how the compiler decides about the unreachable stmt

[NareshAnkuskani Kumar]

--------------------------------------------------------------------------------
hi guys below i am listing the code plz refer it once

class one
{
public static void main(String a[])
{
System.out.println("first stmt");
return;
System.out.println("second stmt");
}
}
i am very much clear that the above code results in compile time error that
the stmt after return stmt is unreachable.

look at the other code below

class two
{ public static void main(......)
{
try
{
return;
}
catch(.....)
{
}
finally
{
System.out.println("due to finaly");
}
System.out.println("out of try-catch-finally stmt");//3
}
}

in the above code (i.e) the second code why i am not facing the same problem which i came across during the compilation of first code.
because i am very sure that the statement marked '//3' is surely not reachable.

[Edwin Dalorzo]

SHORT ANSWER
If an exception happens in the try block, and you handle it, and manage to pass to the next statement out of the try-catch-finally block, then that statement is actually reachable.

public class Test {
public static void main(String args[]) {
doMath(5,0);
}

static int doMath(int a, int b){
try{
return a / b;
}catch(RuntimeException e){
System.out.println("Exception handled");
}
System.out.println("This is reachable");
return 0;
}
}

Not the case of your first example, pal.

Regards,
Edwin Dalorzo

LONG ANSWER

I have an interpretation of this behavior based on the explanations of the JLS chapter 14 section 21.

The JLS determines if a statement is reachable based on two principles

Whether the statement is reachable.

Whether the statement can complete normally.

Now, the JLS states that a break, continue, return, or throw statement cannot complete normally.

Now, this sounds kind of obvious for some statements.

Interpretation #1
That means any statement after any of this statements could be considered unreachable.

Evidence #1

myBlock:
{
break myBlock;
System.out.println("You�ll never get me"); //Unreachable statement.
 
}

for(int i=0;i<10;i++)
{
continue;
System.out.println("You�ll never get me either"); //Unreachable statement.
}

Interpretation #2
Then, JSL suggests that a finally block is reachable if its try statement is reachable. So in this code the finally statement is reachable, no matter that the try block finish abnormally.

Evidence #2

try
{
return;
}
finally
{
System.out.println("Here I am");
}

However, the JLS also states that �[�] Every other statement S in a nonempty block that is not a switch block is reachable if and only if the preceding statement S can complete normally�

Interpretation #3
As the try block in previous statement did not complete normally, the next statement after the try-finally block is unreachable, as this evidence shows.

Evidence #3

try
{
return;
}
finally
{
System.out.println("Here I am");
}
System.out.println("You'll never get me"); //Unreachable statement.

Finally, the JLS states that a Catch block is reachable "[...] if some expression or a throw statement in the try block is reachable and can throw an exception whose type is assignable to the parameter of the catch clause [...]"

Interpretation #4
This statement seems to be override the previous one, because althought the return stament always finish abnormally it might be subject to cause an exception in a try-catch-block, and hence the next evidence is valid, because the cath block is subjet to finish normally if you handle the exception, and if so, the statement after try-catch-finally may be executed. Let's see some evidence

Evidence #4

public class Test {
public static void main(String args[]) {
doMath(5,0);
}

static int doMath(int a, int b){
try{
return a / b;
}catch(RuntimeException e){
System.out.println("Exception handled");
}
System.out.println("This is reachable");
return 0;
}
}

I hope this helps.

Regards,
Edwin Dalorzo

[Murali Mohan]

Thank you Edwin Dalorzo.
Great explanation...

[shandilya popuru]

hats off to your explanation Edwin Dalorzo