Got Object?

I recently coded a fairly tiny application that made use of the MVC pattern. One of the things that I noticed while I was writing it was that I was down-casting a lot. Imagine something like this:

Example

// main type
interface Model {
	void someMethod();
}

// implementation
class Mixer implements Model {
    public void anotherMethod(){
    	System.out.println("hey");
	}
    public void someMethod() {
    	System.out.println("dude");
	}
}

Now If I want to invoke both methods, I’d have to do something like this:

    final Model m = new Mixer();
    m.someMethod();
    // downcasting .
    // imagine this with more model objects
    // with unique methods...
    ((Mixer)m).anotherMethod();

I am really annoyed about this whole downcasting. It does not make my code look that pretty. So I decided to use Generics; more in specific the recursive bounds idiom which was used in the implementation of the Java 5.0′s Enums.

The solution

interface SuperClass<S extends SuperClass<S>> {
    S subclassInstance();
}

interface AnotherType {
    void print();
}

The next step is to implement the previous interfaces. Something like this:

/**
 * subclass A
 */
static
class SubClassA implements
        SuperClass<SubClassA>,
        AnotherType
{
    private final String name;
    SubClassA(){
        final Random random = new Random(19580427);
        name = getClass().getName()
                + random.nextInt();
    }
    public SubClassA subclassInstance(){
    	return this;
	}
    public void print(){
    	System.out.println(name);
	}
}

/**
 * subclass B
 */
static class SubClassB implements SuperClass<SubClassB>,
        AnotherType
{
    private final String name;
    SubClassB(){
        final Random random = new Random(19580427);
        name = getClass().getName()
                + random.nextInt();
    }
    public SubClassB subclassInstance(){
    	return this;
	}
    public void print(){
    	System.out.println(name);
	}
}

Then I can write code like this one and totally avoid dowcasting between objects:

public static void main(String... args){
 final SubClassA      		 sca  = new SubClassA();
 final SuperClass<SubClassA> sc   = sca;
 final SubClassA      sca2    = sca.subclassInstance();
 final SubClassA      sca3    = sc.subclassInstance();

 // no casting..cool huh!
 sca2.print();
 sca3.print();
}

And there you go… I’ve totally solved my downcasting problem!