Question

At first step I run this code:

public class Demo  {
    public static void main(String[] args) {
        String x = "x";
        long start = System.currentTimeMillis();

        for (int i = 0; i < 100000; i++)

        {

            x = x.concat("s");

            // x+="k";

        }

        System.out.println(System.currentTimeMillis() - start);
    }

}

Out: 13579.

On second step I run this code:

public class Demo {
    public static void main(String[] args) {
        String x = "x";
        long start = System.currentTimeMillis();

        for (int i = 0; i < 100000; i++)

        {

            //x = x.concat("s");

             x+="k";

        }

        System.out.println(System.currentTimeMillis() - start);
    }

}

Out: 27328.

And I have two questions:

1. Can I say that my banchmark – correct?
2. Why so big timeline difference between (+) and concat()??? 13.5 sec VS 27 sec. Why?

Answer 1

I think that your microbenchmark is fine, and I can reproduce your result.

On my JVM, the reason x += "k" is twice as slow is that under the covers it does the following:

1. create a new StringBuilder;
2. append x to the StringBuilder;
3. append "k" to the StringBuilder;
4. call StringBuilder.toString() and assign the result to x.

This copies the character data twice (once in step 2 and once in step 4).

On the other hand, x = x.concat("s") only copies the data once.

This double copying makes x += "k" two times slower than the other version.

If you’re curious, here are the bytecodes that my compiler has generated for the += loop:

   10:  goto    36
   13:  new #24; //class java/lang/StringBuilder
   16:  dup
   17:  aload_1
   18:  invokestatic    #26; //Method java/lang/String.valueOf:(Ljava/lang/Object;)Ljava/lang/String;
   21:  invokespecial   #32; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
   24:  ldc #35; //String k
   26:  invokevirtual   #37; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   29:  invokevirtual   #41; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   32:  astore_1
   33:  iinc    4, 1
   36:  iload   4
   38:  ldc #45; //int 100000
   40:  if_icmplt   13

Instructions 21 & 29 are where the two copies are made.