The java.lang Package Certification - Main Classes

• the java.lang Package contains classes that are fundamental to the Java programming language
• it is always implicitly imported
• the most important classes are Object and Class

Object

• the Object class is at the root of the class heirarchy, all other classes inherit it's methods
  
  

  protected Object clone() throws CloneNotSupportedException

  returns an identical copy of an object. The object must implement the Cloneable interface

  public boolean equals(Object obj)

  returns true if obj is the same object as the referenced object

  protected void finalize() throws Throwable

  called by the garbage collector prior to collecting the object

  public final Class getClass()

  returns the runtime class of an object

  public int hashCode()

  returns a distinct integer representing a unique object; supports hash tables

  public final void notify()

  wakes up a single thread waiting on the object's monitor

  public final void notifyAll()

  wakes up all threads waiting on the object's monitor

  public String toString()

  returns a string representation of the object

  public final void wait() throws InterruptedException,
  public final void wait(long timeout) throws InterruptedException,
  public fianl void wait(long timeout, int nanos) throws InterruptedException

  causes the current thread to wait until another thread invokes notify() or notifyAll() for this object, or, the specified time elaspses

Class

• the Class class was introduced in JDK 1.2
• instances of the Class class represent classes and interfaces in a running Java application
• also represents arrays, primitive types and void, all of which are Class instances at runtime
• objects of the Class class are automatically created as classes are loaded by the JVM; they are known as class descriptors
• provides over 30 methods which can be used to obtain information on a running class
• some of the more useful methods are: getName(), toString(), getSuperclass(), isInterface(), newInstance()

Other classes

• Wrapper classes used to represent primitive types as Objects: Boolean, Byte, Short, Character, Integer, Float, Long and Double
• Math class provides commonly used mathematical functions ie cos, sine, tan
• String and StringBuffer classes provide commonly used operations on character strings
• System operation classes: ClassLoader, SecurityManager, Runtime, Process and System which manage the dynamic loading of classes, creation of external processes, security, and host inquiries ie time of day
• Package class is new to JDK 1.2. Provides methods for obtaining package version information stored in the manifest of jar files. Useful methods include: getPackage(), getAllPackages(), which provide package objects that are known to the class loader, and isCompatibleWith() which is used to determine wether a package is comparable to a particular version.
• all the Exception and Error classes, including Throwable

Interfaces

• Cloneable. Contains no methods. Used to differentiate between objects that are cloneable and non-cloneable.
• Comparable, new in JDK 1.2. Defines the compareTo() method. Objects implementing this interface can be compared and sorted.
• Runnable. Defines the run() method which is invoked when a thread is activated.