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Exploring java.lang >
Siva Nookala - 20 Feb 2017
The java.lang.Package class contains version information associated with a package.

Why is package version information becoming important?
  • Because of a rapid increase in the no. of packages in Java library.
  • Sometimes, a Java program may need to know what version of a package is available.

For Example: If you want to know which version of java.util you are currently using, you can pass the package name as an argument to getPackage() method and then invoke getImplementationVersion() on the Package object as shown below:

Package pkg = Package.getPackage( "java.util" );
System.out.println( pkg.getImplementationVersion() );
Output: 1.7.0_51

NOTE: Output indicates the version of java.util package in your system. Here, it is 1.7 and update 51, which is likely to be the version of JDK in your system.

Some important methods defined by Package:
Method Description
String getImplementationTitle() Returns the title of the invoking package
String getImplementationVersion() Returns the version number of the invoking package
String getImplementationVendor() Returns the name of the implementor of the invoking package
static Package[] getPackages() Returns all packages about which the invoking program is currently aware
String getName() Returns the name of the invoking package
static Package getPackage( String pkgName ) Returns a Package objet with the name specified by pkgName
String getSpecificationTitle() Returns the title of the invoking package’s specification
String getSpecificationVendor() Returns the name of the owner of the specification for the invoking package
String getSpecificationVersion() Returns the invoking package’s specification version number


The following program demonstrates getName() and getImplementationVersion() methods of Package class.
Demonstration of getName and getImplementationVersion methods
class PackageClassDemo
{
    public static void main(String arg[])
    {
        // Get all the packages that this program is currently aware of.
        Package[] pkgs = Package.getPackages();    // LINE A
        
        // Display related information about each of these packages in pkgs array
        for( int i = 0; i < pkgs.length; i++ )
        {
            System.out.println( pkgs[i].getName() + " ---> " +
                                    pkgs[i].getImplementationVersion() );
        }
    
    }
}
OUTPUT

javax.swing.tree ---> 1.7.0_51
sun.util.resources ---> 1.7.0_51
java.net ---> 1.7.0_51
sun.awt.image ---> 1.7.0_51
sun.reflect.misc ---> 1.7.0_51
------------------------------------
------------------------------------
------------------------------------

DESCRIPTION

Output is very long. Only a part of it is shown here.
Here, we are collecting the package info. known to this class in a Package array using getPackages() method (LINE A). Then, we are iterating over this array and displaying corresponding name and version info. of each package.

THINGS TO TRY
  • Consider the above program again. Try to use getImplementationTitle() and getImplementationVendor() methods as well in the SOP statement and observe the related info. displayed by these methods.
  • Also try to use getSpecificationTitle(), getSpecificationVendor() and getSpecificationVersion() methods in the same way.
  • Try to obtain information related to a specific package, say javax.swing, by passing it as an argument to getPackage() method as shown earlier.

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