Java for loop performance question

Question

considering this example:

public static void main(final String[] args) {
    final List myList = Arrays.asList(\"A\", \"B\", \"C\", \"D\");
            


        

Answer 1

Personally, I don't think you can draw any meaningful conclusions from a contrived example like this.

But if you really want to know, why not use javap to decompile the code and see what's different? Why guess about what the compiler's doing when you can see for yourself without asking here?

Byte code for the first case:

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

public static void main(java.lang.String[]);
  Code:
   0:   iconst_4
   1:   anewarray       #2; //class java/lang/String
   4:   dup
   5:   iconst_0
   6:   ldc     #3; //String A
   8:   aastore
   9:   dup
   10:  iconst_1
   11:  ldc     #4; //String B
   13:  aastore
   14:  dup
   15:  iconst_2
   16:  ldc     #5; //String C
   18:  aastore
   19:  dup
   20:  iconst_3
   21:  ldc     #6; //String D
   23:  aastore
   24:  invokestatic    #7; //Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List
   27:  astore_1
   28:  invokestatic    #8; //Method java/lang/System.currentTimeMillis:()J
   31:  lstore_2
   32:  ldc     #9; //int 1000000
   34:  istore  4
   36:  iload   4
   38:  aload_1
   39:  invokeinterface #10,  1; //InterfaceMethod java/util/List.size:()I
   44:  if_icmple       61
   47:  getstatic       #11; //Field java/lang/System.out:Ljava/io/PrintStream;
   50:  ldc     #12; //String Hello
   52:  invokevirtual   #13; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   55:  iinc    4, -1
   58:  goto    36
   61:  invokestatic    #8; //Method java/lang/System.currentTimeMillis:()J
   64:  lstore  4
   66:  getstatic       #11; //Field java/lang/System.out:Ljava/io/PrintStream;
   69:  new     #14; //class java/lang/StringBuilder
   72:  dup
   73:  invokespecial   #15; //Method java/lang/StringBuilder."":()V
   76:  ldc     #16; //String Finish:
   78:  invokevirtual   #17; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/la
   81:  lload   4
   83:  lload_2
   84:  lsub
   85:  invokevirtual   #18; //Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
   88:  invokevirtual   #19; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   91:  invokevirtual   #13; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   94:  return
}

Byte code for the second case:

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

public static void main(java.lang.String[]);
  Code:
   0:   iconst_4
   1:   anewarray       #2; //class java/lang/String
   4:   dup
   5:   iconst_0
   6:   ldc     #3; //String A
   8:   aastore
   9:   dup
   10:  iconst_1
   11:  ldc     #4; //String B
   13:  aastore
   14:  dup
   15:  iconst_2
   16:  ldc     #5; //String C
   18:  aastore
   19:  dup
   20:  iconst_3
   21:  ldc     #6; //String D
   23:  aastore
   24:  invokestatic    #7; //Method java/util/Arrays.asList:([Ljava/lang/Object;)Ljava/util/List;
   27:  astore_1
   28:  invokestatic    #8; //Method java/lang/System.currentTimeMillis:()J
   31:  lstore_2
   32:  aload_1
   33:  invokeinterface #9,  1; //InterfaceMethod java/util/List.size:()I
   38:  istore  4
   40:  ldc     #10; //int 1000000
   42:  istore  5
   44:  iload   5
   46:  iload   4
   48:  if_icmple       65
   51:  getstatic       #11; //Field java/lang/System.out:Ljava/io/PrintStream;
   54:  ldc     #12; //String Hello
   56:  invokevirtual   #13; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   59:  iinc    5, -1
   62:  goto    44
   65:  invokestatic    #8; //Method java/lang/System.currentTimeMillis:()J
   68:  lstore  5
   70:  getstatic       #11; //Field java/lang/System.out:Ljava/io/PrintStream;
   73:  new     #14; //class java/lang/StringBuilder
   76:  dup
   77:  invokespecial   #15; //Method java/lang/StringBuilder."":()V
   80:  ldc     #16; //String Finish:
   82:  invokevirtual   #17; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   85:  lload   5
   87:  lload_2
   88:  lsub
   89:  invokevirtual   #18; //Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder;
   92:  invokevirtual   #19; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   95:  invokevirtual   #13; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   98:  return
}

There are differences, but I'm not sure I can make a definitive statement about their effect on performance.

I'd code the second one, because it would mean (on the face of it) one method call as opposed to one per loop iteration. I don't know if the compiler can optimize it away, but I'm certain that I can do it pretty easily. So I do, regardless of its effect on wall time.