CODEWITHSUNDEEP
javaobjectconstructorarchitecture
BerndGit
BerndGit

Reputation: 1660

java constructor to make already correct subclasses

I have a class hirachy like: 

class Main{
    Main(Object input){ /*   */}
}

class SubA extends Main{ 
   SubA(Object input){  
     super(input);
    // ...
    }
}

class SubB extends Main{ 
   SubB(Object input){
       super(input);
       // ...
    }

}

What I'm trying to impement is, that the constructor of Mainalready constructs a subclass depending in the inputparameters. Something like:

// Non working example
class Main{
    Main(Object input){

    if (input.fullfillsSomeCond()) { return new SubA(input); }
    if (input.fullfillsSomeOtherCond()) { return new SubB(input); }

    }
}

This is obvoiously not working, since I'll generate infinite loops due to recursion. Is there a better architecture witch allows that Main somthing = new Main(myInput); already constructs the correct subclass?

Upvotes: 0

Views: 58

Answers (1)

Fildor
Fildor

Reputation: 16084

It is impossible to do this utilizing the Constructor, but you could use a Factory Method:

class Main{
    Main(Object input){}

    public static Main create( Object input ) {
        if (input.fullfillsSomeCond()) { return new SubA(input); }
        if (input.fullfillsSomeOtherCond()) { return new SubB(input); }
        // You might want to handle the case where input does not 
        // meet any of the above criteria.
        throw new IllegalArgumentException("input must be either A or B!");
    }
}

Usage:

// Instead of new Main( input ):
Main oMain = Main.create(myInput);

Along with that you may want to make Main abstract and its CTOR protected.

Drawback here is that Main has to "know" about its subclasses and the conditions. But that were the case, too if it could be done through ctor.

Upvotes: 3

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