Lesson : Interfaces and Packages

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Lesson Interfaces and Packages

• 1. Creating Interfaces
• 2. Creating and Using Packages

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1. Creating Interfaces

• 1. What Is an Interface?
• 2. Defining an Interface
• 3. Implementing the Sleeper Interface
• 4. Using an Interface as a Type
• 5. Warning! Interface Cannot Grow

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What Is an Interface?

• Interface
• A named collection of method definitions (without
  implementations) that can include constant
  declarations.
• Use to define a protocol of behavior that can be
  implemented by any class anywhere in the class
  hierarchy.
• Example
• How to create and use an interface.
• Why you need them.
• How to decide when to use an interface versus
  when to use a class.

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Example AlarmClock and Sleeper

• The AlarmClock class
• A service provider
• Notifies objects after a certain amount of time
  has elapsed.
• To get on AlarmClock's list of sleepers
• Ask the alarm clock to wake it up.
• letMeSleepFor method
• Implement the wakeUp method.
• Sleeper
• GUIClock

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Why Use Interfaces?

• Java doesn't support multiple inheritance
• Hence, you use an interface instead.
• Interface and multiple class inheritance are
  quite different
• A class inherits only constants from an
  interface.
• A class cannot inherit method implementations
  from an interface.
• The interface hierarchy is independent of the
  class hierarchy.
• Java does allow multiple interface inheritance

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Usefulness of Interfaces

• Capturing similarities between unrelated classes
  without artificially forcing a class
  relationship.
• Declaring methods that one or more classes are
  expected to implement.
• Revealing an object's programming interface
  without revealing its class.

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Defining an Interface

• An interface definition has two components
• The interface declaration.
• Declares various attributes about the interface.
  
• The interface body.
• Contains the constant and method declarations
  within that interface.

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The Interface Declaration

• An interface can extend other interfaces just as
  a class.
• An interface can extend any number of
  interfaces.
• An interface inherits all constants and methods
  from its superinterfaces.

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The Interface Body

• The method declaration
• It is not implemented.
• The constant declaration
• Classes that implement an interface can treat the
  constants as though they were inherited.
• Any class can use an interface's constants from
  the name of the interface.

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Implementing the Interface

• To declare a class that implements an interface,
  include an implements clause in the class
  declaration.
• A class can implement more than one interface .
• The implements keyword is followed by a
  comma-separated list of the interfaces
  implemented by the class.
• The class must provide method implementations for
  all of the methods declared in the interface and
  its superinterfaces.

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Using an Interface as a Type

• Defining a new interface
• Defining a new reference data type.
• For example(the AlarmClock class)
• To declare an array of Sleepers
• private Sleeper sleepers new
  SleeperMAX_CAPACITY
• As an argument to letMeSleepFor
• private boolean letMeSleepFor(Sleeper theSleeper,
  int time)
• . . .

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Interfaces Cannot Grow

• Changing Interface
• If you make this change to Sleeper, all classes
  that implement the old Sleeper will break because
  they don't implement the interface anymore!
• To create more interfaces later or to break your
  customer's code.

public interface Sleeper public void
wakeUp() public void beep() public long
ONE_SECOND 1000
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2. Creating and Using Packages

• 1. Creating a Package
• 2. Using Package Members
• 3. Managing Source and Class Files

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Creating and Using Packages

• A package(definition)
• A collection of related classes and interfaces
  that provides access protection and namespace
  management.
• A package used to
• to make classes easier to find and use.
• to avoid naming conflicts.
• to control access.
• The case of JDK
• Applet classes are in java.applet
• I/O classes are in java.io
• GUI widget classes are in java.awt.

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Several Reasons

• Easy to determine that these classes and
  interfaces are related.
• Easy to know where to find classes and interfaces
  that provide graphics-related functions.
• Avoid conflict with class names in other
  packages.
• You can allow classes within the package to have
  unrestricted access to each other.

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Creating a Package

• A package statement
• At the top of the source file in which the class
  or interface is defined
• package graphics
• public class Circle extends Graphic implements
  Draggable
• . . .
• The Circle class is a public member of the
  graphics package.
• Recommend that you use the one-public-class-per-fi
  le convention

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Naming a Package

• If the classes have same name and are in
  different packages, then the compiler allows
  both classes.
• By Convention
• Companies use their reversed Internet domain name
  in their package names, like this
• com.company.package.
• com.company.region.package.

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Using Package Members

• Only public package members are accessible
  outside the package.
• To use a public package member,
• refer to the member by its long (disambiguated)
  name,
• import the package member, or
• import the member's entire package.

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Referring to a Package Member by Name

• Short names
• The name specified in their declaration
• If the code you are writing is in the same
  package as that member.
• If the member's package has been imported.
• Long name
• The name that includes the package name.
• If you want to use a member from a different
  package.
• That package has not been imported.
• graphics.Rectangle

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Importing a Package Member

• To import a specific member into the current
  file, put an import statement.
• import graphics.Circle
• Circle myCircle new Circle()
• This approach works just fine if you use just a
  few members from the graphics package.

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Importing an Entire Package

• To import all of the classes and interfaces
  contained in a particular package, use the import
  statement with the asterisk ().
• import graphics.
• Circle myCircle new Circle()
• Rectangle myRectangle new Rectangle()
• Automatically imported packages
• The default package (the package with no name)
• The java.lang package
• The current package

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Disambiguating a Name

• Two packages have classes that the name is equal
  and both packages are imported, you must refer to
  the member by its long name.
• Example
• The graphics and java.awt packages contains a
  Rectangle class.
• Both graphics and java.awt have been imported.
• Rectangle rect
• graphics.Rectangle rect

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Managing Source and Class Files

• Save the source code
• The short name of the class or interface
• The extension is .java
• Put the source file in a directory
• Directory name reflects the packages name
• graphics.Rectangle //class name
• graphics/Rectangle.java //pathname to file

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Managing Source and Class Files...

• A class path is a list of directories or zip
  files to search for class files.
• The basename of the output file
• The name of the class or interface
• its extension is .class
• A .class file should also be in a series of
  directories that reflect the package name.
• The current directory and the JDK class files are
  automatically in your class path.