By making Structure an interface, we get the following benefits:

• Mobile and Weight can be grouped together as Structures
• Any objects that are Structures will know about the methods in the Structure interface, which makes coding easier--especially when there is pattern matching in an editor like Eclipse!

The technical term for this is polymorphism, which means that a given object can be thought of in multiple ways. In this case a mobile can be thought of as a Mobile, a Structure, or an Object, depending on the need of the programmer. When it comes to writing something like a getWeight() method, thinking of it as a Structure turns out to be really helpful.

public interface Structure {
	public double getWeight();
	public boolean isBalanced();
}