15.17.1

String Concatenation Operator

+

EXPRESSIONS

Now only reference values need to be considered. If the reference is

null

, it is

converted to the string "

null

" (four ASCII characters

n

,

u

,

l

,

l

). Otherwise, the

conversion is performed as if by an invocation of the

toString

 method of the ref 

erenced object with no arguments; but if the result of invoking the

toString

method is

null

, then the string "

null

" is used instead. The

toString

 method

( 20.1.2) is defined by the primordial class

Object

 ( 20.1); many classes override

it, notably

Boolean

,

Character

,

Integer

,

Long

,

Float

,

Double,

 and

String

.

15.17.1.2

Optimization of String Concatenation

An implementation may choose to perform conversion and concatenation in one

step to avoid creating and then discarding an intermediate

String

 object. To

increase the performance of repeated string concatenation, a Java compiler may

use the

StringBuffer

 class ( 20.13) or a similar technique to reduce the number

of intermediate

String

 objects that are created by evaluation of an expression.

For primitive objects, an implementation may also optimize away the creation

of a wrapper object by converting directly from a primitive type to a string.

15.17.1.3

Examples of String Concatenation

The example expression:

"The square root of 2 is " + Math.sqrt(2)

produces the result:

"The square root of 2 is 1.4142135623730952"

The + operator is syntactically left associative, no matter whether it is later

determined by type analysis to represent string concatenation or addition. In some

cases care is required to get the desired result. For example, the expression:

a + b + c

is always regarded as meaning:

(a + b) + c

Therefore the result of the expression:

1 + 2 + " fiddlers"

is:

"3 fiddlers"

but the result of:

"fiddlers " + 1 + 2

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