Java Interfaces

Java has a construct called Interface which is closely related to abstract classes.

You can think of an interface as another level of abstraction that is even more abstract than an abstract class!

Consider the following modified abstract Roster class where the fields and the constructors are removed.

public abstract class Roster {

  public abstract void add(Student s);

  public abstract void remove(Student s);

  public abstract Student find(String email);
}

The above can be rewritten as follows, which is arguably a more compact and cleaner representation of the Roster ADT.

public interface Roster {

  void add(Student s);

  void remove(Student s);

  Student find(String email);
}

Notice the syntax of Java interface:

• We declare it with the keyword interface (instead of class or abstract class)

• It does not contain fields nor constructors.

• Since all methods are public and abstract, there is no need to include those keywords in their declaration.

Java interfaces, like abstract classes, can be used to declare objects but not to instantiate them.