Performance issue ! all java guru invited

Question

hi !
plz let me know is there any performance related difference between these two approachs
mention below.
Ist
public void method1()
for(int i=0;i<20;i++)
String temp = "str"+i;
myObject mobj = new myObject();
2nd
String temp="":
myObject mobj=null;
public void method1(){
for(int i=0;i<20;i++)
temp = "str"+i;
mobj = new myObject();
}

Answer 1

Find out! Get the loop to run 20000 times and time each using System.currentTimeMillis(). Repeat this several times for each, throw away the highest and lowest couple of results and average the rest. Compare this between the two approaches.
Regardless, the performance difference is likely to be minor, if any, compared to the construction and allocation of memory for the new Objects. I would hope that most reasonable compilers would be able to deal with these anyway.
Performance issues aside, many people prefer the former approach since each local variable doesn't outstay its welcome. This makes debugging and refactoring more straightforward. As with all optimisations, it's not worth making them at the expense of clarity if they make no real difference.
Test the performance of your code, in the context of your application, in realistic circumstances. You'll soon find out where your effort should be spent on optimisations.
Hope this helps.