3.9

Keywords

LEXICAL STRUCTURE

A Java letter is a character for which the method

Character.isJavaLetter

( 20.5.17) returns

true

. A Java letter or digit is a character for which the method

Character.isJavaLetterOrDigit

 ( 20.5.18) returns

true

.

The Java letters include uppercase and lowercase ASCII Latin letters

A

Z

(

\u0041

\u005a

), and

a

z (\u0061

\u007a

), and, for historical reasons, the

ASCII underscore (

_

, or

\u005f

) and dollar sign (

$

, or

\u0024

). The

$

 character

should be used only in mechanically generated Java code or, rarely, to access pre 

existing names on legacy systems.

The Java digits include the ASCII digits

0 9

 (

\u0030

\u0039)

.

Two identifiers are the same only if they are identical, that is, have the same

Unicode character for each letter or digit

.

Identifiers that have the same external appearance may yet be different. For

example, the identifiers consisting of the single letters

LATIN CAPITAL LETTER A

(

A

,

\u0041

),

LATIN SMALL LETTER A

 (

a

,

\u0061

),

GREEK CAPITAL LETTER ALPHA

(

A

,

\u0391

), and

CYRILLIC SMALL LETTER A

 (

a

,

\u0430

) are all different.

Unicode composite characters are different from the decomposed characters.

For example, a

LATIN CAPITAL LETTER A ACUTE

(

A, \u00c1)

 could be considered

to be the same as a

LATIN CAPITAL LETTER A

 (

A

,

\u0041)

 immediately followed

by a

NON SPACING ACUTE

 ( ,

\u0301

) when sorting, but these are different in Java

identifiers. See

The Unicode Standard

, Volume 1, pages 412ff for details about

decomposition, and see pages 626 627 of that work for details about sorting.

Examples of identifiers are:

String

i3

MAX_VALUE

isLetterOrDigit

3.9   Keywords

The following character sequences, formed from ASCII letters, are reserved for

use as

keywords

 and cannot be used as identifiers ( 3.8):

Keyword: one of

abstract

default

if

private

throw

boolean

do

implements

protected

throws

break

double

import

public

transient

byte

else

instanceof

return

try

case

extends

int

short

void

catch

final

interface

static

volatile

char

finally

long

super

while

class

float

native

switch

const

for

new

synchronized

continue

goto

package

this

18