Implementing the Decorator pattern using Groovy's @Delegate

Let's say we want to write a decorator around a List to add a creation time to the list. We've all written something like this before, and it usually looks something like:

public class ListDecorator implements List {

   private final List delegate;
   private final Date creationTime = new Date();

   public ListDecorator(List delegate) {
      this.delegate = delegate;
   }

   public Date getCreationTime() { return createTime; }

   public void add(...) {
       delegate.add(...);
   }

   public int indexOf(...) {
      return delegate.indexOf(...);
   }

   // wrap all of the list methods
}

This is very tedious, and we still need to write tests to make sure we actually delegated to the right method.

Enter the groovy @Decorator annotation

// notice we don't need to implement the List interface here
// since we can take advantage of duck typing
class GroovyListDecorator
{
    // all of the calls to the list methods will be delegated to this list
    @Delegate final List delegate
   
    private final Date creationTime = new Date()
   
    GroovyListDecorator(List delegate) {
        this.delegate = delegate;
    }
   
    Date getCreationTime() {
        creationTime
    }
}

And now we can use it like this:
def list = new GroovyListDecorator(new LinkedList())
// these methods delegate to the LinkedList delegate
list.add("abc")
list.add("def")
println list.size() // 2     
println list.creationTime // Mon May 02 19:49:55 EDT 2011

And it's as easy as that.