Package

  • This encapsulates the version data associate with a package.
  • This package version information is important as there are multiple packages and java program may want to know the version of the package that currently using.
  • It have methods like

static packages[] getPackages() - which gets all the packages about the aware of the invoking java program.

String getName()- Returns the name of the invoking package.

String getImplementationTitle()- Returns the title of the invoking package.

String getImplementationVendor()- Returns the name of the inventor of the invoking package.

String getImplementationVersion()- Returns the version of the invoking package.

// Demonstrate Package
public 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()
 );
 }
}

Output:

sun.reflect Java Runtime Environment Oracle Corporation 1.8.0_121
java.util Java Runtime Environment Oracle Corporation 1.8.0_121

RuntimePermission

  • It is relates to the java security mechanism.

Throwable

  • It supports the exception handling system and the class where all the exception classes are derived.

SecurityManager

  • It supports the Java's security system where we can get the current security manager by calling getSecurityManager() which is defined by the System class.

StackTraceElement

  • It defines the individual element of a stack trace when an exception occurs which is called as stack frame.
  • Each stack frame represents the execution point which describes the name of the class, methods, file and the source code line number.
  • It has one constructor as follows.

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

  • An array of stack trace elements can be get by getStackTrace() method.
  • It have methods like getClassName(), getMethodName(), getFileName() and getLineNumber() to return name of the class, method, file and the source code line number.
  • hashCode() returns the hash code of the stack trace element.

Enum

  • An Enum is a generic class where all the enumerations created by inheriting that.

class Enum< E extends Enum <E>>

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

ClassValue

  • It is a generic type which is used to associate a value with a type.

    Class ClassValue<T>

The CharSequence Interface

  • This is a interface which defines methods to grant read-only access to a sequence of characters.
  • This is implemented by Sting, StringBuffer, StringBuilder.

The Comparable Interface

  • A class that implement Comparable have objects in a meaningful manner.

  • It is generic which is defined as follows:

    interface Comparable<T>

  • int compareTo(T obj) is a method which is used to determine what Java calls the natural ordering of instances of a class.

The Appendable Interface

  • Objects of a class that implements Appendable have a character or character sequences appended to it.

  • It defines the following methods.

    Appendable append(char ch) throws IOException //append character to the invoking object.

Appendable append(CharSequence chars) throws IOException //append character sequence to the invoking object.

Appendable append(CharSequence chars, int begin, int end) throws IOException //specifies the start and end of the character sequence.

The Iterable Interface

  • It is an interface which must be implement by the class to use the for-each version of the for loop.

  • It is a generic of the form as follows

    interface Iterable<T>

  • T is the type of the object being iterated.

The Readable Interface

  • It indicates that an object can be used as a source for characters.

int read(CharBuffer buf ) throws IOException

  • This method reads characters into buf. It returns the number of characters read, or –1 if an EOF is encountered.

The AutoCloseable Interface

  • It supports try-with-resources statement which is some times refer as automatic resource management (ARM).
  • It releases the resources when it is no longer needed.
  • It defines only close() method as follows:

void close( ) throws Exception

  • This method closes the invoking object, releasing any resources that it may hold.

The Thread.UncaughtExceptionHandler Interface

  • The classes implement this interface to handle uncaught exceptions.

  • It declares only one method as follows:

    void uncaughtException(Thread thrd, Throwable exc)

  • thrd is a reference to the thread that generated the exception and exc is a reference to the exception.

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