packages. packages in java a package is a collection of related classes and interfaces in java...
TRANSCRIPT
PACKAGES
PACKAGES IN JAVA• A package is a collection of related classes and
interfaces in Java
• Packages help in logical grouping of classes and
interfaces
• All the classes and interfaces that are used for
performing I/O operations can be placed in the same
package
• All the classes and interfaces that are used for writing
network programs can be placed in the same package
CREATING A PACKAGE
• It would be better to group all the classes and
interfaces related to insurance in a package, insurance
• Create a folder insurance
• Place the following program in this folder
package insurance;public class Policy{
//Code goes here}
CREATING A PACKAGE• Compile the program
– class Policy will be created in a package insurance
• The package statement, if existing, should be the very first
statement of the file
• The fully qualified name of a class that is stored in a package is
<packagename>.<ClassName>
– insurance.Policy
insurance.Policy policy = new insurance.Policy();
PACKAGES TO PREVENT NAME CLASH
• Packages prevent the clash of class/interface
names
• More than one class/interface can have the same name
• They should be in two different packages
• The fully qualified names of these classes/interfaces will
be different
IMPORTING A CLASS
• It is difficult to use the fully qualified name of a class through
out the program
• The import statement can be used to import a class into a
program so that the class name need not be fully qualified
import insurance.Policy;class Test{
public static void main(String [] args){Policy policy = new Policy();//Policy means insurance.Policy
}}
IMPORTING ALL CLASSES AND INTERFACES
• The statement import insurance.*
• Imports all the classes and interfaces in the package insurance
• All the classes and interfaces in this package can be used without
qualifying them with package name
PACKAGES AND CLASSPATH• Compiler and JVM must know location of the packages
• An environment variable classpath needs to be set
• The classpath will be pointing to a folder be one level up the package
folder
• If the folder hierarchy is C:\work\java\insurance, the classpath should be
set using the following statement
>set classpath = %classpath%;C:\work\java
SUB PACKAGES• A package can in turn contain another sub package
• To create a sub package policy in insurance
• Create a sub folder policy inside the folder insurance
• Place the following code in the folder policy and compilepackage insurance.policy;public class Policy{
//Code goes here}
• A class Policy will be created in a package insurance.policy
– Fully qualified name will be insurance.policy.Policy
ACCESS MODIFIERS• Java has 4 access control modifiers
• private: Accessible only within the class
• default: No keyword, Accessible only within the package
• protected: Similar to default with the addition that available to all
child classes; that is, even if child class is in a different package
• public: Accessible to all
• Data Members and Methods can have any of these specifier
• Classes and Interfaces can have either the public access or
the default access
USES OF PACKAGES
• Logical grouping of classes and interfaces
• Avoiding clash of names
• Provides an extra level of protection to its members
STANDARD JAVA PACKAGES• java.lang
• Contains classes that form the basis of the design of the Java programming language
• No need to explicitly import this package
• The String class, System class etc, belong to this package
• java.io
• Classes and interfaces to perform I/O operations
• java.util
• Utility classes and interfaces like List, Calendar etc
• java.awt
• Classes and interfaces to create GUI