What does if(false){some code} mean in java

Question

Java code

if(x == null){
 //some code
}
if(false){
 //some code
}

when is if(false){code} executed?

Answer 1

It is never executed. Sometimes people do it when they have some old code they want to remember, or some new code that should not yet be used. like

if(false){fancyNewFunction();}

(as far as i'm concerned, this is bad form and you should not do it, but that doesn't mean it doesn't happen ;) )

Answer 2

This could also be a common way to emulate macro preprocessor directives like #ifdefine. Some people use it to enable or disable logging.

For instance the following code:

public class Sample{

    private static final boolean LOG_ENABLED = true;

    public static void main(String args[]){
        if(LOG_ENABLED){
            System.out.println("Hello World");
        }
    }
}

Produces the following bytecodes:

public class Sample extends java.lang.Object{
public Sample();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc #3; //String Hello World
   5:   invokevirtual   #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return

}

If you disable the flag, you get this bytecodes:

public class Sample extends java.lang.Object{
public Sample();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[]);
  Code:
   0:   return

}

As you can see, no bytecodes were generated for the second case in the main method. So, if you disable logging and recompile the code, you improve underlying bytecodes.