INTERFACES

Initialization of Fields in Interfaces

9.3.1

Every field declaration in the body of an interface is implicitly

public

,

static

, and

final

. It is permitted, but strongly discouraged as a matter of style,

to redundantly specify any or all of these modifiers for such fields.

A constant declaration in an interface must not include any of the modifiers

synchronized

,

transient

, or

volatile

, or a compile time error occurs.

It is possible for an interface to inherit more than one field with the same

name ( 8.3.3.3). Such a situation does not in itself cause a compile time error.

However, any attempt within the body of the interface to refer to either field by its

simple name will result in a compile time error, because such a reference is

ambiguous.

There might be several paths by which the same field declaration might be

inherited from an interface. In such a situation, the field is considered to be inher 

ited only once, and it may be referred to by its simple name without ambiguity.

9.3.1   Initialization of Fields in Interfaces

Every field in the body of an interface must have an initialization expression,

which need not be a constant expression. The variable initializer is evaluated and

the assignment performed exactly once, when the interface is initialized ( 12.4).

A compile time error occurs if an initialization expression for an interface

field contains a reference by simple name to the same field or to another field

whose declaration occurs textually later in the same interface. Thus:

interface Test {

float f = j;

int j = 1;

int k = k+1;

}

causes two compile time errors, because

j

 is referred to in the initialization of

f

before

j

 is declared and because the initialization of

k

 refers to

k

 itself.

(One subtlety here is that, at run time,

fields

 that are initialized with com 

pile time constant values are initialized first. This applies also to

static final

fields in classes ( 8.3.2.1). This means, in particular, that these fields will never be

observed to have their default initial values ( 4.5.4), even by devious programs.

See  12.4.2 and  13.4.8 for more discussion.)

If the keyword

this

 ( 15.7.2) or the keyword

super

 (15.10.2, 15.11) occurs

in an initialization expression for a field of an interface, then a compile time error

occurs.

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