Java solution for C++ style compiler directive

Question

I have a Java array:

    String[] myArray = {"1", "2"};

Depending on a condition that is known at compile time I would like to assign different values:

    String[] myArray = {"A", "B", "C"};

In C++ I would use something like

#ifdef ABC
  // ABC stuff here
#else
  // 123 stuff here
#endif

but what to do in Java?

Not sure, but i think you can pass such things to the jvm using -Dname=value and then use System.getProperty to gets the value of name. — Johannes Schaub - litb
– Johannes Schaub - litb, Commented Jul 20, 2009 at 13:18

dfa · Accepted Answer · 2009-07-20 13:34:15Z

13

class Foo {

   static final boolean ABC = true;

   public void someMehod() {
       if (ABC) {  // #ifdef ABC

       } else {    // #else

       }           // #endif
   } 
}

since ABC is both static and final the compiler evaluates it at compile-time, effectively acting like a pre-processor.

answered Jul 20, 2009 at 13:34

dfa

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1 Comment

James Schek Over a year ago

Use libt's idea from the comments. use System.getProperty("...") to assign ABC.

jjnguy · Accepted Answer · 2009-07-20 14:19:53Z

3

In Java you would need to do that at runtime.

String[] myArray;
if (something)
    myArray = new String[]{"A", "B"};
else
    myArray = new String[]{"A", "B, "C"}

This is going to be the most similar to C++ code. And, if your condition is guaranteed to be true at compile time, Java will optimize out the call.

There are other options available, but they will look nothing like a C++ version.

edited Jul 20, 2009 at 14:19

answered Jul 20, 2009 at 13:17

jjnguy

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4 Comments

MRC Over a year ago

Thanks, this will definitely work - will the compiler optimize away the else part if "something" is always true??

jjnguy Over a year ago

Actually yes. If you have that something so it is always true, it will optimize away the if.

jjnguy Over a year ago

Of course...I totally deserved the downvote. Thanks for noticing!!

MRC Over a year ago

Using this I got "Array constants can only be used in initializers", but myArray = new String[]{"A", "B"}; resolved the issue.

Gregory Mostizky · Accepted Answer · 2009-07-20 13:18:32Z

1

One approach is to re-generate a Constants.java file in the first phase of your build script (ant,maven or whatever else you use). Depending on how complex that file is you can do it manually or use something more heavy like Velocity.

I used something similar in the past to put version and build info inside java classes.

answered Jul 20, 2009 at 13:18

Gregory Mostizky

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Comments

jjnguy · Accepted Answer · 2009-07-20 14:20:32Z

1

String[] myArray;

if (ABC)
myArray = ...
else
myArray = ...

ABC is static final variable, JVM is required to inline condition check.

edited Jul 20, 2009 at 14:20

jjnguy

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answered Jul 20, 2009 at 13:17

stepancheg

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Comments

Jeff Knecht · Accepted Answer · 2009-07-21 19:27:55Z

1

Use a template engine (something like Velocity) to execute a preprocessing step. Your build script could run the template applying properties from the build environment to output Java source code that is ultimately compiled. This is, essentially, what C/C++ preprocessors do.

My guess is that someone has already coded an ant task to do this, but I'm too lazy to Google it for you.

answered Jul 21, 2009 at 19:27

Jeff Knecht

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Comments

robert · Accepted Answer · 2012-06-03 14:25:26Z

1

If you use a var/const as a custom compiler directive in Java don't make it final. You might get lots of warnings because of conditions being "always true" and stuff.

public static boolean ABC = true; // or false

edited Jun 3, 2012 at 14:25

robert

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answered May 6, 2012 at 13:42

miniMe

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1 Comment

sharakan Over a year ago

Fair point about the warnings, but if you don't have that variable be final then the compiler can't optimize away the if/else block.

Jens Erat · Accepted Answer · 2013-04-11 12:11:11Z

You can't really do it...the best way would be to use a template engine...even some compilers will put the code into the end class file - the better ones would remove the code if it is not required...You must make the constants final and also you must set them to an absolute value for the compiler to be able to remove the code.

public static final boolean ABC = true // This will be enough for the compiler to remove code

public static final boolean ABC = [any method call] // This is not enough to work and will leave the code in there

Again, some compilers may still put the code in there even though it is unreachable (that is why some IDE's will warn you that the code is unreachable so that you remove the code that is not needed)

However,

Directives are nasty and most likely you need to be using an implementation of an interface instead of a directive! good for low level coding when needed (to minimize memory foot print) - certain special devices like pace makers and the like but why would you want to use them in java?

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