CONVERSIONS AND PROMOTIONS

Assignment Conversion

5.2

5.2   Assignment Conversion

Assignment conversion

 occurs when the value of an expression is assigned

( 15.25) to a variable: the type of the expression must be converted to the type of

the variable. Assignment contexts allow the use of an identity conversion ( 5.1.1),

a widening primitive conversion ( 5.1.2), or a widening reference conversion

( 5.1.4). In addition, a narrowing primitive conversion may be used if all of the

following conditions are satisfied:

The expression is a constant expression of type

int

.

The type of the variable is

byte

,

short

, or

char

.

The value of the expression (which is known at compile time, because it is a

constant expression) is representable in the type of the variable.

If the type of the expression cannot be converted to the type of the variable by a

conversion permitted in an assignment context, then a compile time error occurs.

If the type of an expression can be converted to the type a variable by assign 

ment conversion, we say the expression (or its value) is

assignable to

 the variable

or, equivalently, that the type of the expression is

assignment compatible with

 the

type of the variable.

An assignment conversion never causes an exception. (Note, however, that an

assignment may result in an exception in a special case involving array elements

 see  10.10 and  15.25.1.)

The compile time narrowing of constants means that code such as:

byte theAnswer = 42;

is allowed. Without the narrowing, the fact that the integer literal

42

 has type

int

would mean that a cast to

byte

 would be required:

byte theAnswer = (byte)42;

//

cast is permitted but not required

A value of primitive type must not be assigned to a variable of reference type;

an attempt to do so will result in a compile time error. A value of type

boolean

can be assigned only to a variable of type

boolean

.

The following test program contains examples of assignment conversion of

primitive values:

class Test {

public static void main(String[] args) {

short s = 12;

//

narrow

12

 to

short

float f = s;

//

widen

short

 to

float

System.out.println("f=" + f);

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