Java Package

1
JavaPackage Interface
2
Package

• Very often, programmers could run out of
  convenient, descriptive names for individual
  classes.
• Package are containers for classes that are used
  to keep the class name space compartmentalized.

3
Package

• You can define classes inside a package that are
  not accessible by code outside that package or
  class members that are only exposed to other
  members of the same package.
• This allows the classes to have intimate
  knowledge of each other, but not expose that
  knowledge to the rest of the world.

4
Package

• In general, a Java source file can contain any of
  the following four internal parts
• (1) A single package statement (optional)
• (2) Any number of import statements
• (optional)
• (3) A single public class declaration (required)
• (4) Any number of classes private to the
• package (optional)

5
Define a Package

• Simply include a package command as the first
  statement in a Java source file.
• Any classes declared within that file will belong
  to the specified package.
• The package statement defines a name space in
  which classes are stored.
• If the package statement is omitted, the class
  names are in the default package, which has no
  name.

6
Define a Package

• Java uses file system directories to store
  packages.
• Any classes declared to be in a particular
  package have to be saved in the a directory named
  after the package.
• For example, classes that belong to the package
  MyPackage have to be in the directory called
  MyPackage.

7
Define a Package

• You can create a hierarchy of packages. The
  general form is
• package.pkg1.pkg2.pkg3
• A package hierarchy must also be reflected in the
  file system of your computer. For example,
  java.awt.image needs to be stored in
  java.awt.image directory. (Package1AccountBalance1
  .java)

8
Access Protection

• Classes and packages are both means of
  encapsulating and containing the name space and
  scope of variables and methods.
• Packages act as containers for classes and other
  subordinate packages.
• Classes act as containers for data and code.

9
Access Protection

• Anything declared public can be accessed from
  anywhere.
• Anything declared private cannot be seen outside
  of its class.
• By default, when no access modifier is specified,
  a class member can be accessed by subclasses and
  other classes in the same package.

10
Access Protection

• If you want a member to be seen outside the
  current package, but only to classes that
  subclass your class directly, declare that member
  protected.

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Access Protection

• Private No Modifier Protected
  Public
• same class Y Y Y Y
• same package
• subclass N Y Y Y
• same package
• non-subclass N Y Y Y
• Different Package
• subclass N N Y Y
• Different Package
• non-subclass N N N Y
• (Package1Protection1.java)

12
Importing Packages

• Java provides import statement to bring certain
  classes, or entire packages, into visibility.
• Once imported, a class can be referred to
  directly.
• Import pkg1.pkg2.classname/
• The may increase compilation time.

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Interfaces

• Normally, in order for a method to be called from
  one classes to another, both classes need to be
  present at compile time so Java can check to
  ensure that the method signatures are compatible.
• In system like this, functionality gets pushed up
  higher and higher in the class hierarchy so that
  the mechanisms will be available for more and
  more subclasses.

14
Interfaces

• Interfaces are designed to support dynamic method
  resolution at run time.
• Interfaces disconnect the definition of a method
  or set of methods from the inheritance hierarchy
  by making it possible for classes that not
  related in terms of the class hierarchy to
  implement the same interface. The idea is similar
  to multiple inheritance in C.

15
Interfaces

• Javas interface classes are classes without
  instance variables and their methods are declared
  without any body.
• Javas interface allows you to fully utilize the
  one-interface, multiple methods aspect of
  polymorphism.
• Interface specifies what a class must do, but not
  how it does it.

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Defining an Interface

• An interface is defined much like a class
• accesmodifier interface interfacename
• returntype methodname(paremeter-list)
• returntype methodname(paremeter-list)
• datatype variable1 value

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Defining an Interface

• The access is either public or not used.
• When the interface is declared public, it can be
  used by any other code.
• When the access modifier is not used, the
  interface is only available to other members of
  the same package.

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Defining an Interface

• The methods are essentially abstract methods.
  Each class that include an interface must
  implement all of the methods.
• Variables are implicitly final and static. They
  must also be initialized with a constant value.
• All methods and variables are implicitly public
  is the interface is declared as public.

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Implementing an Interface

• Once an interface has been defined, one or more
  classes can implement that interface.
• The general form of a class that implementing an
  interface look like this
• accessmodifier class classname implements
  interface ,interface
• classbody
• (InterfaceClass1.java)

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Implementing an Interface

• The methods that implement an interface must be
  declared public.
• The type signature of the implementing method
  must match exactly the type signature specified
  in the interface definition.
• The implementing classes can also define
  additional members of their own.
• (InterfaceClass2.java)

21
Accessing Implementations

• You can declare variables as object references
  that use an interface rather than a class type.
• When you call a method through one of these
  reference, the correct version will be called
  based on the actual instance of the interface
  being referred to.
• (Interface1.java)

22
Accessing Implementations

• An interface reference variable only has
  knowledge of the methods declared by its
  interface declaration, not the ones defined in
  the implementing class.
• (InterfaceClass3.java Interface2.java)
• (Interface3.java Interface4.java)

23
Variables in Interfaces

• You can use interfaces to import shared constants
  into multiple classes by declaring an interface
  that contains variables with desired values.
• Implementing the interfaces with constants is
  similar to importing constant into the class name
  space as final variables.
• (Interface5.java)

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Extending Interfaces

• One interface can inherit another by use of the
  keyword extends.
• The syntax is the same as for inheriting classes.
• When a class implements an interface that
  inherits another interface, it must provide
  implementations for all methods within the
  inheritance chain.
• (Interface6.java)