Lecture 16 - Interfaces Professor Adams

Interfaces are “Java’s substitute for C++’s feature of multiple inheritance… “ “…used when you want to define a certain functionality to be used in several classes, but are not sure exactly how this functionality will be defined by those classes…” Joe Weber and Mike Afergan in Using Java 2nd Edition

Placing classes in an interface outlines common behavior leaves specific implementation to the classes themselves Makes using interfaces instead of classes a better choice when dealing with advanced data handling. Joe Weber and Mike Afergan in Using Java 2nd Edition

Interfaces are underprivileged first cousins of classes define a set of methods and constants to be implemented by another object help define the behavior of an object by declaring a set of characteristics for the object cannot specify any implementation for methods Joe Weber and Mike Afergan in Using Java 2nd Edition

Interfaces A java interface is a collection of constants and abstract methods Remember: an abstract method is a method that does not have an implementation (a body) An interface can not be instantiated Although abstract methods can be preceded by the reserved word abstract, in interfaces they are generally not. Lewis & Loftus pp. 309-310

Implementation of an Interface “A class implements an interface by providing method implementations for each of the abstract methods defined in the interface.” “A class that implements an interface uses the reserved word implements followed by the class name in the class header.” If a class asserts that it implements a particular interface, it must provide a definition for all of the methods in the interface or a compile error will occur. Lewis & Loftus, p. 310

Classes that implement an interface are responsible for specifying the implementation of the methods in the interface must override every method in the interface An Example is on next slide and code is in directory.

Example and notes file is named Product.java public interface Product { static final String MAKER = "My Corp"; static final String PHONE = "555-123-4567"; public int getPrice (int id); } file is named Product.java getPrice is an abstract method – doesn’t require the word abstract and we will omit it. getPrice has no body (nor can it have one) When attributes (fields) are present they must be static and are final Attributes (need to be static because they cant be instantiated)

Shoe.java public class Shoe implements Product { public int getPrice (int id) { if (id == 1) return (5); else return (10); } public String getMaker() { return (MAKER); } }

Store.java public class Store { static Shoe hightop; public static void main (String arg[] ) { init(); getInfo (hightop); orderInfo(hightop); } public static void init () { hightop = new Shoe(); } public static void getInfo (Shoe item) { System.out.println (" This Product is main by " + item.MAKER); System.out.println (" It costs $" + item.getPrice(1) + '\n'); } public static void orderInfo (Product item) { System.out.println (" To order from " + item.MAKER + " call " + item.PHONE + "."); System.out.println (" Each item costs $" + item.getPrice(1)); } } // end class Store

When you create an interface None of your methods may have a body No non-constant variables may be declared Our interfaces will all be public Interface filename must match interface name

extending Interfaces can not extend classes Interfaces can only extend other interfaces If you implement an extended interface, you must override both the methods in the new interface and the methods in the old interface

Remember these Classes implement interfaces to inherit their properties Methods in classes must have bodies unless they are abstract methods (in which case they are in abstract classes) Syntax of a method declaration in an interface is public return_value nameOfMethod (parameterList) throws ExceptionList ;

Interfaces “An interface is similar to a class but there are several important differences: All methods in an interface are abstract; that is, they have name, parameters, and a return type but they don’t have an implementation. All methods in an interface are automatically public An interface doesn’t have instance variables.” Horstman, p. 365

Common Error with interfaces Forgetting to define implementing methods as public

Programs from text //*********************************************** Complexity.java – p. 310 * * @Author: Lewis/Loftus * The interface for an object that can be assigned an * explicit complexity. */ public interface Complexity { public void setComplexity (int complexity); public int getComplexity(); }

Interfaces A class can implement more than one interface. In these cases, the class must provide an implementation for all methods in all interfaces listed. To show that a class implements multiple interfaces, they are listed in the implements clause, separated by commas. Here is an example: Class ManyThings implements interface1, interface2,interface3 { // contains all methods of all interfaces }

Interfaces in the Java standard class library The Comparable interface is defined in java.lang and contains only one method compareTo which takes an object as a parameter and returns an integer. Documentation indicates that integer will be negative if obj1 is less than obj2; 0 if obj1 = = obj2; and positive if obj1 > obj2. Here’s an example of an invocation of the method. If (obj1.compareTo(obj2) < 0) System.out.println (“ obj1 is less than obj2”); The String class implements the Comparable interface and has a compareTo method. Lewis & Loftus, pp. 315-316

Interfaces in the Java standard class library The Iterator interface is another interface defined as part of the Java standard class library. It is used by a class that represents a collection of objects, providing a means to move through the collection one at a time. The two primary methods in the Iterator interface are: hasNext – which returns a boolean result next – which returns an object Lewis & Loftus, pp. 315-316