20

C H A P T E R

The Package

java.lang

T

HE

java.lang

The Package 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

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