CHAPTER 20

The Package java.lang

The java.lang package contains classes that are fundamental to the design of the Java language. The most important classes are Object, which is the root of the class hierarchy, and Class, instances of which represent classes at run time.

Frequently it is necessary to represent a value of primitive type as if it were an object. The wrapper classes Boolean, Character, Integer, Long, Float, and Double serve this purpose. An object of type Double, for example, contains a field whose type is double, representing that value in such a way that a reference to it can be stored in a variable of reference type. These classes also provide a number of methods for converting among primitive values, as well as supporting such standard methods as equals and hashCode.

The class Math provides commonly used mathematical functions such as sine, cosine, and square root. The classes String and StringBuffer similarly provide commonly used operations on character strings.

Classes ClassLoader, Process, Runtime, SecurityManager, and System provide "system operations" that manage the dynamic loading of classes, creation of external processes, host environment inquiries such as the time of day, and enforcement of security policies.

Class Throwable encompasses objects that may be thrown by the throw statement (§14.16). Subclasses of Throwable represent errors and exceptions.

The hierarchy of classes defined in package java.lang is as follows.

Object §20.1	
 interface Cloneable §20.2
	Class §20.3
	Boolean §20.4
	Character §20.5
	Number §20.6
		Integer §20.7
		Long §20.8
		Float §20.9
		Double §20.10
	Math §20.11
	String §20.12
	StringBuffer §20.13
	ClassLoader §20.14
	Process §20.15
	Runtime §20.16
	SecurityManager §20.17
	System §20.18
	interface Runnable §20.19
	Thread §20.20
	ThreadGroup §20.21
	Throwable §20.22
		Error
			LinkageError
				ClassCircularityError
				ClassFormatError
				ExceptionInInitializerError
				IncompatibleClassChangeError
					AbstractMethodError
					IllegalAccessError
					InstantiationError
					NoSuchFieldError
					NoSuchMethodError
				NoClassDefFoundError
				UnsatisfiedLinkError
				VerifyError
			VirtualMachineError
				InternalError
				OutOfMemoryError
				StackOverflowError
				UnknownError
			ThreadDeath
		Exception
			ClassNotFoundException
			CloneNotSupportedException
			IllegalAccessException
			InstantiationException
			InterruptedException
			RuntimeException
				ArithmeticException
				ArrayStoreException
				ClassCastException
				IllegalArgumentException
					IllegalThreadStateException
					NumberFormatException
				IllegalMonitorStateException
				IndexOutOfBoundsException
				NegativeArraySizeException
				NullPointerException
				SecurityException