4

C H A P T E R

Types, Values, and Variables

I send no agent or medium,

offer no representative of value,

but offer the value itself.

 Walt Whitman,

Carol of Occupations

 (1855),

in

Leaves of Grass

J

AVA is a

strongly typed

 language, which means that every variable and every

expression has a type that is known at compile time. Types limit the values that a

variable ( 4.5) can hold or that an expression can produce, limit the operations

supported on those values, and determine the meaning of the operations. Strong

typing helps detect errors at compile time.

The types of the Java language are divided into two categories: primitive types

and reference types. The primitive types ( 4.2) are the

boolean

 type and the

numeric types. The numeric types are the integral types

byte

,

short

,

int

,

long

,

and

char

, and the floating point types

float

 and

double

. The reference types

( 4.3) are class types, interface types, and array types. There is also a special null

type. An object ( 4.3.1) in Java is a dynamically created instance of a class type or

a dynamically created array. The values of a reference type are references to

objects. All objects, including arrays, support the methods of class

Object

( 4.3.2). String literals are represented by

String

 objects ( 4.3.3).

Types are the same ( 4.3.4) if they have the same fully qualified names and

are loaded by the same class loader. Names of types are used ( 4.4) in declara 

tions, in casts, in class instance creation expressions, in array creation expressions,

and in

instanceof

 operator expressions.

A variable ( 4.5) is a storage location. A variable of a primitive type always

holds a value of that exact type. A variable of a class type

T

 can hold a null refer 

ence or a reference to an instance of class

T

 or of any class that is a subclass of

T

.

A variable of an interface type can hold a null reference or a reference to any

instance of any class that implements the interface. If

T

 is a primitive type, then a

variable of type  array of

T

  can hold a null reference or a reference to any array

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