#ifdef #ifndef in Java

private static final boolean enableFast = false;

// ...
if (enableFast) {
  // This is removed at compile time
}

Conditionals like that shown above are evaluated at compile time. If instead you use this

private static final boolean enableFast = "true".equals(System.getProperty("fast"));

Then any conditions dependent on enableFast will be evaluated by the JIT compiler. The overhead for this is negligible.

I think that I've found the solution, It's much simpler.
If I define the boolean variables with "final" modifier Java compiler itself solves the problem. Because it knows in advance what would be the result of testing this condition. For example this code:

    boolean flag1 = true;
    boolean flag2 = false;
    int j=0;
    for(int i=0;i<1000000000;i++){
        if(flag1)
            if(flag2)
                j++;
            else
                j++;
        else
            if(flag2)
                j++;
            else
                j++;
    }

runs about 3 seconds on my computer.
And this one

    final boolean flag1 = true;
    final boolean flag2 = false;
    int j=0;
    for(int i=0;i<1000000000;i++){
        if(flag1)
            if(flag2)
                j++;
            else
                j++;
        else
            if(flag2)
                j++;
            else
                j++;
    }

runs about 1 second. The same time this code takes

    int j=0;
    for(int i=0;i<1000000000;i++){
        j++;
    }

javac will not output compiled code that is unreachable. Use a final variable set to a constant value for your #define and a normal if statement for the #ifdef.

You can use javap to prove that the unreachable code isn't included in the output class file. For example, consider the following code:

public class Test
{
   private static final boolean debug = false;

   public static void main(String[] args)
   {
       if (debug) 
       {
           System.out.println("debug was enabled");
       }
       else
       {
           System.out.println("debug was not enabled");
       }
   }
}

javap -c Test gives the following output, indicating that only one of the two paths was compiled in (and the if statement wasn't):

public static void main(java.lang.String[]);
  Code:
   0:   getstatic       #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #3; //String debug was not enabled
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return