Interfaces and Packages

1
Lesson 4

• Interfaces and Packages

2
Interfaces

• Definition An interface is a named collection of
  method definitions (without implementations). An
  interface can also declare constants.
• An interface defines a protocol of behavior that
  can be implemented by any class anywhere in the
  class hierarchy.
• An interface defines a set of methods but does
  not implement them.
• A class that implements the interface agrees to
  implement all the methods defined in the
  interface, thereby agreeing to certain behavior.

3
Interfaces vs. Abstract Classes

• An interface cannot implement any methods,
  whereas an abstract class can.
• A class can implement many interfaces but can
  have only one superclass.
• An interface is not part of the class hierarchy.
  Unrelated classes can implement the same
  interface.

4
Defining an Interface
5
Implementing an Interface

• When a class implements an interface, it agrees
  to define all methods of that interface. A class
  can implement many different interfaces.
• public class StockApplet extends Applet
  implements StockWatcher
• ...
• public void valueChanged(String tickerSymbol,
  double newValue)
• if (tickerSymbol.equals(sunTicker)) ...
• else if (tickerSymbol.equals(oracleTicker))
  ...
• else if (tickerSymbol.equals(ciscoTicker))
  ...

6
Creating and Using Packages

• A package is a collection of related classes and
  interfaces providing access protection and
  namespace management.

7
Creating and Using Packages

• //in the Graphic.java file
• public abstract class Graphic
• . . .
• //in the Circle.java file
• public class Circle extends Graphic implements
  Draggable
• . . .
• //in the Rectangle.java file
• public class Rectangle extends Graphic implements
  Draggable
• . . .
• //in the Draggable.java file
• public interface Draggable
• . . .

8
Creating a Package

• package graphics
• public class Circle extends Graphic implements
  Draggable
• . . .
• package graphics
• public class Rectangle extends Graphic implements
  Draggable
• . . .

9
Naming a Package

• graphics.Rectangle
• java.awt.Rectangle
• Convention use reversed Internet domain name for
  package names
• E.g., yeditepe.cse.cse252.graphics

10
Using Package Members

• Referring to a Package Member by Name
• graphics.Rectangle myRect new
  graphics.Rectangle()
• Importing a Package Member
• import graphics.Circle
• Circle myCircle new Circle()
• Importing an Entire Package
• import graphics.
• Circle myCircle new Circle()
• Rectangle myRectangle new Rectangle()

11
Managing Source and Class Files
class namegraphics.Rectangle pathname to
filegraphics/Rectangle.java
12
Managing Source and Class Files
13
Managing Source and Class Files
14
Class Path

• A class path is an ordered list of directories or
  ZIP files in which to search for class files.
• By default, the compiler and the interpreter
  search the current directory and the ZIP file
  containing the Java platform class files.

15
Vector Class

• The Vector class implements a growable array of
  objects.
• Like an array, it contains components that can be
  accessed using an integer index.
• However, the size of a Vector can grow or shrink
  as needed to accommodate adding and removing
  items after the Vector has been created.

16
Vectors

• Creation
• Vector myVector new Vector () // or
• Vector myVector new Vector(50)
• Insertion
• for (int i 1 i lt 20 i)
• myVector.addElement( new String ("I am string "
  i))
• Access
• System.out.println("Element"
  myVector.elementAt(5) )
• Traversing
• Enumeration e myVector.elements()
• for (e.hasMoreElements())
• String myString (String) e.nextElement()
  System.out.println(myString)

17
Lab Work

• Open and read a text file.
• Find the words in the text file. Assume that
  words are separated by spaces.
• Sort and print the words.
• Use Vector class to store numbers.