An example of heterogeneous collections using the Visitor pattern

gistfile1.java

public class App
{
    public static void main(String[] args)
    {
        // The main collection
        Collection<Visitable> visitables = new ArrayList<Visitable>();
        
        visitables.add(new VisitableInteger(5));
        visitables.add(new VisitableString("Kevin"));
        visitables.add(new VisitableInteger(17));
        visitables.add(new VisitableString("Rocks"));
        
        // Create a new collection to add to the main collection
        Collection<Visitable> c = new ArrayList<Visitable>();
        
        c.add(new VisitableInteger(15));
        c.add(new VisitableString("Inner collection of death"));
        
        visitables.add(new VisitableCollection(c));
        
        // Iterate over the main collection and "act" on the data (remember that an element can "accept" a visitor, and the visitor will act on the value)
        for (Visitable v : visitables)
        {
            v.accept(new DoSomethingVisitor());
            v.accept(new LazyVisitor());
        }
    }
}

public interface Visitable
{
    public void accept(Visitor v);
}

public interface Visitor
{
    public void visit(VisitableInteger e);
    public void visit(VisitableString e);
    public void visit(VisitableCollection e);
}

public class VisitableCollection implements Visitable
{
    private Collection<Visitable> c;
    
    public VisitableCollection(Collection<Visitable> c)
    {
        this.c = c;
    }
    
    public Collection<Visitable> getValue()
    {
        return c;
    }
    
    public void accept(Visitor v)
    {
        v.visit(this);
    }   
}

public class VisitableInteger implements Visitable
{
    private int i;

    public VisitableInteger(int i)
    {
        this.i = i;
    }
    
    public int getValue()
    {
        return i;
    }

    public void accept(Visitor v)
    {
        v.visit(this);
    }
}

public class VisitableString implements Visitable
{
    private String s;

    public VisitableString(String s)
    {
        this.s = s;
    }
    
    public String getValue()
    {
        return s;
    }

    public void accept(Visitor v)
    {
        v.visit(this);
    }
}

public class DoSomethingVisitor implements Visitor
{
    public void visit(VisitableInteger e)
    {
        System.out.println("I'm going to do all sorts of things with " + e.getValue() + " like double it (" + (e.getValue() * 2) + ")");
    }

    public void visit(VisitableString e)
    {
        System.out.println("Strings are pretty lame so I might as well just print the contents..." + e.getValue());
    }
    
    public void visit(VisitableCollection e)
    {
        System.out.println("What the fuck? Now I have a collection of a collection!");
        
        for (Visitable v : e.getValue())
        {            
            v.accept(this);
        }
    }
}

public class LazyVisitor implements Visitor
{
    public void visit(VisitableInteger e)
    {
        System.out.println("I'm ignoring your int");
    }

    public void visit(VisitableString e)
    {
        System.out.println("I'm ignoring your String");
    }
    
    public void visit(VisitableCollection e)
    {
        System.out.println("Collections are kinda interesting, let's traverse it...");
        
        for (Visitable v : e.getValue())
        {            
            v.accept(this);
        }
    }
}