Local member is faster or instance member

Question

The following code proves that method1 is faster than method2. Can anyone please comment what is the reason for such behavior.

class Trial {
        String _member;
        void method1() {
            for(int i=0;i<30480;i++) {
                _member += "test";
            }
        }
        void method2() {
            String temp="";
            for(int i=0;i<30480;i++) {
                temp += "test";
            }
            _member = temp;
        }

        public static void main(String args[]) {
            Trial t = new Trial();
            long startTime1 = System.currentTimeMillis();
            t.method1();
            long endTime1 = System.currentTimeMillis();
            long startTime2 = System.currentTimeMillis();
            t.method2();
            long endTime2 = System.currentTimeMillis();
            System.out.println(endTime1 - startTime1);
            System.out.println(endTime2 - startTime2);
        }
    }

Answer 1

The following code proves that method1 is faster than method2

No. It does not prove it.

It depends on many factors. When I run this code, I get

1403
1248

So in my environment, your code "proves" that method1 is slower than method2.

When doing benchmarking, you need to take care of effects like caching and JVM warmup.

See also

• How do I write a correct micro-benchmark in Java?
• Avoid jvm warmup

for more information.

---

I slightly refactored the main method:

...

static void doBenchmark() {
   Trial t = new Trial();

   long startTime1 = System.currentTimeMillis();
   t.method1();
   long endTime1 = System.currentTimeMillis();

   long startTime2 = System.currentTimeMillis();
   t.method2();
   long endTime2 = System.currentTimeMillis();

   System.out.println(endTime1 - startTime1);
   System.out.println(endTime2 - startTime2);
}

public static void main(String args[]) {

   for (int i = 0;  i < 20;  i++) {
      doBenchmark();
      System.out.println("----");
   }
}

This results in similar values for the first iteration of the for-loop, but then the results converge and do not differ significantly anymore:

1396
1133
----
1052
1070
----
688
711
----
728
726
----
715
709
----
...

Even, sometimes method1 seems faster and sometimes method2 - this is most likely due to measurement inaccuracy.