFoo() { . . . } public void setOldFoo() { . . . }

This could be a clear way to resolve the problem, but i would resolve this problem in the constructor..

3) Could static variables usefull in this context?

public static final String "YOUNG"; public static final String "ADULT"; public static final String "OLD"; public Foo(String field) { }

I do not know how fill this constructor because I have never used static final variables (I have seen them used in some Java classes like Calendar even if not used in the contructor).

Could you please comment these three option to underline what are their disadvantages because I am quite sure that they can not be a good solution..

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  • A constructor is not a method and vice versa. Commented Jan 25, 2013 at 22:19
  • constructors should be self contained. they can call methods outside the class but calling class methods in a constructor does not make sense (since the class doesn't yet exist until return from the constructor) Commented Jan 25, 2013 at 22:22
  • this is completely wrong code also, you confuse methods and constructors therefor giving me the impression you dont understand the concept of classes and objects. You should start with java basics and object oriented programming. Commented Jan 25, 2013 at 22:25
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    @ KevinDTimm I have no used methods in the constructor, in none of the three possibilities. Commented Jan 25, 2013 at 22:27
  • @fork ok I edit my question to correct it. Commented Jan 25, 2013 at 22:28

2 Answers 2

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Use method 1, but with an enum instead of an int. You can even inline the enum in the class, just make it public static.

class Foo { public static enum fooState { young, adult, old; } public Foo(fooState i) { switch(i) { case young: ... break; case adult: ... break; case old: ... break; default: throw new InvalidStateException(); } ... }

Edit: You asked for opinions on your 3 options, here are mine:

  1. Option 1 technically does the same as my version, but uses magic numbers (i.e. numbers without assigned name), which makes it harder to understand (both for other people and for your future self)
  2. Option 2 leaves the class in an undefined or even invalid state between the Object creation and the call to set*Foo(). If you can make the transition to Young/Adult/Old at any time, it would be an option to set the Object to e.g. Young state in the constructor and let the user use the already necessary functions to switch to another state. Depends on the situation if this is a good option, but it is a valid one.
  3. Option 3 is basically the same as Option 1, just slower because it involves string comparison. The code is easier to read if you call "new Foo(Foo.YOUNG)", but it is not intuitive that you have to write it that way. Also, it would be possible to write "new Foo("Alligator")", which would in the best case create a runtime error.
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5 Comments

Agreed. Best option of the three (though using an enum is similar to option 3).
@Aurand True, but it is much less error prone than option 3 and also faster (integer comparison against string comparison).
@Jennis Froese excellent solution. Only one clarification: could creating the enum class out of this class be usefull? (not now but in the future this enum could be used in other classes) Or creating it as an innner enum has some advantages?
if it's used nowhere else, inner public static enums just reduce the number of files. Also if somebody else reads the code of your class, he will likely want to see the code for that enum, so why not put it right there. But apart from that there are no advantages, and you could consider it a matter of preferred style.
@JannisFroese Thank you very much! With last edit you have gained the accepted answer! :)
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Shouldnt you actually have three classes for Young, Adult and Old and all of them can implement a common interface or extend an abstract class to tie them together?

Make a static factory method in the parent abstract class which created the object of the desired childclass and gives it back? Why dont you want to make three classes? They will be referenced from a reference type of the parent class so they will "seem" like the same class.

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I do not want to create three classes (even if all child of the same parent class) because they are only three status of one class. You could imagine this class like a class Man and "YOUNG", "ADULT" and "OLD" status referring to his age. If I create a "YOUNG" man I have to set some charateristic in a certain way, but the class will evolve in time and it will become "ADULT" and then "OLD", so I think that create three separate words is conceptual wrong.
You could use composition. Have a containing class for a person, then have either Young Adult and Old inside that. the person class is responsible for checking the class inside it to see when it should swap out, and each of the young, adult and old provide methods to help with that.

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