EXECUTION

When Initialization Occurs

12.4.1

12.4   Initialization of Classes and Interfaces

Initialization of a class consists of executing its static initializers and the initializ 

ers for

static

 fields (class variables) declared in the class. Initialization of an

interface consists of executing the initializers for fields (constants) declared there.

Before a class is initialized, its superclass must be initialized, but interfaces

implemented by the class need not be initialized. Similarly, the superinterfaces of

an interface need not be initialized before the interface is initialized.

12.4.1   When Initialization Occurs

A class or interface type

T

 will be

initialized

 at its first

active use

, which occurs if:

T

 is a class and a method actually declared in

T

 (rather than inherited from a

superclass) is invoked.

T

 is a class and a constructor for class

T

 is invoked, or

U

 is an array with ele 

ment type

T

, and an array of type

U

 is created.

A non constant field declared in

T

 (rather than inherited from a superclass or

superinterface) is used or assigned. A

constant field

 is one that is (explicitly or

implicitly) both

final

 and

static

, and that is initialized with the value of a

compile time constant expression ( 15.27). Java specifies that a reference to a

constant field must be resolved at compile time to a copy of the compile time

constant value, so uses of such a field are never active uses. See  13.4.8 for a

further discussion.

All other uses of a type are

passive uses

.

The intent here is that a class or interface type has a set of initializers that put

it in a consistent state, and that this state is the first state that is observed by other

classes. The static initializers and class variable initializers are executed in textual

order, and may not refer to class variables declared in the class whose declarations

appear textually after the use, even though these class variables are in scope

( 8.5). This restriction is designed to detect, at compile time, most circular or oth 

erwise malformed initializations.

As shown in an example in  8.5, the fact that initialization code is unrestricted

allows examples to be constructed where the value of a class variable can be

observed when it still has its initial default value, before its initializing expression

is evaluated, but such examples are rare in practice. (Such examples can be also

constructed for instance variable initialization; see the example at the end of

 12.5). Java provides the full power of the language in these initializers; program 

mers must exercise some care. This power places an extra burden on code genera 

tors, but this burden would arise in any case because Java is concurrent ( 12.4.3).

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