What Are Interfaces?

A Java interface is a set of constants and method declarations without an implementation, as shown in Listing 9.1.

Code Listing 9.1. Steerable.java—An Interface for Steerable Vehicles
public interface Steerable {

  String NORTH_DIRECTION = "North";
  String SOUTH_DIRECTION = "South";
  String EAST_DIRECTION  = "East";
  String WEST_DIRECTION  = "West";

  void turnLeft();
  void turnRight();
  String getCurrentDirection();
}

This concept might not sound too impressive at this point, but interfaces are a powerful feature of the Java language. Successful object-oriented programming depends on separating the functionality needed by one part of your system from how that functionality is ultimately implemented by another part. Interfaces are ...

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