Why does Java have an unreachable statement compiler error

Question

I often find when debugging a program it is convenient   although arguably bad practice  to insert a return statement inside a block of code  I might try something like this in Java       class Test           public static void main String args                      System out println  hello world                    return                  System out println  i think this line might cause a problem                  of course  this would yield the compiler error      Test java 7  unreachable statement   I could understand why a warning might be justified as having unused code is bad practice  But I don t understand why this needs to generate an error    Is this just Java trying to be a Nanny  or is there a good reason to make this a compiler error

User · Answer

It is certainly a good thing to complain the more stringent the compiler is the better  as far as it allows you to do what you need  Usually the small price to pay is to comment the code out  the gain is that when you compile your code works  A general example is Haskell about which people screams until they realize that their test debugging is main test only and short one  I personally in Java do almost no debugging while being   in fact on purpose  not attentive

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While I think this compiler error is a good thing  there is a way you can work around it  Use a condition you know will be true   public void myMethod         someCodeHere         if 1  lt  2  return     compiler isn t smart enough to complain about this      moreCodeHere         The compiler is not smart enough to complain about that

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Because unreachable code is meaningless to the compiler  Whilst making code meaningful to people is both paramount and harder than making it meaningful to a compiler  the compiler is the essential consumer of code  The designers of Java take the viewpoint that code that is not meaningful to the compiler is an error  Their stance is that if you have some unreachable code  you have made a mistake that needs to be fixed   There is a similar question here  Unreachable code  error or warning   in which the author says  Personally I strongly feel it should be an error  if the programmer writes a piece of code  it should always be with the intention of actually running it in some scenario   Obviously the language designers of Java agree   Whether unreachable code should prevent compilation is a question on which there will never be consensus  But this is why the Java designers did it     A number of people in comments point out that there are many classes of unreachable code Java doesn t prevent compiling  If I understand the consequences of G  del correctly  no compiler can possibly catch all classes of unreachable code   Unit tests cannot catch every single bug  We don t use this as an argument against their value  Likewise a compiler can t catch all problematic code  but it is still valuable for it to prevent compilation of bad code when it can   The Java language designers consider unreachable code an error  So preventing it compiling when possible is reasonable      Before you downvote  the question is not whether or not Java should have an unreachable statement compiler error  The question is why Java has an unreachable statement compiler error  Don t downvote me just because you think Java made the wrong design decision

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I only just noticed this question  and wanted to add my   02 to this   In case of Java  this is not actually an option  The  unreachable code  error doesn t come from the fact that JVM developers thought to protect developers from anything  or be extra vigilant  but from the requirements of the JVM specification   Both Java compiler  and JVM  use what is called  stack maps  - a definite information about all of the items on the stack  as allocated for the current method  The type of each and every slot of the stack must be known  so that a JVM instruction doesn t mistreat item of one type for another type  This is mostly important for preventing having a numeric value ever being used as a pointer  It s possible  using Java assembly  to try to push store a number  but then pop load an object reference  However  JVM will reject this code during class validation - that is when stack maps are being created and tested for consistency   To verify the stack maps  the VM has to walk through all the code paths that exist in a method  and make sure that no matter which code path will ever be executed  the stack data  for every instruction agrees with what any previous code has pushed stored in the stack  So  in simple case of   Object a  if  something    a   new Object      else   a   new String      System out println a     at line 3  JVM will check that both branches of  if  have only stored into a  which is just local var 0  something that is compatible with Object  since that s how code from line 3 and on will treat local var 0    When compiler gets to an unreachable code  it doesn t quite know what state the stack might be at that point  so it can t verify its state  It can t quite compile the code anymore at that point  as it can t keep track of local variables either  so instead of leaving this ambiguity in the class file  it produces a fatal error   Of course a simple condition like if  1 lt 2  will fool it  but it s not really fooling - it s giving it a potential branch that can lead to the code  and at least both the compiler and the VM can determine  how the stack items can be used from there on   P S  I don t know what  NET does in this case  but I believe it will fail compilation as well  This normally will not be a problem for any machine code compilers  C  C    Obj-C  etc

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One of the goals of compilers is to rule out classes of errors   Some unreachable code is there by accident  it s nice that javac rules out that class of error at compile time   For every rule that catches erroneous code  someone will want the compiler to accept it because they know what they re doing   That s the penalty of compiler checking  and getting the balance right is one of the tricker points of language design   Even with the strictest checking there s still an infinite number of programs that can be written  so things can t be that bad

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There is no definitive reason why unreachable statements must be not be allowed  other languages allow them without problems  For your specific need  this is the usual trick   if  true  return    It looks nonsensical  anyone who reads the code will guess that it must have been done deliberately  not a careless mistake of leaving the rest of  statements unreachable   Java has a little bit support for  conditional compilation   http   java sun com docs books jls third edition html statements html 14 21   if  false    x 3          does not result in a compile-time   error  An optimizing compiler may   realize that the statement x 3  will   never be executed and may choose to   omit the code for that statement from   the generated class file  but the   statement x 3  is not regarded as    unreachable  in the technical sense   specified here       The rationale for this differing   treatment is to allow programmers to   define  flag variables  such as   static final boolean DEBUG   false        and then write code such as   if  DEBUG    x 3          The idea is that it should be possible   to change the value of DEBUG from   false to true or from true to false   and then compile the code correctly   with no other changes to the program   text

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It is Nanny   I feel  Net got this one right - it raises a warning for unreachable code  but not an error  It is good to be warned about it  but I see no reason to prevent compilation  especially during debugging sessions where it is nice to throw a return in to bypass some code

User · Answer

If the reason for allowing if  aBooleanVariable  return  someMoreCode  is to allow flags  then the fact that if  true  return  someMoreCode  does not generate a compile time error seems like inconsistency in the policy of generating CodeNotReachable exception  since the compiler  knows  that true is not a flag  not a variable    Two other ways which might be interesting  but don t apply to switching off part of a method s code as well as if  true  return   Now  instead of saying if  true  return  you might want to say assert false and add -ea OR -ea package OR -ea className to the jvm arguments  The good point is that this allows for some granularity and requires adding an extra parameter to the jvm invocation so there is no need of setting a DEBUG flag in the code  but by added argument at runtime  which is useful when the target is not the developer machine and recompiling  amp  transferring bytecode takes time   There is also the System exit 0  way  but this might be an overkill  if you put it in Java in a JSP then it will terminate the server   Apart from that Java is by-design a  nanny  language  I would rather use something native like C C   for more control

[java] Why does Java have an "unreachable statement" compiler error?

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