BLOCKS AND STATEMENTS

Unreachable Statements

14.19

14.19   Unreachable Statements

That looks like a path.

Is that the way to reach the top from here?

 Robert Frost,

The Mountain

 (1915)

It is a compile time error if a statement cannot be executed because it is

unreach 

able

. Every Java compiler must carry out the conservative flow analysis specified

here to make sure all statements are reachable.

This section is devoted to a precise explanation of the word  reachable.  The

idea is that there must be some possible execution path from the beginning of the

constructor, method, or static initializer that contains the statement to the state 

ment itself. The analysis takes into account the structure of statements. Except for

the special treatment of

while

,

do

, and

for

 statements whose condition expres 

sion has the constant value

true

, the values of expressions are not taken into

account in the flow analysis. For example, a Java compiler will accept the code:

{

int n = 5;

while (n > 7) n = 2;

}

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

 can never be executed. A Java

compiler must operate according to the rules laid out in this section.

The rules in this section define two technical terms:

whether a statement is

reachable

whether a statement

can complete normally

The definitions here allow a statement to complete normally only if it is reachable.

To shorten the description of the rules, the customary abbreviation  iff  is

used to mean  if and only if. 

The rules are as follows:

The block that is the body of a constructor, method, or static initializer is

reachable.

An empty block that is not a switch block can complete normally iff it is

reachable. A nonempty block that is not a switch block can complete nor 

mally iff the last statement in it can complete normally. The first statement in

a nonempty block that is not a switch block is reachable iff the block is reach 

able. Every other statement

S

 in a nonempty block that is not a switch block is

reachable iff the statement preceding

S

 can complete normally.

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