Java: How will JVM optimise the call to a void and empty function?

Question

Let\'s imagine we have the following classes:

public class Message extends Object {}

public class Logger implements ILogger {
 public void log(Message m) {/         


        

Answer 1

Disassembling the following file (with javap -c) suggests they are not stripped out by the 1.7.0 compiler when compiling down to bytecode:

public class Program
{
    public static class Message extends Object {}

    public interface ILogger {
        void log(Message m);
    }

    public static class Logger implements ILogger {
        public void log(Message m) { /* empty */ }
    }

    public static void main(String[] args) {
        ILogger l = new Logger();
        l.log((Message)null); // a)
        l.log(new Message()); // b)
    }
}

The result is below. The key bits are the invokes on lines 13 and 26.

Compiled from "Program.java"
public class Program {
  public Program();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class Program$Logger
       3: dup
       4: invokespecial #3                  // Method Program$Logger."":()V
       7: astore_1
       8: aload_1
       9: aconst_null
      10: checkcast     #4                  // class Program$Message
      13: invokeinterface #5,  2            // InterfaceMethod Program$ILogger.log:(LProgram$Message;)V
      18: aload_1
      19: new           #4                  // class Program$Message
      22: dup
      23: invokespecial #6                  // Method Program$Message."":()V
      26: invokeinterface #5,  2            // InterfaceMethod Program$ILogger.log:(LProgram$Message;)V
      31: return
}

EDIT: However, as @mikera pointed out, it's likely that the JIT compiler will do further optimizations when the program runs, which may be able to eliminate the calls. I don't know enough about the details to comment on that unfortunately.

SIDE NOTE: You might be interested in this link, which deals with performance techniques used by the Hotspot JVM:

https://wikis.oracle.com/display/HotSpotInternals/PerformanceTechniques