Java SE > Java SE Specifications > Java Language Specification
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&&||!? :? :boolean++ and --if Statementsassert Statementsswitch Statementswhile Statementsdo Statementsfor Statementsbreak, continue, return, and throw Statementssynchronized Statementstry StatementsEach local variable
(§14.4) and every blank final field
(§4.12.4, §8.3.1.2) must
have a definitely assigned value when any access
of its value occurs.
An access to
its value consists of the simple name of the variable
(or, for a field, the simple name of the field qualified by
this) occurring anywhere in an expression except as the
left-hand operand of the simple assignment operator =
(§15.26.1).
For every
access of a local variable or blank final field x, x must be
definitely assigned before the access, or a compile-time error
occurs.
Similarly, every blank final
variable must be assigned at most once; it must be
definitely unassigned when an assignment to it
occurs.
Such an
assignment is defined to occur if and only if either the simple name
of the variable (or, for a field, its simple name qualified by this)
occurs on the left hand side of an assignment operator.
For every
assignment to a blank final variable, the variable must be
definitely unassigned before the assignment, or a compile-time error
occurs.
The remainder of this chapter is devoted to a precise explanation of the words "definitely assigned before" and "definitely unassigned before".
The idea behind definite
assignment is that an assignment to the local variable or blank
final field must occur on every possible execution path to the
access. Similarly, the idea behind definite unassignment is that no
other assignment to the blank final variable is permitted to occur
on any possible execution path to an assignment.
The analysis takes into account
the structure of statements and expressions; it also provides a
special treatment of the expression operators !,
&&, ||, and ? :, and of boolean-valued constant
expressions.
Except for the special treatment
of the conditional boolean operators &&, ||, and ? :
and of boolean-valued constant expressions, the values of expressions
are not taken into account in the flow analysis.
Example 16-1. Definite Assignment Considers Structure of Statements and Expressions
A Java compiler recognizes that k
is definitely assigned before its access (as an argument of a method
invocation) in the code:
{
int k;
if (v > 0 && (k = System.in.read()) >= 0)
System.out.println(k);
}
because the access occurs only if the value of the expression:
v > 0 && (k = System.in.read()) >= 0
is true, and the value can be true only if the
assignment to k is executed (more properly,
evaluated).
Similarly, a Java compiler will recognize that in the code:
{
int k;
while (true) {
k = n;
if (k >= 5) break;
n = 6;
}
System.out.println(k);
}
the variable k is definitely
assigned by the while statement because the condition expression
true never has the value false, so only the break statement can
cause the while statement to complete normally,
and k is definitely assigned before the break
statement.
On the other hand, the code:
{
int k;
while (n < 4) {
k = n;
if (k >= 5) break;
n = 6;
}
System.out.println(k); /* k is not "definitely assigned"
before this statement */
}
must be rejected by a Java compiler, because in this
case the while statement is not guaranteed to execute its body as
far as the rules of definite assignment are concerned.
Example 16-2. Definite Assignment Does Not Consider Values of Expressions
A Java compiler must produce a compile-time error for the code:
{
int k;
int n = 5;
if (n > 2)
k = 3;
System.out.println(k); /* k is not "definitely assigned"
before this statement */
}
even though the value of n is
known at compile time, and in principle it can be known at compile
time that the assignment to k will always be
executed (more properly, evaluated). A Java compiler must operate
according to the rules laid out in this section. The rules recognize
only constant expressions; in this example, the expression n
> 2 is not a constant expression as defined in
§15.28.
As another example, a Java compiler will accept the code:
void flow(boolean flag) {
int k;
if (flag)
k = 3;
else
k = 4;
System.out.println(k);
}
as far as definite assignment
of k is concerned, because the rules outlined in
this section allow it to tell that k is assigned no
matter whether the flag is true or false. But
the rules do not accept the variation:
void flow(boolean flag) {
int k;
if (flag)
k = 3;
if (!flag)
k = 4;
System.out.println(k); /* k is not "definitely assigned"
before this statement */
}
and so compiling this program must cause a compile-time error to occur.
Example 16-3. Definite Unassignment
A Java compiler will accept the code:
void unflow(boolean flag) {
final int k;
if (flag) {
k = 3;
System.out.println(k);
}
else {
k = 4;
System.out.println(k);
}
}
as far as definite unassignment
of k is concerned, because the rules outlined in
this section allow it to tell that k is assigned at
most once (indeed, exactly once) no matter whether
the flag is true or false. But the rules do not
accept the variation:
void unflow(boolean flag) {
final int k;
if (flag) {
k = 3;
System.out.println(k);
}
if (!flag) {
k = 4;
System.out.println(k); /* k is not "definitely unassigned"
before this statement */
}
}
and so compiling this program must cause a compile-time error to occur.
In order to precisely specify all the cases of definite assignment, the rules in this section define several technical terms:
whether a variable is definitely assigned before a statement or expression;
whether a variable is definitely unassigned before a statement or expression;
whether a variable is definitely assigned after a statement or expression; and
whether a variable is definitely unassigned after a statement or expression.
For boolean-valued expressions, the last two are refined into four cases:
whether a variable is definitely assigned after the expression when true;
whether a variable is definitely unassigned after the expression when true;
whether a variable is definitely assigned after the expression when false; and
whether a variable is definitely unassigned after the expression when false.
Here, when true and when false refer to the value of the expression.
For example, the local variable k
is definitely assigned a value after evaluation of the
expression:
a && ((k=m) > 5)
when the expression is true but not when the
expression is false (because if a is false, then
the assignment to k is not necessarily executed
(more properly, evaluated)).
The phrase "V is definitely assigned after X" (where V is a local variable and X is a statement or expression) means "V is definitely assigned after X if X completes normally". If X completes abruptly, the assignment need not have occurred, and the rules stated here take this into account.
A peculiar consequence of this definition is that
"V is definitely assigned after break;" is always
true! Because a break statement never completes normally, it is
vacuously true that V has been assigned a value if the break
statement completes normally.
The statement "V is definitely unassigned after X" (where V is a variable and X is a statement or expression) means "V is definitely unassigned after X if X completes normally".
An even more peculiar consequence of this definition
is that "V is definitely unassigned after break;"
is always true! Because a break statement never completes normally,
it is vacuously true that V has not been assigned a value if the
break statement completes normally. (For that matter, it is also
vacuously true that the moon is made of green cheese if the break
statement completes normally.)
In all, there are four possibilities for a variable V after a statement or expression has been executed:
V is definitely assigned and is not definitely unassigned.
(The flow analysis rules prove that an assignment to V has occurred.)
V is definitely unassigned and is not definitely assigned.
(The flow analysis rules prove that an assignment to V has not occurred.)
V is not definitely assigned and is not definitely unassigned.
(The rules cannot prove whether or not an assignment to V has occurred.)
V is definitely assigned and is definitely unassigned.
(It is impossible for the statement or expression to complete normally.)
To shorten the rules, the customary abbreviation "iff" is used to mean "if and only if". We also use an abbreviation convention: if a rule contains one or more occurrences of "[un]assigned" then it stands for two rules, one with every occurrence of "[un]assigned" replaced by "definitely assigned" and one with every occurrence of "[un]assigned" replaced by "definitely unassigned".
For example:
V is [un]assigned after an empty statement iff it is [un]assigned before the empty statement.
should be understood to stand for two rules:
V is definitely assigned after an empty statement iff it is definitely assigned before the empty statement.
V is definitely unassigned after an empty statement iff it is definitely unassigned before the empty statement.
Throughout the rest of this chapter, we
will, unless explicitly stated otherwise, write V to represent a
local variable or blank final field which is in scope
(§6.3).
Likewise, we will use a, b, c, and e to represent expressions,
and S and T to represent statements. We will use the phrase "a
is V" to mean that a is either the simple name of the variable
V, or V's simple name qualified by this (ignoring
parentheses). We will use the phrase "a is not V" to mean the
negation of "a is V".
The definite unassignment analysis of loop
statements raises a special problem. Consider the
statement while (. In order to determine
whether V is definitely unassigned within some subexpression of e) Se,
we need to determine whether V is definitely unassigned before
e. One might argue, by analogy with the rule for definite assignment
(§16.2.10), that V is definitely unassigned
before e iff it is definitely unassigned before the while
statement. However, such a rule is inadequate for our purposes. If e
evaluates to true, the statement S will be executed. Later, if V
is assigned by S, then in the following iteration(s) V will have
already been assigned when e is evaluated. Under the rule suggested
above, it would be possible to assign V multiple times, which is
exactly what we have sought to avoid by introducing these
rules.
A revised rule would be: "V is definitely
unassigned before e iff it is definitely unassigned before the
while statement and definitely unassigned after S". However, when
we formulate the rule for S, we find: "V is definitely unassigned
before S iff it is definitely unassigned after e when true". This
leads to a circularity. In effect, V is definitely
unassigned before the loop condition e only if
it is unassigned after the loop as a
whole!
We break this vicious circle using a hypothetical
analysis of the loop condition and body. For example, if we assume
that V is definitely unassigned before e (regardless of whether
V really is definitely unassigned before e), and can then prove
that V was definitely unassigned after e then we know that e
does not assign V. This is stated more formally as:
Assuming V is definitely unassigned before e,
V is definitely unassigned after e.
Variations on the above analysis are used to define well founded definite unassignment rules for all loop statements in the Java programming language.
V
is [un]assigned after any constant expression
(§15.28) whose value is true when
false.
V
is [un]assigned after any constant expression whose value is
false when true.
V
is [un]assigned after any constant expression whose value is
true when true iff V is [un]assigned before the constant
expression.
V
is [un]assigned after any constant expression whose value is
false when false iff V is [un]assigned before the constant
expression.
V
is [un]assigned after a boolean-valued constant expression e iff
V is [un]assigned after e when true and V is [un]assigned
after e when false.
This is equivalent to saying that V is
[un]assigned after e iff V is [un]assigned before e.
Because a constant expression whose value is true
never has the value false, and a constant expression whose value is
false never has the value true, the first two rules are vacuously
satisfied. They are helpful in analyzing expressions involving the
operators && (§16.1.2), ||
(§16.1.3), !
(§16.1.4), and ? :
(§16.1.5).
V
is [un]assigned after a && b
(§15.23) when true iff V is [un]assigned
after b when true.
V
is [un]assigned after a && b when false iff V is
[un]assigned after a when false and V is [un]assigned after
b when false.
V
is [un]assigned before a iff V is [un]assigned before a
&& b.
V
is [un]assigned before b iff V is [un]assigned after a when
true.
V
is [un]assigned after a && b iff V is [un]assigned
after a && b when true and V is [un]assigned after
a && b when false.
V
is [un]assigned after a || b
(§15.24) when true iff V is [un]assigned
after a when true and V is [un]assigned after b when
true.
V
is [un]assigned after a || b when false iff V is
[un]assigned after b when false.
V
is [un]assigned before a iff V is [un]assigned before a
|| b.
V
is [un]assigned before b iff V is [un]assigned after a when
false.
V
is [un]assigned after a || b iff V is [un]assigned
after a || b when true and V is [un]assigned after a
|| b when false.
V
is [un]assigned after !a (§15.15.6)
when true iff V is [un]assigned after a when false.
V
is [un]assigned after !a when false iff V is
[un]assigned after a when true.
V
is [un]assigned after !a iff V is [un]assigned after
!a when true and V is [un]assigned after !a when
false.
This is equivalent to saying that V is
[un]assigned after !a iff V is [un]assigned
after a.
Suppose
that b and c are boolean-valued expressions.
V
is [un]assigned after a ? b : c
(§15.25) when true iff V is [un]assigned
after b when true and V is [un]assigned after c when
true.
V
is [un]assigned after a ? b : c when false iff
V is [un]assigned after b when false and V is [un]assigned
after c when false.
V
is [un]assigned before a iff V is [un]assigned before a
? b : c.
V
is [un]assigned before b iff V is [un]assigned after a when
true.
V
is [un]assigned before c iff V is [un]assigned after a when
false.
V
is [un]assigned after a ? b : c iff V is
[un]assigned after a ? b : c when true and V is
[un]assigned after a ? b : c when false.
Suppose
that b and c are expressions that are not boolean-valued.
V
is [un]assigned after a ? b : c
(§15.25) iff V is [un]assigned after b
and V is [un]assigned after c.
V
is [un]assigned before a iff V is [un]assigned before a
? b : c.
V
is [un]assigned before b iff V is [un]assigned after a when
true.
V
is [un]assigned before c iff V is [un]assigned after a when
false.
Suppose
that e is an expression of type boolean and is not a boolean
constant expression, logical complement expression !a,
conditional-and expression a && b, conditional-or
expression a || b, or conditional expression a ? b
: c.
Consider
an assignment expression a = b, a += b, a
-= b, a *= b, a /= b,
a %= b, a <<= b, a >>=
b, a >>>= b, a &= b, a |=
b, or a ^= b (§15.26).
Note that if a is V and V is not definitely
assigned before a compound assignment such as
a &= b, then a compile-time error will
necessarily occur. The first rule for definite assignment stated above
includes the disjunct "a is V" even for compound assignment
expressions, not just simple assignments, so that V will be
considered to have been definitely assigned at later points in the
code. Including the disjunct "a is V" does not affect the binary
decision as to whether a program is acceptable or will result in a
compile-time error, but it affects how many different points in the
code may be regarded as erroneous, and so in practice it can improve
the quality of error reporting. A similar remark applies to the
inclusion of the conjunct "a is not V" in the first rule for
definite unassignment stated above.
V
is definitely assigned after ++a
(§15.15.1), --a
(§15.15.2), a++
(§15.14.2), or a--
(§15.14.3) iff either a is V or V is
definitely assigned after the operand expression.
V
is definitely unassigned after ++a, --a,
a++, or a-- iff a is not V and V is
definitely unassigned after the operand expression.
V
is [un]assigned before a iff V is [un]assigned before
++a, --a, a++, or
a--.
If an
expression is not a boolean constant expression, and is not a
preincrement expression ++a, predecrement expression
--a, postincrement expression a++, postdecrement
expression a--, logical complement expression !a,
conditional-and expression a && b, conditional-or
expression a || b, conditional expression a ? b
: c, or assignment expression, then the following rules
apply:
If the expression has no subexpressions, V is [un]assigned after the expression iff V is [un]assigned before the expression.
This case applies to literals, names, this
(both qualified and unqualified), unqualified class instance
creation expressions with no arguments, initialized array creation
expressions whose initializers contain no expressions, unqualified
superclass field access expressions, named method invocations with
no arguments, and unqualified superclass method invocations with
no arguments.
If the expression has subexpressions, V is [un]assigned after the expression iff V is [un]assigned after its rightmost immediate subexpression.
There is a piece of subtle reasoning behind the
assertion that a variable V can be known to be definitely unassigned
after a method invocation. Taken by itself, at face value and without
qualification, such an assertion is not always true, because an
invoked method can perform assignments. But it must be remembered
that, for the purposes of the Java programming language, the concept of definite
unassignment is applied only to blank final variables. If V is a
blank final local variable, then only the method to which its
declaration belongs can perform assignments to V. If V is a blank
final field, then only a constructor or an initializer for the class
containing the declaration for V can perform assignments to V; no
method can perform assignments to V. Finally, explicit constructor
invocations (§8.8.7.1) are handled specially
(§16.9); although they are syntactically similar
to expression statements containing method invocations, they are not
expression statements and therefore the rules of this section do not
apply to explicit constructor invocations.
For any
immediate subexpression y of an expression x, V is [un]assigned
before y iff one of the following is true:
y is the leftmost immediate
subexpression of x and V is [un]assigned before x.
y is the right-hand operand of a
binary operator and V is [un]assigned after the left-hand
operand.
x is an array access, y is the
subexpression within the brackets, and V is [un]assigned after
the subexpression before the brackets.
x is a primary method invocation
expression, y is the first argument expression in the method
invocation expression, and V is [un]assigned after the primary
expression that computes the target object.
x is a method invocation expression
or a class instance creation expression; y is an argument
expression, but not the first; and V is [un]assigned after the
argument expression to the left of y.
x is a qualified class instance
creation expression, y is the first argument expression in the
class instance creation expression, and V is [un]assigned after
the primary expression that computes the qualifying object.
x is an array instance creation
expression; y is a dimension expression, but not the first; and
V is [un]assigned after the dimension expression to the left of
y.
x is an array instance creation
expression initialized via an array initializer; y is the array
initializer in x; and V is [un]assigned after the dimension
expression to the left of y.
V is [un]assigned after an empty statement (§14.6) iff it is [un]assigned before the empty statement.
A blank
final member field V is definitely assigned (and moreover is
not definitely unassigned) before the block
(§14.2) that is the body of any method in the
scope of V and before the declaration of any class
declared within the scope of V.
A local variable V is definitely unassigned (and moreover is not definitely assigned) before the block that is the body of the constructor, method, instance initializer or static initializer that declares V.
Let C be a class declared within the scope of V. Then V is definitely assigned before the block that is the body of any constructor, method, instance initializer, or static initializer declared in C iff V is definitely assigned before the declaration of C.
Note that there are no rules that would allow us to conclude that V is definitely unassigned before the block that is the body of any constructor, method, instance initializer, or static initializer declared in C. We can informally conclude that V is not definitely unassigned before the block that is the body of any constructor, method, instance initializer, or static initializer declared in C, but there is no need for such a rule to be stated explicitly.
V is [un]assigned after an empty block iff V is [un]assigned before the empty block.
V is [un]assigned after a non-empty block iff V is [un]assigned after the last statement in the block.
V is [un]assigned before the first statement of the block iff V is [un]assigned before the block.
V is [un]assigned before any other statement S of the block iff V is [un]assigned after the statement immediately preceding S in the block.
We say that V is definitely unassigned everywhere in a block B iff:
These conditions are counterintuitive and require
some explanation. Consider a simple assignment V = e. If
V is definitely assigned after e, then either:
The assignment occurs in dead code, and V is vacuously definitely assigned. In this case, the assignment will not actually take place, and we can assume that V is not being assigned by the assignment expression. Or:
V was already assigned by an earlier
expression prior to e. In this case the current assignment will
cause a compile-time error.
So, we can conclude that if the conditions are met by a program that causes no compile time error, then any assignments to V in B will not actually take place at run time.
V is [un]assigned after a local class declaration statement (§14.3) iff V is [un]assigned before the local class declaration statement.
V is [un]assigned after a local variable declaration statement (§14.4) that contains no variable initializers iff V is [un]assigned before the local variable declaration statement.
V is definitely assigned after a local variable declaration statement that contains at least one variable initializer iff either V is definitely assigned after the last variable initializer in the local variable declaration statement or the last variable initializer in the declaration is in the declarator that declares V.
V is definitely unassigned after a local variable declaration statement that contains at least one variable initializer iff V is definitely unassigned after the last variable initializer in the local variable declaration statement and the last variable initializer in the declaration is not in the declarator that declares V.
V is [un]assigned before the first variable initializer in a local variable declaration statement iff V is [un]assigned before the local variable declaration statement.
V
is definitely assigned before any variable initializer e other
than the first one in the local variable declaration statement iff
either V is definitely assigned after the variable initializer
to the left of e or the initializer expression to the left of
e is in the declarator that declares V.
V
is definitely unassigned before any variable initializer e other
than the first one in the local variable declaration statement iff
V is definitely unassigned after the variable initializer to the
left of e and the initializer expression to the left of e is
not in the declarator that declares V.
V
is [un]assigned after a labeled statement L : S (where
L is a label) (§14.7) iff V is
[un]assigned after S and V is [un]assigned before every
break statement that may exit the labeled statement L :
S.
V
is [un]assigned before S iff V is [un]assigned before L
: S.
V
is [un]assigned after an expression statement e;
(§14.8) iff it is [un]assigned after
e.
V
is [un]assigned before e iff it is [un]assigned before
e;.
The
following rules apply to a statement if (
(§14.9.1):
e) S
The
following rules apply to a statement if (e) S else
T (§14.9.2):
V
is [un]assigned after if ( iff
V is [un]assigned after S and V is [un]assigned after
T.
e) S else T
V
is [un]assigned before e iff V is [un]assigned
before if (.
e) S else T
V
is [un]assigned before S iff V is [un]assigned after e when
true.
V
is [un]assigned before T iff V is [un]assigned after e when
false.
The
following rules apply both to a statement assert
and to a statement e1assert (§14.10):
e1 :
e2
V
is [un]assigned before e1 iff V is
[un]assigned before the assert statement.
V
is definitely assigned after the assert statement iff V is
definitely assigned before the assert statement.
V
is definitely unassigned after the assert statement iff V is
definitely unassigned before the assert statement and V is
definitely unassigned after e1 when true.
V
is [un]assigned after a switch statement
(§14.11) iff all of the following are
true:
Either there is a default label
in the switch block or V is [un]assigned after the switch
expression.
Either there are no switch labels
in the switch block that do not begin a block-statement-group
(that is, there are no switch labels immediately before the
"}" that ends the switch block) or V is
[un]assigned after the switch expression.
Either the switch block contains no block-statement-groups or V is [un]assigned after the last block-statement of the last block-statement-group.
V is [un]assigned before every
break statement that may exit the switch statement.
V
is [un]assigned before the switch expression iff V is
[un]assigned before the switch statement.
If a switch block contains at least one block-statement-group, then the following rules also apply:
V is [un]assigned before the first block-statement of the first block-statement-group in the switch block iff V is [un]assigned after the switch expression.
V is [un]assigned before the first block-statement of any block-statement-group other than the first iff V is [un]assigned after the switch expression and V is [un]assigned after the preceding block-statement.
V is [un]assigned
after while (
(§14.12) iff V is [un]assigned after e) Se
when false and V is [un]assigned before every break statement
for which the while statement is the break target.
V is definitely assigned before e
iff V is definitely assigned before the while
statement.
V is definitely unassigned before e
iff all of the following are true:
V is [un]assigned before S iff V
is [un]assigned after e when true.
V is [un]assigned after do
S while ( (§14.13) iff V
is [un]assigned after e);e when false and V is [un]assigned
before every break statement for which the do statement is the
break target.
V is definitely assigned before S
iff V is definitely assigned before the do statement.
V is definitely unassigned before S iff all of the following are true:
V is [un]assigned before e iff V
is [un]assigned after S and V is [un]assigned before every
continue statement for which the do statement is the continue
target.
The
rules herein cover the basic for statement
(§14.14.1). Since the enhanced for statement
(§14.14.2) is defined by translation to a basic
for statement, no special rules need to be provided for it.
V is [un]assigned after a for
statement iff both of the following are true:
V is [un]assigned before the
initialization part of the for statement iff V is [un]assigned
before the for statement.
V is definitely assigned before the
condition part of the for statement iff V is definitely
assigned after the initialization part of the for
statement.
V is definitely unassigned before the
condition part of the for statement iff all of the following
are true:
V is definitely unassigned
after the initialization part of the for statement.
Assuming V is definitely
unassigned before the condition part of the for statement,
V is definitely unassigned after the contained
statement.
Assuming V is definitely
unassigned before the contained statement, V is definitely
unassigned before every continue statement for which the
for statement is the continue target.
V is [un]assigned before the contained statement iff either of the following is true:
V is [un]assigned before the
incrementation part of the for statement iff V is [un]assigned
after the contained statement and V is [un]assigned before every
continue statement for which the for statement is the continue
target.
If the initialization part of the
for statement is a local variable declaration statement, the
rules of §16.2.4 apply.
Otherwise, if the initialization part is empty, then V is [un]assigned after the initialization part iff V is [un]assigned before the initialization part.
V is [un]assigned after the initialization part iff V is [un]assigned after the last expression statement in the initialization part.
V is [un]assigned before the first expression statement in the initialization part iff V is [un]assigned before the initialization part.
V is [un]assigned before an expression statement S other than the first in the initialization part iff V is [un]assigned after the expression statement immediately preceding S.
If the incrementation part of the
for statement is empty, then V is [un]assigned after the
incrementation part iff V is [un]assigned before the
incrementation part.
V is [un]assigned after the incrementation part iff V is [un]assigned after the last expression statement in the incrementation part.
V is [un]assigned before the first expression statement in the incrementation part iff V is [un]assigned before the incrementation part.
V is [un]assigned before an expression statement S other than the first in the incrementation part iff V is [un]assigned after the expression statement immediately preceding S.
By
convention, we say that V is [un]assigned after any break,
continue, return, or throw statement
(§14.15, §14.16,
§14.17, §14.18).
The notion that a variable is "[un]assigned
after" a statement or expression really means "is [un]assigned
after the statement or expression completes normally". Because a
break, continue, return, or throw statement never
completes normally, it vacuously satisfies this notion.
In
a return statement with an expression e or a throw statement
with an expression e, V is [un]assigned before e iff V is
[un]assigned before the return or throw statement.
V is [un]assigned
after synchronized (
(§14.19) iff V is [un]assigned after
S.
e) S
V is [un]assigned before e iff V
is [un]assigned before the statement synchronized (.
e)
S
These
rules apply to every try statement (§14.20),
whether or not it has a finally block:
V is [un]assigned before the try
block iff V is [un]assigned before the try statement.
V is definitely assigned before a
catch block iff V is definitely assigned before the try
block.
V is definitely unassigned before a
catch block iff all of the following are true:
V is definitely unassigned
before every return statement that belongs to the try
block.
V is definitely unassigned
after e in every statement of the form throw e that
belongs to the try block.
V is definitely
unassigned after every assert statement that occurs in the
try block.
V is definitely unassigned
before every break statement that belongs to the try block
and whose break target contains (or is) the try
statement.
V is definitely unassigned
before every continue statement that belongs to the try
block and whose continue target contains the try
statement.
If a
try statement does not have a finally block, then this rule also
applies:
If a
try statement does have a finally block, then these rules also
apply:
V is definitely assigned after the
try statement iff at least one of the following is true:
V is definitely assigned before the
finally block iff V is definitely assigned before the try
statement.
V is definitely unassigned before the
finally block iff all of the following are true:
V is definitely unassigned
before every return statement that belongs to the try
block.
V is definitely unassigned
after e in every statement of the form throw e that
belongs to the try block.
V is definitely
unassigned after every assert statement that occurs in the
try block.
V is definitely unassigned
before every break statement that belongs to the try block
and whose break target contains (or is) the try
statement.
V is definitely unassigned
before every continue statement that belongs to the try
block and whose continue target contains the try
statement.
V is definitely unassigned
after every catch block of the try statement.
A formal parameter V of a method or constructor (§8.4.1, §8.8.1) is definitely assigned (and moreover is not definitely unassigned) before the body of the method or constructor.
An
exception parameter V of a catch clause
(§14.20) is definitely assigned (and moreover
is not definitely unassigned) before the body of the catch
clause.
V is [un]assigned after an empty array initializer (§10.6) iff V is [un]assigned before the empty array initializer.
V is [un]assigned after a non-empty array initializer iff V is [un]assigned after the last variable initializer in the array initializer.
V is [un]assigned before the first variable initializer of the array initializer iff V is [un]assigned before the array initializer.
V
is [un]assigned before any other variable initializer e of the
array initializer iff V is [un]assigned after the variable
initializer to the left of e in the array initializer.
The rules determining when a variable is definitely assigned or definitely unassigned before an enum constant (§8.9.1) are given in §16.8.
This is because an enum constant is essentially a
static final field (§8.3.1.1,
§8.3.1.2) that is initialized with a class
instance creation expression (§15.9).
V is definitely assigned before the declaration of a class body of an enum constant with no arguments that is declared within the scope of V iff V is definitely assigned before the enum constant.
V is definitely assigned before the declaration of a class body of an enum constant with arguments that is declared within the scope of V iff V is definitely assigned after the last argument expression of the enum constant
The definite assignment/unassignment status of any construct within the class body of an enum constant is governed by the usual rules for classes.
V is definitely assigned before an anonymous class declaration (§15.9.5) that is declared within the scope of V iff V is definitely assigned after the class instance creation expression that declares the anonymous class.
It should be clear that if an anonymous class is implicitly defined by an enum constant, the rules of §16.5 apply.
Let C be a class declared within the scope of V. Then:
V is definitely assigned before an enum constant (§8.9.1) or static variable initializer (§8.3.2) of C iff V is definitely assigned before the declaration of C.
Note that there are no rules that would allow us to conclude that V is definitely unassigned before a static variable initializer or enum constant. We can informally conclude that V is not definitely unassigned before any static variable initializer of C, but there is no need for such a rule to be stated explicitly.
Let C be a class, and let V be a blank static final member
field of C, declared in C. Then:
V is definitely unassigned (and moreover is not definitely assigned) before the leftmost enum constant, static initializer (§8.7), or static variable initializer of C.
V is [un]assigned before an enum constant, static initializer, or static variable initializer of C other than the leftmost iff V is [un]assigned after the preceding enum constant, static initializer, or static variable initializer of C.
Let C be
a class, and let V be a blank static final member field of C,
declared in a superclass of C. Then:
V is definitely assigned (and moreover is not definitely unassigned) before every enum constant of C.
V is definitely assigned (and moreover is not definitely unassigned) before the block that is the body of a static initializer of C.
V is definitely assigned (and moreover is not definitely unassigned) before every static variable initializer of C.
Let C be a class declared within the scope of V. Then:
V is definitely assigned before an instance variable initializer (§8.3.2) of C iff V is definitely assigned before the declaration of C.
Note that there are no rules that would allow us to conclude that V is definitely unassigned before an instance variable initializer. We can informally conclude that V is not definitely unassigned before any instance variable initializer of C, but there is no need for such a rule to be stated explicitly.
Let C be
a class, and let V be a blank final non-static member field of
C, declared in C. Then:
V is definitely unassigned (and moreover is not definitely assigned) before the leftmost instance initializer (§8.6) or instance variable initializer of C.
V is [un]assigned before an instance initializer or instance variable initializer of C other than the leftmost iff V is [un]assigned after the preceding instance initializer or instance variable initializer of C.
The following rules hold within the constructors (§8.8.7) of class C:
V is definitely assigned (and moreover is not definitely unassigned) after an alternate constructor invocation (§8.8.7.1).
V is definitely unassigned (and moreover is not definitely assigned) before an explicit or implicit superclass constructor invocation (§8.8.7.1).
If C has no instance initializers or instance variable initializers, then V is not definitely assigned (and moreover is definitely unassigned) after an explicit or implicit superclass constructor invocation.
If C has at least one instance initializer or instance variable initializer then V is [un]assigned after an explicit or implicit superclass constructor invocation iff V is [un]assigned after the rightmost instance initializer or instance variable initializer of C.
Let C be
a class, and let V be a blank final member field of C, declared
in a superclass of C. Then: