Interfaces

• An interface can only contain abstract methods and static final data.
• Best used for "has a" relationships, as opposed to classes (inheritance), which are best used for "is a" relationships.
• Interface methods are abstract by default (unimplemented).
• Interface data is static and final by default.
• If an interface is public, it must be in it's own .java file.
  

Implementing

class SubClass extends SuperClass implements Interface1, Interface2 {
    ...
}

Extending

interface SubInterface extends SuperInterface {
    ...
}