Package, StackTraceElement, Enum - java.lang

Package

Package encapsulates version data associated with a package. Package version information is becoming more important because of the proliferation of packages and because a Java program may need to know what version of a package is available. The methods defined by Package are shown in Table 17-20. The following program demonstrates Package, displaying the packages about which the program currently is aware:

// Demonstrate Package

class PkgTest {

public static void main(String args[]) { Package pkgs[];

pkgs = Package.getPackages();

for(int i=0; i < pkgs.length; i++) System.out.println(

pkgs[i].getName() + " " + pkgs[i].getImplementationTitle() + " " +

pkgs[i].getImplementationVendor() + " " + pkgs[i].getImplementationVersion()

);

}

}

RuntimePermission

RuntimePermission relates to Java’s security mechanism and is not examined further here.

Throwable

The Throwable class supports Java’s exception-handling system and is the class from which all exception classes are derived. It is discussed in Chapter 10.

SecurityManager

SecurityManager supports Java’s security system. A reference to the current security manager can be obtained by calling getSecurityManager( ) defined by the System class.

StackTraceElement

The StackTraceElement class describes a single stack frame, which is an individual element of a stack trace when an exception occurs. Each stack frame represents an execution point, which includes such things as the name of the class, the name of the method, the name of the file, and the source-code line number. An array of StackTraceElements is returned by the getStackTrace( ) method of the Throwable class.

StackTraceElement has one constructor:

StackTraceElement(String className, String methName, string fileName, int line)

Here, the name of the class is specified by className, the name of the method is specified in methName, the name of the file is specified by fileName, and the line number is passed in line. If there is no valid line number, use a negative value for line. Furthermore, a value of –2 for line indicates that this frame refers to a native method.

The methods supported by StackTraceElement are shown in Table 17-21. These methods give you programmatic access to a stack trace.

Enum

As described in Chapter 12, an enumeration is a list of named constants. (Recall that an enumeration is created by using the keyword enum.) All enumerations automatically inherit Enum. Enum is a generic class that is declared as shown here:

class Enum<E extends Enum<E>>

Here, E stands for the enumeration type. Enum has no public constructors.

Enum defines several methods that are available for use by all enumerations, which are shown in Table 17-22.

ClassValue

ClassValue can be used to associate a value with a type. It is a generic type defined like this: Class ClassValue<T>

It is designed for highly specialized uses, not for normal programming.