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?

Answer 1

String[] myArray;

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

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

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.