Throw Exception in constructor of a subclass

Question

so my issue is that I have a class and its subclass.

-> class Kassenbon -> class KassenbonVerbessert extends Kassenbon

in my "Kassenbon" class I have this constructor:

   public Kassenbon(int max) {
        produkte = new String[max];
        preise = new Integer[max];
    }

and then the constructor of my subclass looks like this:

   public KassenbonVerbessert(int max) {
        super(max);
    }

My issue now is that I want to check the parameter max and if it is negative then I wanna throw an exception because that would mean that the constructor would create 2 arrays with a negative length which isnt possible. But how do I do it simply because the super call must be the first statement in the constructor right?: But how do I implement a:

if(max < 0){
   throw new IllegalArgumentException("Error");
}

Answer 1

First, it looks like that check would make more sense in the Kassenbon constructor. But if you don't want to do that, you can inline a method call to check the parameter:

public KassenbonVerbessert(int max) {
    super(checkMax(max));
}

private static int checkMax(int max) {
    if (max < 0) {
        throw new IllegalArgumentException("max cannot be negative.");
    }
    return max;
} 

Answer 2

There's no compulsion that you need to put the check in the child class. You can directly put the check inside the constructor of your parent class.

Answer 3

When you are unable to apply the check to the parent constructor, you have to inline the check, as suggested here and here . But you don't have to write your own method for that, you can take java.util.Objects.checkIndex() for that purpose:

public KassenbonVerbessert( int max ) 
{
    super( java.util.Objects.checkIndex( max, Integer.MAX_VALUE ) );
}

The second argument is the maximum value for max , just if you want to provide an upper border as well …

And of course you can add a static import for that method: import static java.util.Objects.checkIndex; .

Answer 4

In addition to what @khelwood said, you could also force creation of a class using factory methods. Usually when creation of an object gets even more complex - especially in regards to checks and Exceptions - you start using those:

package stackoverflow.kassensystem;

public class Kassenbon {

    private final String[]  produkte;
    private final Integer[] preise;

    public Kassenbon(final int max) {
        produkte = new String[max];
        preise = new Integer[max];
    }

}

and

package stackoverflow.kassensystem;

public class KassenbonVerbessert extends Kassenbon {

    static public KassenbonVerbessert createInstance(final int max) {
        if (max < 0) throw new IllegalArgumentException("Parameter 'max' is out of valid bounds! Expected max>0, but got '" + max + "' instead.");
        return new KassenbonVerbessert(max);
    }

    private KassenbonVerbessert(final int max) {
        super(max);
    }

}

How to use:

package stackoverflow.kassensystem;

public class KassenSystem {

    public static void main(final String[] args) {
        System.out.println("KassenSystem.main() startet...");
        // KassenbonVerbessert kb1 = new KassenbonVerbessert(20); this line cannot work, because KassenbonVerbessert CTOR is private. Must instanciate via static method
        final KassenbonVerbessert kb2 = KassenbonVerbessert.createInstance(20); // this works alright
        final KassenbonVerbessert kb3 = KassenbonVerbessert.createInstance(-666); // this will fail
    }

}