Java Generic Classes, how to define constructor for possible Parameter types

Question

I am trying to define a Java class which could be used for generic types, but I want to define the constructors for possible parameter types like Character, or an object type I can manually define.

The class I am trying to create is called:

public class Repeated<T> implements Pict {

private T[][] elements;

private double scale;

public Repeated(T[][] elems) {
    elements = elems;
}
}

So I want to add a constructor that adds a Character Array and does something specific if only T is the Type Character. Is it possible? Or should I make this an Interface, then implement it later?

It's not possible with constructors, but it may well be possible if you use static factory methods instead! — Louis Wasserman
– Louis Wasserman, Commented Nov 11, 2012 at 22:51
Due to type erasure, this isn't possible. Also, remember the KISS principle - if you really need generics, I'd look at what @Paul Bellora has described, otherwise leave it out. — wulfgarpro
– wulfgarpro, Commented Nov 12, 2012 at 10:47

Paul Bellora · Accepted Answer · 2012-11-11 23:01:20Z

2

So I want to add a constructor that adds a Character Array and does something specific if only T is the Type Character

Sounds like a good case for polymorphism:

public class RepeatedChars extends Repeated<Character> {

    public RepeatedChars(Character[][] chars) {
        super(chars);
        //do special stuff
    }

    //other character-specific logic if applicable
}

You could even hide this class as an implementation detail:

public class Repeated<T> implements Pict {

    ...

    private static class RepeatedChars extends Repeated<Character> {
        //see above
    }

    public static Repeated<Character> makeForChars(Character[][] chars) {
        return new RepeatedChars(chars);
    }
}

answered Nov 11, 2012 at 23:01

Paul Bellora

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1 Comment

Ted Hopp Over a year ago

If you're going to do this with factory methods, you don't need subclasses. Just define a (most likely private) constructor Repeated(T[][] elems, boolean isCharsFlag). The factory methods can then pass the appropriate flag value to the constructor.

Ted Hopp · Accepted Answer · 2012-11-12 16:42:24Z

2

You should be able to do this:

public Repeated(T[][] elems) {
    elements = elems;
    boolean isCharacterArrayArray = false;
    if (elems != null) {
        for (T[] elt : elems) {
            if (elt != null && elt.length > 0) {
                isCharacterArrayArray = elt[0] instanceof Character;
                break;
            }
        }
    }
    if (isCharacterArrayArray) {
        // received a Character[][]
    }
}

However, because of type erasure, I don't think you can distinguish these calls:

new Repeated((Character[][]) null);
new Repeated((Integer[][]) null);

without passing some sort of explicit class argument.

EDIT With a class argument, it all becomes much simpler:

public Repeated(T[][] elems, Class<T> cls) {
    elements = elems;
    if (cls.equals(Character.class)) {
        // passed a Character[][]
    }
}

edited Nov 12, 2012 at 16:42

answered Nov 11, 2012 at 22:48

Ted Hopp

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2 Comments

Paul Bellora Over a year ago

I'm not crazy about your solution, but you could just use Class.getComponentType() - that would account for empty arrays.

Ted Hopp Over a year ago

@PaulBellora - Excellent idea. It still wouldn't work for null arrays, though, since there's no object there to call getClass() on. I don't like my solution, either; passing a Class object fixes everything, though.

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