This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
An interface is essentially a collection of constants and abstract methods. To make use of an interface, you implement the interface in a class - that is, you declare that the class implements the interface and you write the code for each of the methods declared in the interface as part of the class definition. Check Learning the Java Language What is an Interface? Implementing an Interface