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 compile-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.